Merge patch series "nstree: listns()"

Christian Brauner <brauner@kernel.org> says:

As announced a while ago this is the next step building on the nstree
work from prior cycles. There's a bunch of fixes and semantic cleanups
in here and a ton of tests.

Currently listns() is relying on active namespace reference counts which
are introduced alongside this series.

While a namespace is on the namespace trees with a valid reference count
it is possible to reopen it through a namespace file handle. This is all
fine but has some issues that should be addressed.

On current kernels a namespace is visible to userspace in the
following cases:

(1) The namespace is in use by a task.
(2) The namespace is persisted through a VFS object (namespace file
    descriptor or bind-mount).
    Note that (2) only cares about direct persistence of the namespace
    itself not indirectly via e.g., file->f_cred file references or
    similar.
(3) The namespace is a hierarchical namespace type and is the parent of
    a single or multiple child namespaces.

Case (3) is interesting because it is possible that a parent namespace
might not fulfill any of (1) or (2), i.e., is invisible to userspace but
it may still be resurrected through the NS_GET_PARENT ioctl().

Currently namespace file handles allow much broader access to namespaces
than what is currently possible via (1)-(3). The reason is that
namespaces may remain pinned for completely internal reasons yet are
inaccessible to userspace.

For example, a user namespace my remain pinned by get_cred() calls to
stash the opener's credentials into file->f_cred. As it stands file
handles allow to resurrect such a users namespace even though this
should not be possible via (1)-(3). This is a fundamental uapi change
that we shouldn't do if we don't have to.

Consider the following insane case: Various architectures support the
CONFIG_MMU_LAZY_TLB_REFCOUNT option which uses lazy TLB destruction.
When this option is set a userspace task's struct mm_struct may be used
for kernel threads such as the idle task and will only be destroyed once
the cpu's runqueue switches back to another task. But because of ptrace()
permission checks struct mm_struct stashes the user namespace of the
task that struct mm_struct originally belonged to. The kernel thread
will take a reference on the struct mm_struct and thus pin it.

So on an idle system user namespaces can be persisted for arbitrary
amounts of time which also means that they can be resurrected using
namespace file handles. That makes no sense whatsoever. The problem is
of course excarabted on large systems with a huge number of cpus.

To handle this nicely we introduce an active reference count which
tracks (1)-(3). This is easy to do as all of these things are already
managed centrally. Only (1)-(3) will count towards the active reference
count and only namespaces which are active may be opened via namespace
file handles.

The problem is that namespaces may be resurrected. Which means that they
can become temporarily inactive and will be reactived some time later.
Currently the only example of this is the SIOGCSKNS socket ioctl. The
SIOCGSKNS ioctl allows to open a network namespace file descriptor based
on a socket file descriptor.

If a socket is tied to a network namespace that subsequently becomes
inactive but that socket is persisted by another process in another
network namespace (e.g., via SCM_RIGHTS of pidfd_getfd()) then the
SIOCGSKNS ioctl will resurrect this network namespace.

So calls to open_related_ns() and open_namespace() will end up
resurrecting the corresponding namespace tree.

Note that the active reference count does not regulate the lifetime of
the namespace itself. This is still done by the normal reference count.
The active reference count can only be elevated if the regular reference
count is elevated.

The active reference count also doesn't regulate the presence of a
namespace on the namespace trees. It only regulates its visiblity to
namespace file handles (and in later patches to listns()).

A namespace remains on the namespace trees from creation until its
actual destruction. This will allow the kernel to always reach any
namespace trivially and it will also enable subsystems like bpf to walk
the namespace lists on the system for tracing or general introspection
purposes.

Note that different namespaces have different visibility lifetimes on
current kernels. While most namespace are immediately released when the
last task using them exits, the user- and pid namespace are persisted
and thus both remain accessible via /proc/<pid>/ns/<ns_type>.

The user namespace lifetime is aliged with struct cred and is only
released through exit_creds(). However, it becomes inaccessible to
userspace once the last task using it is reaped, i.e., when
release_task() is called and all proc entries are flushed. Similarly,
the pid namespace is also visible until the last task using it has been
reaped and the associated pid numbers are freed.

The active reference counts of the user- and pid namespace are
decremented once the task is reaped.

Based on the namespace trees and the active reference count, a new
listns() system call that allows userspace to iterate through namespaces
in the system. This provides a programmatic interface to discover and
inspect namespaces, enhancing existing namespace apis.

Currently, there is no direct way for userspace to enumerate namespaces
in the system. Applications must resort to scanning /proc/<pid>/ns/
across all processes, which is:

1. Inefficient - requires iterating over all processes
2. Incomplete - misses inactive namespaces that aren't attached to any
   running process but are kept alive by file descriptors, bind mounts,
   or parent namespace references
3. Permission-heavy - requires access to /proc for many processes
4. No ordering or ownership.
5. No filtering per namespace type: Must always iterate and check all
   namespaces.

The list goes on. The listns() system call solves these problems by
providing direct kernel-level enumeration of namespaces. It is similar
to listmount() but obviously tailored to namespaces.

/*
 * @req: Pointer to struct ns_id_req specifying search parameters
 * @ns_ids: User buffer to receive namespace IDs
 * @nr_ns_ids: Size of ns_ids buffer (maximum number of IDs to return)
 * @flags: Reserved for future use (must be 0)
 */
ssize_t listns(const struct ns_id_req *req, u64 *ns_ids,
               size_t nr_ns_ids, unsigned int flags);

Returns:
- On success: Number of namespace IDs written to ns_ids
- On error: Negative error code

/*
 * @size: Structure size
 * @ns_id: Starting point for iteration; use 0 for first call, then
 *         use the last returned ID for subsequent calls to paginate
 * @ns_type: Bitmask of namespace types to include (from enum ns_type):
 *           0: Return all namespace types
 *           MNT_NS: Mount namespaces
 *           NET_NS: Network namespaces
 *           USER_NS: User namespaces
 *           etc. Can be OR'd together
 * @user_ns_id: Filter results to namespaces owned by this user namespace:
 *              0: Return all namespaces (subject to permission checks)
 *              LISTNS_CURRENT_USER: Namespaces owned by caller's user namespace
 *              Other value: Namespaces owned by the specified user namespace ID
 */
struct ns_id_req {
        __u32 size;         /* sizeof(struct ns_id_req) */
        __u32 spare;        /* Reserved, must be 0 */
        __u64 ns_id;        /* Last seen namespace ID (for pagination) */
        __u32 ns_type;      /* Filter by namespace type(s) */
        __u32 spare2;       /* Reserved, must be 0 */
        __u64 user_ns_id;   /* Filter by owning user namespace */
};

Example 1: List all namespaces

void list_all_namespaces(void)
{
	struct ns_id_req req = {
		.size = sizeof(req),
		.ns_id = 0,      /* Start from beginning */
		.ns_type = 0,    /* All types */
		.user_ns_id = 0, /* All user namespaces */
	};
	uint64_t ids[100];
	ssize_t ret;

	printf("All namespaces in the system:\n");
	do {
		ret = listns(&req, ids, 100, 0);
		if (ret < 0) {
			perror("listns");
			break;
		}

		for (ssize_t i = 0; i < ret; i++)
			printf("  Namespace ID: %llu\n", (unsigned long long)ids[i]);

		/* Continue from last seen ID */
		if (ret > 0)
			req.ns_id = ids[ret - 1];
	} while (ret == 100); /* Buffer was full, more may exist */
}

Example 2 : List network namespaces only

void list_network_namespaces(void)
{
	struct ns_id_req req = {
		.size = sizeof(req),
		.ns_id = 0,
		.ns_type = NET_NS, /* Only network namespaces */
		.user_ns_id = 0,
	};
	uint64_t ids[100];
	ssize_t ret;

	ret = listns(&req, ids, 100, 0);
	if (ret < 0) {
		perror("listns");
		return;
	}

	printf("Network namespaces: %zd found\n", ret);
	for (ssize_t i = 0; i < ret; i++)
		printf("  netns ID: %llu\n", (unsigned long long)ids[i]);
}

Example 3 : List namespaces owned by current user namespace

void list_owned_namespaces(void)
{
	struct ns_id_req req = {
		.size = sizeof(req),
		.ns_id = 0,
		.ns_type = 0,                      /* All types */
		.user_ns_id = LISTNS_CURRENT_USER, /* Current userns */
	};
	uint64_t ids[100];
	ssize_t ret;

	ret = listns(&req, ids, 100, 0);
	if (ret < 0) {
		perror("listns");
		return;
	}

	printf("Namespaces owned by my user namespace: %zd\n", ret);
	for (ssize_t i = 0; i < ret; i++)
		printf("  ns ID: %llu\n", (unsigned long long)ids[i]);
}

Example 4 : List multiple namespace types

void list_network_and_mount_namespaces(void)
{
	struct ns_id_req req = {
		.size = sizeof(req),
		.ns_id = 0,
		.ns_type = NET_NS | MNT_NS, /* Network and mount */
		.user_ns_id = 0,
	};
	uint64_t ids[100];
	ssize_t ret;

	ret = listns(&req, ids, 100, 0);
	printf("Network and mount namespaces: %zd found\n", ret);
}

Example 5 : Pagination through large namespace sets

void list_all_with_pagination(void)
{
	struct ns_id_req req = {
		.size = sizeof(req),
		.ns_id = 0,
		.ns_type = 0,
		.user_ns_id = 0,
	};
	uint64_t ids[50];
	size_t total = 0;
	ssize_t ret;

	printf("Enumerating all namespaces with pagination:\n");

	while (1) {
		ret = listns(&req, ids, 50, 0);
		if (ret < 0) {
			perror("listns");
			break;
		}
		if (ret == 0)
			break; /* No more namespaces */

		total += ret;
		printf("  Batch: %zd namespaces\n", ret);

		/* Last ID in this batch becomes start of next batch */
		req.ns_id = ids[ret - 1];

		if (ret < 50)
			break; /* Partial batch = end of results */
	}

	printf("Total: %zu namespaces\n", total);
}

listns() respects namespace isolation and capabilities:

(1) Global listing (user_ns_id = 0):
    - Requires CAP_SYS_ADMIN in the namespace's owning user namespace
    - OR the namespace must be in the caller's namespace context (e.g.,
      a namespace the caller is currently using)
    - User namespaces additionally allow listing if the caller has
      CAP_SYS_ADMIN in that user namespace itself
(2) Owner-filtered listing (user_ns_id != 0):
    - Requires CAP_SYS_ADMIN in the specified owner user namespace
    - OR the namespace must be in the caller's namespace context
    - This allows unprivileged processes to enumerate namespaces they own
(3) Visibility:
    - Only "active" namespaces are listed
    - A namespace is active if it has a non-zero __ns_ref_active count
    - This includes namespaces used by running processes, held by open
      file descriptors, or kept active by bind mounts
    - Inactive namespaces (kept alive only by internal kernel
      references) are not visible via listns()

* patches from https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-0-2e6f823ebdc0@kernel.org: (74 commits)
  selftests/namespace: test listns() pagination
  selftests/namespace: add stress test
  selftests/namespace: commit_creds() active reference tests
  selftests/namespace: third threaded active reference count test
  selftests/namespace: second threaded active reference count test
  selftests/namespace: first threaded active reference count test
  selftests/namespaces: twelth inactive namespace resurrection test
  selftests/namespaces: eleventh inactive namespace resurrection test
  selftests/namespaces: tenth inactive namespace resurrection test
  selftests/namespaces: ninth inactive namespace resurrection test
  selftests/namespaces: eigth inactive namespace resurrection test
  selftests/namespaces: seventh inactive namespace resurrection test
  selftests/namespaces: sixth inactive namespace resurrection test
  selftests/namespaces: fifth inactive namespace resurrection test
  selftests/namespaces: fourth inactive namespace resurrection test
  selftests/namespaces: third inactive namespace resurrection test
  selftests/namespaces: second inactive namespace resurrection test
  selftests/namespaces: first inactive namespace resurrection test
  selftests/namespaces: seventh listns() permission test
  selftests/namespaces: sixth listns() permission test
  ...

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-0-2e6f823ebdc0@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

56 files changed, 8858 insertions, 105 deletions

diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index 16dca28ebf17..3fed97478058 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -509,3 +509,4 @@
 577	common	open_tree_attr			sys_open_tree_attr
 578	common	file_getattr			sys_file_getattr
 579	common	file_setattr			sys_file_setattr
+580	common	listns				sys_listns
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index b07e699aaa3c..fd09afae72a2 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -484,3 +484,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/arm64/tools/syscall_32.tbl b/arch/arm64/tools/syscall_32.tbl
index 8d9088bc577d..8cdfe5d4dac9 100644
--- a/arch/arm64/tools/syscall_32.tbl
+++ b/arch/arm64/tools/syscall_32.tbl
@@ -481,3 +481,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index f41d38dfbf13..871a5d67bf41 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -469,3 +469,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index 580af574fe73..022fc85d94b3 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -475,3 +475,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
index d824ffe9a014..8cedc83c3266 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
@@ -408,3 +408,4 @@
 467	n32	open_tree_attr			sys_open_tree_attr
 468	n32	file_getattr			sys_file_getattr
 469	n32	file_setattr			sys_file_setattr
+470	n32	listns				sys_listns
diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index 7a7049c2c307..9b92bddf06b5 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
@@ -384,3 +384,4 @@
 467	n64	open_tree_attr			sys_open_tree_attr
 468	n64	file_getattr			sys_file_getattr
 469	n64	file_setattr			sys_file_setattr
+470	n64	listns				sys_listns
diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
index d330274f0601..f810b8a55716 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
@@ -457,3 +457,4 @@
 467	o32	open_tree_attr			sys_open_tree_attr
 468	o32	file_getattr			sys_file_getattr
 469	o32	file_setattr			sys_file_setattr
+470	o32	listns				sys_listns
diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index 88a788a7b18d..39bdacaa530b 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl
@@ -468,3 +468,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index b453e80dfc00..ec4458cdb97b 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -560,3 +560,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index 8a6744d658db..5863787ab036 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl
@@ -472,3 +472,4 @@
 467  common	open_tree_attr		sys_open_tree_attr		sys_open_tree_attr
 468  common	file_getattr		sys_file_getattr		sys_file_getattr
 469  common	file_setattr		sys_file_setattr		sys_file_setattr
+470  common	listns			sys_listns			sys_listns
diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
index 5e9c9eff5539..969c11325ade 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -473,3 +473,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index ebb7d06d1044..39aa26b6a50b 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -515,3 +515,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 4877e16da69a..e979a3eac7a3 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -475,3 +475,4 @@
 467	i386	open_tree_attr		sys_open_tree_attr
 468	i386	file_getattr		sys_file_getattr
 469	i386	file_setattr		sys_file_setattr
+470	i386	listns			sys_listns
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index ced2a1deecd7..8a4ac4841be6 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -394,6 +394,7 @@
 467	common	open_tree_attr		sys_open_tree_attr
 468	common	file_getattr		sys_file_getattr
 469	common	file_setattr		sys_file_setattr
+470	common	listns			sys_listns
 
 #
 # Due to a historical design error, certain syscalls are numbered differently
diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
index 374e4cb788d8..438a3b170402 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -440,3 +440,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/fs/libfs.c b/fs/libfs.c
index ce8c496a6940..4bb4d8a313e7 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -680,6 +680,7 @@ static int pseudo_fs_fill_super(struct super_block *s, struct fs_context *fc)
 	s->s_export_op = ctx->eops;
 	s->s_xattr = ctx->xattr;
 	s->s_time_gran = 1;
+	s->s_d_flags |= ctx->s_d_flags;
 	root = new_inode(s);
 	if (!root)
 		return -ENOMEM;
diff --git a/fs/namespace.c b/fs/namespace.c
index d82910f33dc4..eded33eeb647 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4094,7 +4094,7 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns, bool a
 		return ERR_PTR(ret);
 	}
 	if (!anon)
-		ns_tree_gen_id(&new_ns->ns);
+		ns_tree_gen_id(new_ns);
 	refcount_set(&new_ns->passive, 1);
 	new_ns->mounts = RB_ROOT;
 	init_waitqueue_head(&new_ns->poll);
@@ -5985,11 +5985,8 @@ SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req,
 }
 
 struct mnt_namespace init_mnt_ns = {
-	.ns.inum	= ns_init_inum(&init_mnt_ns),
-	.ns.ops		= &mntns_operations,
+	.ns		= NS_COMMON_INIT(init_mnt_ns, 1),
 	.user_ns	= &init_user_ns,
-	.ns.__ns_ref	= REFCOUNT_INIT(1),
-	.ns.ns_type	= ns_common_type(&init_mnt_ns),
 	.passive	= REFCOUNT_INIT(1),
 	.mounts		= RB_ROOT,
 	.poll		= __WAIT_QUEUE_HEAD_INITIALIZER(init_mnt_ns.poll),
diff --git a/fs/nsfs.c b/fs/nsfs.c
index 79b026a36fb6..ba6c8975c82e 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -58,6 +58,8 @@ const struct dentry_operations ns_dentry_operations = {
 static void nsfs_evict(struct inode *inode)
 {
 	struct ns_common *ns = inode->i_private;
+
+	__ns_ref_active_put(ns);
 	clear_inode(inode);
 	ns->ops->put(ns);
 }
@@ -408,6 +410,7 @@ static const struct super_operations nsfs_ops = {
 	.statfs = simple_statfs,
 	.evict_inode = nsfs_evict,
 	.show_path = nsfs_show_path,
+	.drop_inode = inode_just_drop,
 };
 
 static int nsfs_init_inode(struct inode *inode, void *data)
@@ -418,6 +421,16 @@ static int nsfs_init_inode(struct inode *inode, void *data)
 	inode->i_mode |= S_IRUGO;
 	inode->i_fop = &ns_file_operations;
 	inode->i_ino = ns->inum;
+
+	/*
+	 * Bring the namespace subtree back to life if we have to. This
+	 * can happen when e.g., all processes using a network namespace
+	 * and all namespace files or namespace file bind-mounts have
+	 * died but there are still sockets pinning it. The SIOCGSKNS
+	 * ioctl on such a socket will resurrect the relevant namespace
+	 * subtree.
+	 */
+	__ns_ref_active_resurrect(ns);
 	return 0;
 }
 
@@ -458,6 +471,45 @@ static int nsfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 	return FILEID_NSFS;
 }
 
+bool is_current_namespace(struct ns_common *ns)
+{
+	switch (ns->ns_type) {
+#ifdef CONFIG_CGROUPS
+	case CLONE_NEWCGROUP:
+		return current_in_namespace(to_cg_ns(ns));
+#endif
+#ifdef CONFIG_IPC_NS
+	case CLONE_NEWIPC:
+		return current_in_namespace(to_ipc_ns(ns));
+#endif
+	case CLONE_NEWNS:
+		return current_in_namespace(to_mnt_ns(ns));
+#ifdef CONFIG_NET_NS
+	case CLONE_NEWNET:
+		return current_in_namespace(to_net_ns(ns));
+#endif
+#ifdef CONFIG_PID_NS
+	case CLONE_NEWPID:
+		return current_in_namespace(to_pid_ns(ns));
+#endif
+#ifdef CONFIG_TIME_NS
+	case CLONE_NEWTIME:
+		return current_in_namespace(to_time_ns(ns));
+#endif
+#ifdef CONFIG_USER_NS
+	case CLONE_NEWUSER:
+		return current_in_namespace(to_user_ns(ns));
+#endif
+#ifdef CONFIG_UTS_NS
+	case CLONE_NEWUTS:
+		return current_in_namespace(to_uts_ns(ns));
+#endif
+	default:
+		VFS_WARN_ON_ONCE(true);
+		return false;
+	}
+}
+
 static struct dentry *nsfs_fh_to_dentry(struct super_block *sb, struct fid *fh,
 					int fh_len, int fh_type)
 {
@@ -483,18 +535,35 @@ static struct dentry *nsfs_fh_to_dentry(struct super_block *sb, struct fid *fh,
 		return NULL;
 	}
 
+	if (!fid->ns_id)
+		return NULL;
+	/* Either both are set or both are unset. */
+	if (!fid->ns_inum != !fid->ns_type)
+		return NULL;
+
 	scoped_guard(rcu) {
 		ns = ns_tree_lookup_rcu(fid->ns_id, fid->ns_type);
 		if (!ns)
 			return NULL;
 
 		VFS_WARN_ON_ONCE(ns->ns_id != fid->ns_id);
-		VFS_WARN_ON_ONCE(ns->ns_type != fid->ns_type);
 
-		if (ns->inum != fid->ns_inum)
+		if (fid->ns_inum && (fid->ns_inum != ns->inum))
+			return NULL;
+		if (fid->ns_type && (fid->ns_type != ns->ns_type))
 			return NULL;
 
-		if (!__ns_ref_get(ns))
+		/*
+		 * This is racy because we're not actually taking an
+		 * active reference. IOW, it could happen that the
+		 * namespace becomes inactive after this check.
+		 * We don't care because nsfs_init_inode() will just
+		 * resurrect the relevant namespace tree for us. If it
+		 * has been active here we just allow it's resurrection.
+		 * We could try to take an active reference here and
+		 * then drop it again. But really, why bother.
+		 */
+		if (!ns_get_unless_inactive(ns))
 			return NULL;
 	}
 
@@ -590,6 +659,8 @@ static int nsfs_init_fs_context(struct fs_context *fc)
 	struct pseudo_fs_context *ctx = init_pseudo(fc, NSFS_MAGIC);
 	if (!ctx)
 		return -ENOMEM;
+	fc->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;
+	ctx->s_d_flags |= DCACHE_DONTCACHE;
 	ctx->ops = &nsfs_ops;
 	ctx->eops = &nsfs_export_operations;
 	ctx->dops = &ns_dentry_operations;
@@ -612,3 +683,27 @@ void __init nsfs_init(void)
 	nsfs_root_path.mnt = nsfs_mnt;
 	nsfs_root_path.dentry = nsfs_mnt->mnt_root;
 }
+
+void nsproxy_ns_active_get(struct nsproxy *ns)
+{
+	ns_ref_active_get(ns->mnt_ns);
+	ns_ref_active_get(ns->uts_ns);
+	ns_ref_active_get(ns->ipc_ns);
+	ns_ref_active_get(ns->pid_ns_for_children);
+	ns_ref_active_get(ns->cgroup_ns);
+	ns_ref_active_get(ns->net_ns);
+	ns_ref_active_get(ns->time_ns);
+	ns_ref_active_get(ns->time_ns_for_children);
+}
+
+void nsproxy_ns_active_put(struct nsproxy *ns)
+{
+	ns_ref_active_put(ns->mnt_ns);
+	ns_ref_active_put(ns->uts_ns);
+	ns_ref_active_put(ns->ipc_ns);
+	ns_ref_active_put(ns->pid_ns_for_children);
+	ns_ref_active_put(ns->cgroup_ns);
+	ns_ref_active_put(ns->net_ns);
+	ns_ref_active_put(ns->time_ns);
+	ns_ref_active_put(ns->time_ns_for_children);
+}
diff --git a/fs/pidfs.c b/fs/pidfs.c
index 0ef5b47d796a..db236427fc2c 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -1022,6 +1022,7 @@ static int pidfs_init_fs_context(struct fs_context *fc)
 
 	fc->s_iflags |= SB_I_NOEXEC;
 	fc->s_iflags |= SB_I_NODEV;
+	ctx->s_d_flags |= DCACHE_DONTCACHE;
 	ctx->ops = &pidfs_sops;
 	ctx->eops = &pidfs_export_operations;
 	ctx->dops = &pidfs_dentry_operations;
diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index f5b68b8abb54..bd4492ef6ffc 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -4,7 +4,9 @@
 
 #include <linux/refcount.h>
 #include <linux/rbtree.h>
+#include <linux/vfsdebug.h>
 #include <uapi/linux/sched.h>
+#include <uapi/linux/nsfs.h>
 
 struct proc_ns_operations;
 
@@ -37,6 +39,67 @@ extern const struct proc_ns_operations cgroupns_operations;
 extern const struct proc_ns_operations timens_operations;
 extern const struct proc_ns_operations timens_for_children_operations;
 
+/*
+ * Namespace lifetimes are managed via a two-tier reference counting model:
+ *
+ * (1) __ns_ref (refcount_t): Main reference count tracking memory
+ *     lifetime. Controls when the namespace structure itself is freed.
+ *     It also pins the namespace on the namespace trees whereas (2)
+ *     only regulates their visibility to userspace.
+ *
+ * (2) __ns_ref_active (atomic_t): Reference count tracking active users.
+ *     Controls visibility of the namespace in the namespace trees.
+ *     Any live task that uses the namespace (via nsproxy or cred) holds
+ *     an active reference. Any open file descriptor or bind-mount of
+ *     the namespace holds an active reference. Once all tasks have
+ *     called exited their namespaces and all file descriptors and
+ *     bind-mounts have been released the active reference count drops
+ *     to zero and the namespace becomes inactive. IOW, the namespace
+ *     cannot be listed or opened via file handles anymore.
+ *
+ *     Note that it is valid to transition from active to inactive and
+ *     back from inactive to active e.g., when resurrecting an inactive
+ *     namespace tree via the SIOCGSKNS ioctl().
+ *
+ * Relationship and lifecycle states:
+ *
+ * - Active (__ns_ref_active > 0):
+ *   Namespace is actively used and visible to userspace. The namespace
+ *   can be reopened via /proc/<pid>/ns/<ns_type>, via namespace file
+ *   handles, or discovered via listns().
+ *
+ * - Inactive (__ns_ref_active == 0, __ns_ref > 0):
+ *   No tasks are actively using the namespace and it isn't pinned by
+ *   any bind-mounts or open file descriptors anymore. But the namespace
+ *   is still kept alive by internal references. For example, the user
+ *   namespace could be pinned by an open file through file->f_cred
+ *   references when one of the now defunct tasks had opened a file and
+ *   handed the file descriptor off to another process via a UNIX
+ *   sockets. Such references keep the namespace structure alive through
+ *   __ns_ref but will not hold an active reference.
+ *
+ * - Destroyed (__ns_ref == 0):
+ *   No references remain. The namespace is removed from the tree and freed.
+ *
+ * State transitions:
+ *
+ * Active -> Inactive:
+ *   When the last task using the namespace exits it drops its active
+ *   references to all namespaces. However, user and pid namespaces
+ *   remain accessible until the task has been reaped.
+ *
+ * Inactive -> Active:
+ *   An inactive namespace tree might be resurrected due to e.g., the
+ *   SIOCGSKNS ioctl() on a socket.
+ *
+ * Inactive -> Destroyed:
+ *   When __ns_ref drops to zero the namespace is removed from the
+ *   namespaces trees and the memory is freed (after RCU grace period).
+ *
+ * Initial namespaces:
+ *   Boot-time namespaces (init_net, init_pid_ns, etc.) start with
+ *   __ns_ref_active = 1 and remain active forever.
+ */
 struct ns_common {
 	u32 ns_type;
 	struct dentry *stashed;
@@ -46,15 +109,37 @@ struct ns_common {
 	union {
 		struct {
 			u64 ns_id;
-			struct rb_node ns_tree_node;
-			struct list_head ns_list_node;
+			struct /* global namespace rbtree and list */ {
+				struct rb_node ns_unified_tree_node;
+				struct list_head ns_unified_list_node;
+			};
+			struct /* per type rbtree and list */ {
+				struct rb_node ns_tree_node;
+				struct list_head ns_list_node;
+			};
+			struct /* namespace ownership rbtree and list */ {
+				struct rb_root ns_owner_tree; /* rbtree of namespaces owned by this namespace */
+				struct list_head ns_owner; /* list of namespaces owned by this namespace */
+				struct rb_node ns_owner_tree_node; /* node in the owner namespace's rbtree */
+				struct list_head ns_owner_entry; /* node in the owner namespace's ns_owned list */
+			};
+			atomic_t __ns_ref_active; /* do not use directly */
 		};
 		struct rcu_head ns_rcu;
 	};
 };
 
+bool is_current_namespace(struct ns_common *ns);
 int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_operations *ops, int inum);
 void __ns_common_free(struct ns_common *ns);
+struct ns_common *__must_check ns_owner(struct ns_common *ns);
+
+static __always_inline bool is_initial_namespace(struct ns_common *ns)
+{
+	VFS_WARN_ON_ONCE(ns->inum == 0);
+	return unlikely(in_range(ns->inum, MNT_NS_INIT_INO,
+				 IPC_NS_INIT_INO - MNT_NS_INIT_INO + 1));
+}
 
 #define to_ns_common(__ns)                                    \
 	_Generic((__ns),                                      \
@@ -97,6 +182,17 @@ void __ns_common_free(struct ns_common *ns);
 		struct user_namespace *:   &init_user_ns,   \
 		struct uts_namespace *:    &init_uts_ns)
 
+#define ns_init_id(__ns)						\
+	_Generic((__ns),						\
+		struct cgroup_namespace *:	CGROUP_NS_INIT_ID,	\
+		struct ipc_namespace *:		IPC_NS_INIT_ID,		\
+		struct mnt_namespace *:		MNT_NS_INIT_ID,		\
+		struct net *:			NET_NS_INIT_ID,		\
+		struct pid_namespace *:		PID_NS_INIT_ID,		\
+		struct time_namespace *:	TIME_NS_INIT_ID,	\
+		struct user_namespace *:	USER_NS_INIT_ID,	\
+		struct uts_namespace *:		UTS_NS_INIT_ID)
+
 #define to_ns_operations(__ns)                                                                         \
 	_Generic((__ns),                                                                               \
 		struct cgroup_namespace *: (IS_ENABLED(CONFIG_CGROUPS) ? &cgroupns_operations : NULL), \
@@ -119,6 +215,21 @@ void __ns_common_free(struct ns_common *ns);
 		struct user_namespace *:   CLONE_NEWUSER,   \
 		struct uts_namespace *:    CLONE_NEWUTS)
 
+#define NS_COMMON_INIT(nsname, refs)							\
+{											\
+	.ns_type		= ns_common_type(&nsname),				\
+	.ns_id			= ns_init_id(&nsname),					\
+	.inum			= ns_init_inum(&nsname),				\
+	.ops			= to_ns_operations(&nsname),				\
+	.stashed		= NULL,							\
+	.__ns_ref		= REFCOUNT_INIT(refs),					\
+	.__ns_ref_active	= ATOMIC_INIT(1),					\
+	.ns_list_node		= LIST_HEAD_INIT(nsname.ns.ns_list_node),		\
+	.ns_owner_entry		= LIST_HEAD_INIT(nsname.ns.ns_owner_entry),		\
+	.ns_owner		= LIST_HEAD_INIT(nsname.ns.ns_owner),			\
+	.ns_unified_list_node	= LIST_HEAD_INIT(nsname.ns.ns_unified_list_node),	\
+}
+
 #define ns_common_init(__ns)                     \
 	__ns_common_init(to_ns_common(__ns),     \
 			 ns_common_type(__ns),   \
@@ -133,21 +244,91 @@ void __ns_common_free(struct ns_common *ns);
 
 #define ns_common_free(__ns) __ns_common_free(to_ns_common((__ns)))
 
+static __always_inline __must_check int __ns_ref_active_read(const struct ns_common *ns)
+{
+	return atomic_read(&ns->__ns_ref_active);
+}
+
 static __always_inline __must_check bool __ns_ref_put(struct ns_common *ns)
 {
-	return refcount_dec_and_test(&ns->__ns_ref);
+	if (refcount_dec_and_test(&ns->__ns_ref)) {
+		VFS_WARN_ON_ONCE(__ns_ref_active_read(ns));
+		return true;
+	}
+	return false;
 }
 
 static __always_inline __must_check bool __ns_ref_get(struct ns_common *ns)
 {
-	return refcount_inc_not_zero(&ns->__ns_ref);
+	if (refcount_inc_not_zero(&ns->__ns_ref))
+		return true;
+	VFS_WARN_ON_ONCE(__ns_ref_active_read(ns));
+	return false;
+}
+
+static __always_inline __must_check int __ns_ref_read(const struct ns_common *ns)
+{
+	return refcount_read(&ns->__ns_ref);
 }
 
-#define ns_ref_read(__ns) refcount_read(&to_ns_common((__ns))->__ns_ref)
+#define ns_ref_read(__ns) __ns_ref_read(to_ns_common((__ns)))
 #define ns_ref_inc(__ns) refcount_inc(&to_ns_common((__ns))->__ns_ref)
 #define ns_ref_get(__ns) __ns_ref_get(to_ns_common((__ns)))
 #define ns_ref_put(__ns) __ns_ref_put(to_ns_common((__ns)))
 #define ns_ref_put_and_lock(__ns, __lock) \
 	refcount_dec_and_lock(&to_ns_common((__ns))->__ns_ref, (__lock))
 
+#define ns_ref_active_read(__ns) \
+	((__ns) ? __ns_ref_active_read(to_ns_common(__ns)) : 0)
+
+void __ns_ref_active_get_owner(struct ns_common *ns);
+
+static __always_inline void __ns_ref_active_get(struct ns_common *ns)
+{
+	WARN_ON_ONCE(atomic_add_negative(1, &ns->__ns_ref_active));
+	VFS_WARN_ON_ONCE(is_initial_namespace(ns) && __ns_ref_active_read(ns) <= 0);
+}
+#define ns_ref_active_get(__ns) \
+	do { if (__ns) __ns_ref_active_get(to_ns_common(__ns)); } while (0)
+
+static __always_inline bool __ns_ref_active_get_not_zero(struct ns_common *ns)
+{
+	if (atomic_inc_not_zero(&ns->__ns_ref_active)) {
+		VFS_WARN_ON_ONCE(!__ns_ref_read(ns));
+		return true;
+	}
+	return false;
+}
+
+#define ns_ref_active_get_owner(__ns) \
+	do { if (__ns) __ns_ref_active_get_owner(to_ns_common(__ns)); } while (0)
+
+void __ns_ref_active_put_owner(struct ns_common *ns);
+
+static __always_inline void __ns_ref_active_put(struct ns_common *ns)
+{
+	if (atomic_dec_and_test(&ns->__ns_ref_active)) {
+		VFS_WARN_ON_ONCE(is_initial_namespace(ns));
+		VFS_WARN_ON_ONCE(!__ns_ref_read(ns));
+		__ns_ref_active_put_owner(ns);
+	}
+}
+#define ns_ref_active_put(__ns) \
+	do { if (__ns) __ns_ref_active_put(to_ns_common(__ns)); } while (0)
+
+static __always_inline struct ns_common *__must_check ns_get_unless_inactive(struct ns_common *ns)
+{
+	VFS_WARN_ON_ONCE(__ns_ref_active_read(ns) && !__ns_ref_read(ns));
+	if (!__ns_ref_active_read(ns))
+		return NULL;
+	if (!__ns_ref_get(ns))
+		return NULL;
+	return ns;
+}
+
+void __ns_ref_active_resurrect(struct ns_common *ns);
+
+#define ns_ref_active_resurrect(__ns) \
+	do { if (__ns) __ns_ref_active_resurrect(to_ns_common(__ns)); } while (0)
+
 #endif
diff --git a/include/linux/nsfs.h b/include/linux/nsfs.h
index e5a5fa83d36b..731b67fc2fec 100644
--- a/include/linux/nsfs.h
+++ b/include/linux/nsfs.h
@@ -37,4 +37,7 @@ void nsfs_init(void);
 
 #define current_in_namespace(__ns) (__current_namespace_from_type(__ns) == __ns)
 
+void nsproxy_ns_active_get(struct nsproxy *ns);
+void nsproxy_ns_active_put(struct nsproxy *ns);
+
 #endif /* _LINUX_NSFS_H */
diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
index bd118a187dec..ac825eddec59 100644
--- a/include/linux/nsproxy.h
+++ b/include/linux/nsproxy.h
@@ -93,7 +93,10 @@ static inline struct cred *nsset_cred(struct nsset *set)
  */
 
 int copy_namespaces(u64 flags, struct task_struct *tsk);
-void exit_task_namespaces(struct task_struct *tsk);
+void switch_cred_namespaces(const struct cred *old, const struct cred *new);
+void exit_nsproxy_namespaces(struct task_struct *tsk);
+void get_cred_namespaces(struct task_struct *tsk);
+void exit_cred_namespaces(struct task_struct *tsk);
 void switch_task_namespaces(struct task_struct *tsk, struct nsproxy *new);
 int exec_task_namespaces(void);
 void free_nsproxy(struct nsproxy *ns);
diff --git a/include/linux/nstree.h b/include/linux/nstree.h
index 8b8636690473..38674c6fa4f7 100644
--- a/include/linux/nstree.h
+++ b/include/linux/nstree.h
@@ -1,4 +1,5 @@
 /* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (c) 2025 Christian Brauner <brauner@kernel.org> */
 #ifndef _LINUX_NSTREE_H
 #define _LINUX_NSTREE_H
 
@@ -8,6 +9,7 @@
 #include <linux/seqlock.h>
 #include <linux/rculist.h>
 #include <linux/cookie.h>
+#include <uapi/linux/nsfs.h>
 
 extern struct ns_tree cgroup_ns_tree;
 extern struct ns_tree ipc_ns_tree;
@@ -29,7 +31,11 @@ extern struct ns_tree uts_ns_tree;
 		struct user_namespace *:   &(user_ns_tree),	\
 		struct uts_namespace *:    &(uts_ns_tree))
 
-u64 ns_tree_gen_id(struct ns_common *ns);
+#define ns_tree_gen_id(__ns)                 \
+	__ns_tree_gen_id(to_ns_common(__ns), \
+			 (((__ns) == ns_init_ns(__ns)) ? ns_init_id(__ns) : 0))
+
+u64 __ns_tree_gen_id(struct ns_common *ns, u64 id);
 void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree);
 void __ns_tree_remove(struct ns_common *ns, struct ns_tree *ns_tree);
 struct ns_common *ns_tree_lookup_rcu(u64 ns_id, int ns_type);
@@ -37,9 +43,9 @@ struct ns_common *__ns_tree_adjoined_rcu(struct ns_common *ns,
 					 struct ns_tree *ns_tree,
 					 bool previous);
 
-static inline void __ns_tree_add(struct ns_common *ns, struct ns_tree *ns_tree)
+static inline void __ns_tree_add(struct ns_common *ns, struct ns_tree *ns_tree, u64 id)
 {
-	ns_tree_gen_id(ns);
+	__ns_tree_gen_id(ns, id);
 	__ns_tree_add_raw(ns, ns_tree);
 }
 
@@ -59,7 +65,9 @@ static inline void __ns_tree_add(struct ns_common *ns, struct ns_tree *ns_tree)
  * This function assigns a new id to the namespace and adds it to the
  * appropriate namespace tree and list.
  */
-#define ns_tree_add(__ns) __ns_tree_add(to_ns_common(__ns), to_ns_tree(__ns))
+#define ns_tree_add(__ns)                                   \
+	__ns_tree_add(to_ns_common(__ns), to_ns_tree(__ns), \
+		      (((__ns) == ns_init_ns(__ns)) ? ns_init_id(__ns) : 0))
 
 /**
  * ns_tree_remove - Remove a namespace from a namespace tree
diff --git a/include/linux/pseudo_fs.h b/include/linux/pseudo_fs.h
index 2503f7625d65..a651e60d9410 100644
--- a/include/linux/pseudo_fs.h
+++ b/include/linux/pseudo_fs.h
@@ -9,6 +9,7 @@ struct pseudo_fs_context {
 	const struct xattr_handler * const *xattr;
 	const struct dentry_operations *dops;
 	unsigned long magic;
+	unsigned int s_d_flags;
 };
 
 struct pseudo_fs_context *init_pseudo(struct fs_context *fc,
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 66c06fcdfe19..cf84d98964b2 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -77,6 +77,7 @@ struct cachestat_range;
 struct cachestat;
 struct statmount;
 struct mnt_id_req;
+struct ns_id_req;
 struct xattr_args;
 struct file_attr;
 
@@ -437,6 +438,9 @@ asmlinkage long sys_statmount(const struct mnt_id_req __user *req,
 asmlinkage long sys_listmount(const struct mnt_id_req __user *req,
 			      u64 __user *mnt_ids, size_t nr_mnt_ids,
 			      unsigned int flags);
+asmlinkage long sys_listns(const struct ns_id_req __user *req,
+			   u64 __user *ns_ids, size_t nr_ns_ids,
+			   unsigned int flags);
 asmlinkage long sys_truncate(const char __user *path, long length);
 asmlinkage long sys_ftruncate(unsigned int fd, off_t length);
 #if BITS_PER_LONG == 32
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 9a9aebbf96b9..9c3be157397e 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -166,13 +166,13 @@ static inline void set_userns_rlimit_max(struct user_namespace *ns,
 	ns->rlimit_max[type] = max <= LONG_MAX ? max : LONG_MAX;
 }
 
-#ifdef CONFIG_USER_NS
-
 static inline struct user_namespace *to_user_ns(struct ns_common *ns)
 {
 	return container_of(ns, struct user_namespace, ns);
 }
 
+#ifdef CONFIG_USER_NS
+
 static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
 {
 	if (ns)
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 04e0077fb4c9..942370b3f5d2 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -857,9 +857,11 @@ __SYSCALL(__NR_open_tree_attr, sys_open_tree_attr)
 __SYSCALL(__NR_file_getattr, sys_file_getattr)
 #define __NR_file_setattr 469
 __SYSCALL(__NR_file_setattr, sys_file_setattr)
+#define __NR_listns 470
+__SYSCALL(__NR_listns, sys_listns)
 
 #undef __NR_syscalls
-#define __NR_syscalls 470
+#define __NR_syscalls 471
 
 /*
  * 32 bit systems traditionally used different
diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
index e098759ec917..a25e38d1c874 100644
--- a/include/uapi/linux/nsfs.h
+++ b/include/uapi/linux/nsfs.h
@@ -67,4 +67,62 @@ struct nsfs_file_handle {
 #define NSFS_FILE_HANDLE_SIZE_VER0 16 /* sizeof first published struct */
 #define NSFS_FILE_HANDLE_SIZE_LATEST sizeof(struct nsfs_file_handle) /* sizeof latest published struct */
 
+enum init_ns_id {
+	IPC_NS_INIT_ID		= 1ULL,
+	UTS_NS_INIT_ID		= 2ULL,
+	USER_NS_INIT_ID		= 3ULL,
+	PID_NS_INIT_ID		= 4ULL,
+	CGROUP_NS_INIT_ID	= 5ULL,
+	TIME_NS_INIT_ID		= 6ULL,
+	NET_NS_INIT_ID		= 7ULL,
+	MNT_NS_INIT_ID		= 8ULL,
+#ifdef __KERNEL__
+	NS_LAST_INIT_ID		= MNT_NS_INIT_ID,
+#endif
+};
+
+enum ns_type {
+	TIME_NS    = (1ULL << 7),  /* CLONE_NEWTIME */
+	MNT_NS     = (1ULL << 17), /* CLONE_NEWNS */
+	CGROUP_NS  = (1ULL << 25), /* CLONE_NEWCGROUP */
+	UTS_NS     = (1ULL << 26), /* CLONE_NEWUTS */
+	IPC_NS     = (1ULL << 27), /* CLONE_NEWIPC */
+	USER_NS    = (1ULL << 28), /* CLONE_NEWUSER */
+	PID_NS     = (1ULL << 29), /* CLONE_NEWPID */
+	NET_NS     = (1ULL << 30), /* CLONE_NEWNET */
+};
+
+/**
+ * struct ns_id_req - namespace ID request structure
+ * @size: size of this structure
+ * @spare: reserved for future use
+ * @filter: filter mask
+ * @ns_id: last namespace id
+ * @user_ns_id: owning user namespace ID
+ *
+ * Structure for passing namespace ID and miscellaneous parameters to
+ * statns(2) and listns(2).
+ *
+ * For statns(2) @param represents the request mask.
+ * For listns(2) @param represents the last listed mount id (or zero).
+ */
+struct ns_id_req {
+	__u32 size;
+	__u32 spare;
+	__u64 ns_id;
+	struct /* listns */ {
+		__u32 ns_type;
+		__u32 spare2;
+		__u64 user_ns_id;
+	};
+};
+
+/*
+ * Special @user_ns_id value that can be passed to listns()
+ */
+#define LISTNS_CURRENT_USER 0xffffffffffffffff /* Caller's userns */
+
+/* List of all ns_id_req versions. */
+#define NS_ID_REQ_SIZE_VER0 32 /* sizeof first published struct */
+
 #endif /* __LINUX_NSFS_H */
diff --git a/init/version-timestamp.c b/init/version-timestamp.c
index d071835121c2..56ded64fdfe4 100644
--- a/init/version-timestamp.c
+++ b/init/version-timestamp.c
@@ -8,8 +8,7 @@
 #include <linux/utsname.h>
 
 struct uts_namespace init_uts_ns = {
-	.ns.ns_type = ns_common_type(&init_uts_ns),
-	.ns.__ns_ref = REFCOUNT_INIT(2),
+	.ns = NS_COMMON_INIT(init_uts_ns, 2),
 	.name = {
 		.sysname	= UTS_SYSNAME,
 		.nodename	= UTS_NODENAME,
@@ -19,10 +18,6 @@ struct uts_namespace init_uts_ns = {
 		.domainname	= UTS_DOMAINNAME,
 	},
 	.user_ns = &init_user_ns,
-	.ns.inum = ns_init_inum(&init_uts_ns),
-#ifdef CONFIG_UTS_NS
-	.ns.ops = &utsns_operations,
-#endif
 };
 
 /* FIXED STRINGS! Don't touch! */
diff --git a/ipc/msgutil.c b/ipc/msgutil.c
index 7a03f6d03de3..55a908ec0674 100644
--- a/ipc/msgutil.c
+++ b/ipc/msgutil.c
@@ -27,13 +27,8 @@ DEFINE_SPINLOCK(mq_lock);
  * and not CONFIG_IPC_NS.
  */
 struct ipc_namespace init_ipc_ns = {
-	.ns.__ns_ref = REFCOUNT_INIT(1),
+	.ns = NS_COMMON_INIT(init_ipc_ns, 1),
 	.user_ns = &init_user_ns,
-	.ns.inum = ns_init_inum(&init_ipc_ns),
-#ifdef CONFIG_IPC_NS
-	.ns.ops = &ipcns_operations,
-#endif
-	.ns.ns_type = ns_common_type(&init_ipc_ns),
 };
 
 struct msg_msgseg {
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 6ae5f48cf64e..b758a9dd7526 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -250,12 +250,9 @@ bool cgroup_enable_per_threadgroup_rwsem __read_mostly;
 
 /* cgroup namespace for init task */
 struct cgroup_namespace init_cgroup_ns = {
-	.ns.__ns_ref	= REFCOUNT_INIT(2),
+	.ns		= NS_COMMON_INIT(init_cgroup_ns, 2),
 	.user_ns	= &init_user_ns,
-	.ns.ops		= &cgroupns_operations,
-	.ns.inum	= ns_init_inum(&init_cgroup_ns),
 	.root_cset	= &init_css_set,
-	.ns.ns_type	= ns_common_type(&init_cgroup_ns),
 };
 
 static struct file_system_type cgroup2_fs_type;
@@ -1522,9 +1519,9 @@ static struct cgroup *current_cgns_cgroup_dfl(void)
 	} else {
 		/*
 		 * NOTE: This function may be called from bpf_cgroup_from_id()
-		 * on a task which has already passed exit_task_namespaces() and
-		 * nsproxy == NULL. Fall back to cgrp_dfl_root which will make all
-		 * cgroups visible for lookups.
+		 * on a task which has already passed exit_nsproxy_namespaces()
+		 * and nsproxy == NULL. Fall back to cgrp_dfl_root which will
+		 * make all cgroups visible for lookups.
 		 */
 		return &cgrp_dfl_root.cgrp;
 	}
diff --git a/kernel/cgroup/namespace.c b/kernel/cgroup/namespace.c
index fdbe57578e68..db9617556dd7 100644
--- a/kernel/cgroup/namespace.c
+++ b/kernel/cgroup/namespace.c
@@ -30,7 +30,6 @@ static struct cgroup_namespace *alloc_cgroup_ns(void)
 	ret = ns_common_init(new_ns);
 	if (ret)
 		return ERR_PTR(ret);
-	ns_tree_add(new_ns);
 	return no_free_ptr(new_ns);
 }
 
@@ -86,6 +85,7 @@ struct cgroup_namespace *copy_cgroup_ns(u64 flags,
 	new_ns->ucounts = ucounts;
 	new_ns->root_cset = cset;
 
+	ns_tree_add(new_ns);
 	return new_ns;
 }
 
diff --git a/kernel/cred.c b/kernel/cred.c
index dbf6b687dc5c..a6e7f580df14 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -306,6 +306,7 @@ int copy_creds(struct task_struct *p, u64 clone_flags)
 		kdebug("share_creds(%p{%ld})",
 		       p->cred, atomic_long_read(&p->cred->usage));
 		inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
+		get_cred_namespaces(p);
 		return 0;
 	}
 
@@ -343,6 +344,8 @@ int copy_creds(struct task_struct *p, u64 clone_flags)
 
 	p->cred = p->real_cred = get_cred(new);
 	inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
+	get_cred_namespaces(p);
+
 	return 0;
 
 error_put:
@@ -435,10 +438,13 @@ int commit_creds(struct cred *new)
 	 */
 	if (new->user != old->user || new->user_ns != old->user_ns)
 		inc_rlimit_ucounts(new->ucounts, UCOUNT_RLIMIT_NPROC, 1);
+
 	rcu_assign_pointer(task->real_cred, new);
 	rcu_assign_pointer(task->cred, new);
 	if (new->user != old->user || new->user_ns != old->user_ns)
 		dec_rlimit_ucounts(old->ucounts, UCOUNT_RLIMIT_NPROC, 1);
+	if (new->user_ns != old->user_ns)
+		switch_cred_namespaces(old, new);
 
 	/* send notifications */
 	if (!uid_eq(new->uid,   old->uid)  ||
diff --git a/kernel/exit.c b/kernel/exit.c
index 9f74e8f1c431..988e16efd66b 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -291,6 +291,7 @@ repeat:
 	write_unlock_irq(&tasklist_lock);
 	/* @thread_pid can't go away until free_pids() below */
 	proc_flush_pid(thread_pid);
+	exit_cred_namespaces(p);
 	add_device_randomness(&p->se.sum_exec_runtime,
 			      sizeof(p->se.sum_exec_runtime));
 	free_pids(post.pids);
@@ -962,7 +963,7 @@ void __noreturn do_exit(long code)
 	exit_fs(tsk);
 	if (group_dead)
 		disassociate_ctty(1);
-	exit_task_namespaces(tsk);
+	exit_nsproxy_namespaces(tsk);
 	exit_task_work(tsk);
 	exit_thread(tsk);
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 3da0f08615a9..f1857672426e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2453,7 +2453,7 @@ bad_fork_cleanup_io:
 	if (p->io_context)
 		exit_io_context(p);
 bad_fork_cleanup_namespaces:
-	exit_task_namespaces(p);
+	exit_nsproxy_namespaces(p);
 bad_fork_cleanup_mm:
 	if (p->mm) {
 		mm_clear_owner(p->mm, p);
@@ -2487,6 +2487,7 @@ bad_fork_cleanup_delayacct:
 	delayacct_tsk_free(p);
 bad_fork_cleanup_count:
 	dec_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
+	exit_cred_namespaces(p);
 	exit_creds(p);
 bad_fork_free:
 	WRITE_ONCE(p->__state, TASK_DEAD);
diff --git a/kernel/nscommon.c b/kernel/nscommon.c
index c1fb2bad6d72..6fe1c747fa46 100644
--- a/kernel/nscommon.c
+++ b/kernel/nscommon.c
@@ -1,7 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2025 Christian Brauner <brauner@kernel.org> */
 
 #include <linux/ns_common.h>
 #include <linux/proc_ns.h>
+#include <linux/user_namespace.h>
 #include <linux/vfsdebug.h>
 
 #ifdef CONFIG_DEBUG_VFS
@@ -52,13 +54,21 @@ static void ns_debug(struct ns_common *ns, const struct proc_ns_operations *ops)
 
 int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_operations *ops, int inum)
 {
+	int ret;
+
 	refcount_set(&ns->__ns_ref, 1);
 	ns->stashed = NULL;
 	ns->ops = ops;
 	ns->ns_id = 0;
 	ns->ns_type = ns_type;
 	RB_CLEAR_NODE(&ns->ns_tree_node);
+	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
+	RB_CLEAR_NODE(&ns->ns_owner_tree_node);
 	INIT_LIST_HEAD(&ns->ns_list_node);
+	INIT_LIST_HEAD(&ns->ns_unified_list_node);
+	ns->ns_owner_tree = RB_ROOT;
+	INIT_LIST_HEAD(&ns->ns_owner);
+	INIT_LIST_HEAD(&ns->ns_owner_entry);
 
 #ifdef CONFIG_DEBUG_VFS
 	ns_debug(ns, ops);
@@ -68,10 +78,219 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
 		ns->inum = inum;
 		return 0;
 	}
-	return proc_alloc_inum(&ns->inum);
+	ret = proc_alloc_inum(&ns->inum);
+	if (ret)
+		return ret;
+	/*
+	 * Tree ref starts at 0. It's incremented when namespace enters
+	 * active use (installed in nsproxy) and decremented when all
+	 * active uses are gone. Initial namespaces are always active.
+	 */
+	if (is_initial_namespace(ns))
+		atomic_set(&ns->__ns_ref_active, 1);
+	else
+		atomic_set(&ns->__ns_ref_active, 0);
+	return 0;
 }
 
 void __ns_common_free(struct ns_common *ns)
 {
 	proc_free_inum(ns->inum);
 }
+
+struct ns_common *__must_check ns_owner(struct ns_common *ns)
+{
+	struct user_namespace *owner;
+
+	if (unlikely(!ns->ops))
+		return NULL;
+	VFS_WARN_ON_ONCE(!ns->ops->owner);
+	owner = ns->ops->owner(ns);
+	VFS_WARN_ON_ONCE(!owner && ns != to_ns_common(&init_user_ns));
+	if (!owner)
+		return NULL;
+	/* Skip init_user_ns as it's always active */
+	if (owner == &init_user_ns)
+		return NULL;
+	return to_ns_common(owner);
+}
+
+void __ns_ref_active_get_owner(struct ns_common *ns)
+{
+	ns = ns_owner(ns);
+	if (ns)
+		WARN_ON_ONCE(atomic_add_negative(1, &ns->__ns_ref_active));
+}
+
+/*
+ * The active reference count works by having each namespace that gets
+ * created take a single active reference on its owning user namespace.
+ * That single reference is only released once the child namespace's
+ * active count itself goes down.
+ *
+ * A regular namespace tree might look as follow:
+ * Legend:
+ * + : adding active reference
+ * - : dropping active reference
+ * x : always active (initial namespace)
+ *
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        +      +
+ *                        user_ns1 (2)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   +   +
+ *                        user_ns2 (3)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * If both net_ns and pid_ns put their last active reference on
+ * themselves it will cascade to user_ns1 dropping its own active
+ * reference and dropping one active reference on user_ns2:
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        -      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   -   +
+ *                        user_ns2 (2)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * The iteration stops once we reach a namespace that still has active
+ * references.
+ */
+void __ns_ref_active_put_owner(struct ns_common *ns)
+{
+	for (;;) {
+		ns = ns_owner(ns);
+		if (!ns)
+			return;
+		if (!atomic_dec_and_test(&ns->__ns_ref_active))
+			return;
+	}
+}
+
+/*
+ * The active reference count works by having each namespace that gets
+ * created take a single active reference on its owning user namespace.
+ * That single reference is only released once the child namespace's
+ * active count itself goes down. This makes it possible to efficiently
+ * resurrect a namespace tree:
+ *
+ * A regular namespace tree might look as follow:
+ * Legend:
+ * + : adding active reference
+ * - : dropping active reference
+ * x : always active (initial namespace)
+ *
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        +      +
+ *                        user_ns1 (2)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   +   +
+ *                        user_ns2 (3)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * If both net_ns and pid_ns put their last active reference on
+ * themselves it will cascade to user_ns1 dropping its own active
+ * reference and dropping one active reference on user_ns2:
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        -      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   -   +
+ *                        user_ns2 (2)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * Assume the whole tree is dead but all namespaces are still active:
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        -      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        -   -   -
+ *                        user_ns2 (0)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * Now assume the net_ns gets resurrected (.e.g., via the SIOCGSKNS ioctl()):
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        +      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        -   +   -
+ *                        user_ns2 (0)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * If net_ns had a zero reference count and we bumped it we also need to
+ * take another reference on its owning user namespace. Similarly, if
+ * pid_ns had a zero reference count it also needs to take another
+ * reference on its owning user namespace. So both net_ns and pid_ns
+ * will each have their own reference on the owning user namespace.
+ *
+ * If the owning user namespace user_ns1 had a zero reference count then
+ * it also needs to take another reference on its owning user namespace
+ * and so on.
+ */
+void __ns_ref_active_resurrect(struct ns_common *ns)
+{
+	/* If we didn't resurrect the namespace we're done. */
+	if (atomic_fetch_add(1, &ns->__ns_ref_active))
+		return;
+
+	/*
+	 * We did resurrect it. Walk the ownership hierarchy upwards
+	 * until we found an owning user namespace that is active.
+	 */
+	for (;;) {
+		ns = ns_owner(ns);
+		if (!ns)
+			return;
+
+		if (atomic_fetch_add(1, &ns->__ns_ref_active))
+			return;
+	}
+}
diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index 19aa64ab08c8..94c2cfe0afa1 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -26,6 +26,7 @@
 #include <linux/syscalls.h>
 #include <linux/cgroup.h>
 #include <linux/perf_event.h>
+#include <linux/nstree.h>
 
 static struct kmem_cache *nsproxy_cachep;
 
@@ -179,12 +180,15 @@ int copy_namespaces(u64 flags, struct task_struct *tsk)
 	if ((flags & CLONE_VM) == 0)
 		timens_on_fork(new_ns, tsk);
 
+	nsproxy_ns_active_get(new_ns);
 	tsk->nsproxy = new_ns;
 	return 0;
 }
 
 void free_nsproxy(struct nsproxy *ns)
 {
+	nsproxy_ns_active_put(ns);
+
 	put_mnt_ns(ns->mnt_ns);
 	put_uts_ns(ns->uts_ns);
 	put_ipc_ns(ns->ipc_ns);
@@ -232,6 +236,9 @@ void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
 
 	might_sleep();
 
+	if (new)
+		nsproxy_ns_active_get(new);
+
 	task_lock(p);
 	ns = p->nsproxy;
 	p->nsproxy = new;
@@ -241,11 +248,27 @@ void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
 		put_nsproxy(ns);
 }
 
-void exit_task_namespaces(struct task_struct *p)
+void exit_nsproxy_namespaces(struct task_struct *p)
 {
 	switch_task_namespaces(p, NULL);
 }
 
+void switch_cred_namespaces(const struct cred *old, const struct cred *new)
+{
+	ns_ref_active_get(new->user_ns);
+	ns_ref_active_put(old->user_ns);
+}
+
+void get_cred_namespaces(struct task_struct *tsk)
+{
+	ns_ref_active_get(tsk->real_cred->user_ns);
+}
+
+void exit_cred_namespaces(struct task_struct *tsk)
+{
+	ns_ref_active_put(tsk->real_cred->user_ns);
+}
+
 int exec_task_namespaces(void)
 {
 	struct task_struct *tsk = current;
diff --git a/kernel/nstree.c b/kernel/nstree.c
index b24a320a11a6..4a8838683b6b 100644
--- a/kernel/nstree.c
+++ b/kernel/nstree.c
@@ -1,34 +1,38 @@
 // SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2025 Christian Brauner <brauner@kernel.org> */
 
 #include <linux/nstree.h>
 #include <linux/proc_ns.h>
+#include <linux/rculist.h>
 #include <linux/vfsdebug.h>
+#include <linux/syscalls.h>
+#include <linux/user_namespace.h>
+
+static __cacheline_aligned_in_smp DEFINE_SEQLOCK(ns_tree_lock);
+static struct rb_root ns_unified_tree = RB_ROOT; /* protected by ns_tree_lock */
+static LIST_HEAD(ns_unified_list); /* protected by ns_tree_lock */
 
 /**
  * struct ns_tree - Namespace tree
  * @ns_tree: Rbtree of namespaces of a particular type
  * @ns_list: Sequentially walkable list of all namespaces of this type
- * @ns_tree_lock: Seqlock to protect the tree and list
  * @type: type of namespaces in this tree
  */
 struct ns_tree {
-       struct rb_root ns_tree;
-       struct list_head ns_list;
-       seqlock_t ns_tree_lock;
-       int type;
+	struct rb_root ns_tree;
+	struct list_head ns_list;
+	int type;
 };
 
 struct ns_tree mnt_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(mnt_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(mnt_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWNS,
 };
 
 struct ns_tree net_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(net_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(net_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWNET,
 };
 EXPORT_SYMBOL_GPL(net_ns_tree);
@@ -36,47 +40,39 @@ EXPORT_SYMBOL_GPL(net_ns_tree);
 struct ns_tree uts_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(uts_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(uts_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWUTS,
 };
 
 struct ns_tree user_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(user_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(user_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWUSER,
 };
 
 struct ns_tree ipc_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(ipc_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(ipc_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWIPC,
 };
 
 struct ns_tree pid_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(pid_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(pid_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWPID,
 };
 
 struct ns_tree cgroup_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(cgroup_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(cgroup_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWCGROUP,
 };
 
 struct ns_tree time_ns_tree = {
 	.ns_tree = RB_ROOT,
 	.ns_list = LIST_HEAD_INIT(time_ns_tree.ns_list),
-	.ns_tree_lock = __SEQLOCK_UNLOCKED(time_ns_tree.ns_tree_lock),
 	.type = CLONE_NEWTIME,
 };
 
-DEFINE_COOKIE(namespace_cookie);
-
 static inline struct ns_common *node_to_ns(const struct rb_node *node)
 {
 	if (!node)
@@ -84,30 +80,54 @@ static inline struct ns_common *node_to_ns(const struct rb_node *node)
 	return rb_entry(node, struct ns_common, ns_tree_node);
 }
 
-static inline int ns_cmp(struct rb_node *a, const struct rb_node *b)
+static inline struct ns_common *node_to_ns_unified(const struct rb_node *node)
 {
-	struct ns_common *ns_a = node_to_ns(a);
-	struct ns_common *ns_b = node_to_ns(b);
-	u64 ns_id_a = ns_a->ns_id;
-	u64 ns_id_b = ns_b->ns_id;
+	if (!node)
+		return NULL;
+	return rb_entry(node, struct ns_common, ns_unified_tree_node);
+}
 
-	if (ns_id_a < ns_id_b)
+static inline struct ns_common *node_to_ns_owner(const struct rb_node *node)
+{
+	if (!node)
+		return NULL;
+	return rb_entry(node, struct ns_common, ns_owner_tree_node);
+}
+
+static int ns_id_cmp(u64 id_a, u64 id_b)
+{
+	if (id_a < id_b)
 		return -1;
-	if (ns_id_a > ns_id_b)
+	if (id_a > id_b)
 		return 1;
 	return 0;
 }
 
+static int ns_cmp(struct rb_node *a, const struct rb_node *b)
+{
+	return ns_id_cmp(node_to_ns(a)->ns_id, node_to_ns(b)->ns_id);
+}
+
+static int ns_cmp_unified(struct rb_node *a, const struct rb_node *b)
+{
+	return ns_id_cmp(node_to_ns_unified(a)->ns_id, node_to_ns_unified(b)->ns_id);
+}
+
+static int ns_cmp_owner(struct rb_node *a, const struct rb_node *b)
+{
+	return ns_id_cmp(node_to_ns_owner(a)->ns_id, node_to_ns_owner(b)->ns_id);
+}
+
 void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 {
 	struct rb_node *node, *prev;
+	const struct proc_ns_operations *ops = ns->ops;
 
 	VFS_WARN_ON_ONCE(!ns->ns_id);
-
-	write_seqlock(&ns_tree->ns_tree_lock);
-
 	VFS_WARN_ON_ONCE(ns->ns_type != ns_tree->type);
 
+	write_seqlock(&ns_tree_lock);
+
 	node = rb_find_add_rcu(&ns->ns_tree_node, &ns_tree->ns_tree, ns_cmp);
 	/*
 	 * If there's no previous entry simply add it after the
@@ -119,22 +139,83 @@ void __ns_tree_add_raw(struct ns_common *ns, struct ns_tree *ns_tree)
 	else
 		list_add_rcu(&ns->ns_list_node, &node_to_ns(prev)->ns_list_node);
 
-	write_sequnlock(&ns_tree->ns_tree_lock);
+	/* Add to unified tree and list */
+	rb_find_add_rcu(&ns->ns_unified_tree_node, &ns_unified_tree, ns_cmp_unified);
+	prev = rb_prev(&ns->ns_unified_tree_node);
+	if (!prev)
+		list_add_rcu(&ns->ns_unified_list_node, &ns_unified_list);
+	else
+		list_add_rcu(&ns->ns_unified_list_node, &node_to_ns_unified(prev)->ns_unified_list_node);
+
+	if (ops) {
+		struct user_namespace *user_ns;
+
+		VFS_WARN_ON_ONCE(!ops->owner);
+		user_ns = ops->owner(ns);
+		if (user_ns) {
+			struct ns_common *owner = &user_ns->ns;
+			VFS_WARN_ON_ONCE(owner->ns_type != CLONE_NEWUSER);
+
+			/* Insert into owner's rbtree */
+			rb_find_add_rcu(&ns->ns_owner_tree_node, &owner->ns_owner_tree, ns_cmp_owner);
+
+			/* Insert into owner's list in sorted order */
+			prev = rb_prev(&ns->ns_owner_tree_node);
+			if (!prev)
+				list_add_rcu(&ns->ns_owner_entry, &owner->ns_owner);
+			else
+				list_add_rcu(&ns->ns_owner_entry, &node_to_ns_owner(prev)->ns_owner_entry);
+		} else {
+			/* Only the initial user namespace doesn't have an owner. */
+			VFS_WARN_ON_ONCE(ns != to_ns_common(&init_user_ns));
+		}
+	}
+	write_sequnlock(&ns_tree_lock);
 
 	VFS_WARN_ON_ONCE(node);
+
+	/*
+	 * Take an active reference on the owner namespace. This ensures
+	 * that the owner remains visible while any of its child namespaces
+	 * are active. For init namespaces this is a no-op as ns_owner()
+	 * returns NULL for namespaces owned by init_user_ns.
+	 */
+	__ns_ref_active_get_owner(ns);
 }
 
 void __ns_tree_remove(struct ns_common *ns, struct ns_tree *ns_tree)
 {
+	const struct proc_ns_operations *ops = ns->ops;
+	struct user_namespace *user_ns;
+
 	VFS_WARN_ON_ONCE(RB_EMPTY_NODE(&ns->ns_tree_node));
 	VFS_WARN_ON_ONCE(list_empty(&ns->ns_list_node));
 	VFS_WARN_ON_ONCE(ns->ns_type != ns_tree->type);
 
-	write_seqlock(&ns_tree->ns_tree_lock);
+	write_seqlock(&ns_tree_lock);
 	rb_erase(&ns->ns_tree_node, &ns_tree->ns_tree);
-	list_bidir_del_rcu(&ns->ns_list_node);
 	RB_CLEAR_NODE(&ns->ns_tree_node);
-	write_sequnlock(&ns_tree->ns_tree_lock);
+
+	list_bidir_del_rcu(&ns->ns_list_node);
+
+	rb_erase(&ns->ns_unified_tree_node, &ns_unified_tree);
+	RB_CLEAR_NODE(&ns->ns_unified_tree_node);
+
+	list_bidir_del_rcu(&ns->ns_unified_list_node);
+
+	/* Remove from owner's rbtree if this namespace has an owner */
+	if (ops) {
+		user_ns = ops->owner(ns);
+		if (user_ns) {
+			struct ns_common *owner = &user_ns->ns;
+			rb_erase(&ns->ns_owner_tree_node, &owner->ns_owner_tree);
+			RB_CLEAR_NODE(&ns->ns_owner_tree_node);
+		}
+
+		list_bidir_del_rcu(&ns->ns_owner_entry);
+	}
+
+	write_sequnlock(&ns_tree_lock);
 }
 EXPORT_SYMBOL_GPL(__ns_tree_remove);
 
@@ -150,6 +231,17 @@ static int ns_find(const void *key, const struct rb_node *node)
 	return 0;
 }
 
+static int ns_find_unified(const void *key, const struct rb_node *node)
+{
+	const u64 ns_id = *(u64 *)key;
+	const struct ns_common *ns = node_to_ns_unified(node);
+
+	if (ns_id < ns->ns_id)
+		return -1;
+	if (ns_id > ns->ns_id)
+		return 1;
+	return 0;
+}
 
 static struct ns_tree *ns_tree_from_type(int ns_type)
 {
@@ -175,31 +267,49 @@ static struct ns_tree *ns_tree_from_type(int ns_type)
 	return NULL;
 }
 
-struct ns_common *ns_tree_lookup_rcu(u64 ns_id, int ns_type)
+static struct ns_common *__ns_unified_tree_lookup_rcu(u64 ns_id)
 {
-	struct ns_tree *ns_tree;
 	struct rb_node *node;
 	unsigned int seq;
 
-	RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "suspicious ns_tree_lookup_rcu() usage");
+	do {
+		seq = read_seqbegin(&ns_tree_lock);
+		node = rb_find_rcu(&ns_id, &ns_unified_tree, ns_find_unified);
+		if (node)
+			break;
+	} while (read_seqretry(&ns_tree_lock, seq));
+
+	return node_to_ns_unified(node);
+}
+
+static struct ns_common *__ns_tree_lookup_rcu(u64 ns_id, int ns_type)
+{
+	struct ns_tree *ns_tree;
+	struct rb_node *node;
+	unsigned int seq;
 
 	ns_tree = ns_tree_from_type(ns_type);
 	if (!ns_tree)
 		return NULL;
 
 	do {
-		seq = read_seqbegin(&ns_tree->ns_tree_lock);
+		seq = read_seqbegin(&ns_tree_lock);
 		node = rb_find_rcu(&ns_id, &ns_tree->ns_tree, ns_find);
 		if (node)
 			break;
-	} while (read_seqretry(&ns_tree->ns_tree_lock, seq));
+	} while (read_seqretry(&ns_tree_lock, seq));
 
-	if (!node)
-		return NULL;
+	return node_to_ns(node);
+}
+
+struct ns_common *ns_tree_lookup_rcu(u64 ns_id, int ns_type)
+{
+	RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "suspicious ns_tree_lookup_rcu() usage");
 
-	VFS_WARN_ON_ONCE(node_to_ns(node)->ns_type != ns_type);
+	if (ns_type)
+		return __ns_tree_lookup_rcu(ns_id, ns_type);
 
-	return node_to_ns(node);
+	return __ns_unified_tree_lookup_rcu(ns_id);
 }
 
 /**
@@ -233,15 +343,416 @@ struct ns_common *__ns_tree_adjoined_rcu(struct ns_common *ns,
 /**
  * ns_tree_gen_id - generate a new namespace id
  * @ns: namespace to generate id for
+ * @id: if non-zero, this is the initial namespace and this is a fixed id
  *
  * Generates a new namespace id and assigns it to the namespace. All
  * namespaces types share the same id space and thus can be compared
  * directly. IOW, when two ids of two namespace are equal, they are
  * identical.
  */
-u64 ns_tree_gen_id(struct ns_common *ns)
+u64 __ns_tree_gen_id(struct ns_common *ns, u64 id)
 {
-	guard(preempt)();
-	ns->ns_id = gen_cookie_next(&namespace_cookie);
+	static atomic64_t namespace_cookie = ATOMIC64_INIT(NS_LAST_INIT_ID + 1);
+
+	if (id)
+		ns->ns_id = id;
+	else
+		ns->ns_id = atomic64_inc_return(&namespace_cookie);
 	return ns->ns_id;
 }
+
+struct klistns {
+	u64 __user *uns_ids;
+	u32 nr_ns_ids;
+	u64 last_ns_id;
+	u64 user_ns_id;
+	u32 ns_type;
+	struct user_namespace *user_ns;
+	bool userns_capable;
+	struct ns_common *first_ns;
+};
+
+static void __free_klistns_free(const struct klistns *kls)
+{
+	if (kls->user_ns_id != LISTNS_CURRENT_USER)
+		put_user_ns(kls->user_ns);
+	if (kls->first_ns && kls->first_ns->ops)
+		kls->first_ns->ops->put(kls->first_ns);
+}
+
+#define NS_ALL (PID_NS | USER_NS | MNT_NS | UTS_NS | IPC_NS | NET_NS | CGROUP_NS | TIME_NS)
+
+static int copy_ns_id_req(const struct ns_id_req __user *req,
+			  struct ns_id_req *kreq)
+{
+	int ret;
+	size_t usize;
+
+	BUILD_BUG_ON(sizeof(struct ns_id_req) != NS_ID_REQ_SIZE_VER0);
+
+	ret = get_user(usize, &req->size);
+	if (ret)
+		return -EFAULT;
+	if (unlikely(usize > PAGE_SIZE))
+		return -E2BIG;
+	if (unlikely(usize < NS_ID_REQ_SIZE_VER0))
+		return -EINVAL;
+	memset(kreq, 0, sizeof(*kreq));
+	ret = copy_struct_from_user(kreq, sizeof(*kreq), req, usize);
+	if (ret)
+		return ret;
+	if (kreq->spare != 0)
+		return -EINVAL;
+	if (kreq->ns_type & ~NS_ALL)
+		return -EOPNOTSUPP;
+	return 0;
+}
+
+static inline int prepare_klistns(struct klistns *kls, struct ns_id_req *kreq,
+				  u64 __user *ns_ids, size_t nr_ns_ids)
+{
+	kls->last_ns_id = kreq->ns_id;
+	kls->user_ns_id = kreq->user_ns_id;
+	kls->nr_ns_ids	= nr_ns_ids;
+	kls->ns_type	= kreq->ns_type;
+	kls->uns_ids	= ns_ids;
+	return 0;
+}
+
+/*
+ * Lookup a namespace owned by owner with id >= ns_id.
+ * Returns the namespace with the smallest id that is >= ns_id.
+ */
+static struct ns_common *lookup_ns_owner_at(u64 ns_id, struct ns_common *owner)
+{
+	struct ns_common *ret = NULL;
+	struct rb_node *node;
+
+	VFS_WARN_ON_ONCE(owner->ns_type != CLONE_NEWUSER);
+
+	read_seqlock_excl(&ns_tree_lock);
+	node = owner->ns_owner_tree.rb_node;
+
+	while (node) {
+		struct ns_common *ns;
+
+		ns = node_to_ns_owner(node);
+		if (ns_id <= ns->ns_id) {
+			ret = ns;
+			if (ns_id == ns->ns_id)
+				break;
+			node = node->rb_left;
+		} else {
+			node = node->rb_right;
+		}
+	}
+
+	if (ret)
+		ret = ns_get_unless_inactive(ret);
+	read_sequnlock_excl(&ns_tree_lock);
+	return ret;
+}
+
+static struct ns_common *lookup_ns_id(u64 mnt_ns_id, int ns_type)
+{
+	struct ns_common *ns;
+
+	guard(rcu)();
+	ns = ns_tree_lookup_rcu(mnt_ns_id, ns_type);
+	if (!ns)
+		return NULL;
+
+	if (!ns_get_unless_inactive(ns))
+		return NULL;
+
+	return ns;
+}
+
+static inline bool __must_check ns_requested(const struct klistns *kls,
+					     const struct ns_common *ns)
+{
+	return !kls->ns_type || (kls->ns_type & ns->ns_type);
+}
+
+static inline bool __must_check may_list_ns(const struct klistns *kls,
+					    struct ns_common *ns)
+{
+	if (kls->user_ns) {
+		if (kls->userns_capable)
+			return true;
+	} else {
+		struct ns_common *owner;
+		struct user_namespace *user_ns;
+
+		owner = ns_owner(ns);
+		if (owner)
+			user_ns = to_user_ns(owner);
+		else
+			user_ns = &init_user_ns;
+		if (ns_capable_noaudit(user_ns, CAP_SYS_ADMIN))
+			return true;
+	}
+
+	if (is_current_namespace(ns))
+		return true;
+
+	if (ns->ns_type != CLONE_NEWUSER)
+		return false;
+
+	if (ns_capable_noaudit(to_user_ns(ns), CAP_SYS_ADMIN))
+		return true;
+
+	return false;
+}
+
+static void __ns_put(struct ns_common *ns)
+{
+	if (ns->ops)
+		ns->ops->put(ns);
+}
+
+DEFINE_FREE(ns_put, struct ns_common *, if (!IS_ERR_OR_NULL(_T)) __ns_put(_T))
+
+static inline struct ns_common *__must_check legitimize_ns(const struct klistns *kls,
+							   struct ns_common *candidate)
+{
+	struct ns_common *ns __free(ns_put) = NULL;
+
+	if (!ns_requested(kls, candidate))
+		return NULL;
+
+	ns = ns_get_unless_inactive(candidate);
+	if (!ns)
+		return NULL;
+
+	if (!may_list_ns(kls, ns))
+		return NULL;
+
+	return no_free_ptr(ns);
+}
+
+static ssize_t do_listns_userns(struct klistns *kls)
+{
+	u64 __user *ns_ids = kls->uns_ids;
+	size_t nr_ns_ids = kls->nr_ns_ids;
+	struct ns_common *ns = NULL, *first_ns = NULL;
+	const struct list_head *head;
+	ssize_t ret;
+
+	VFS_WARN_ON_ONCE(!kls->user_ns_id);
+
+	if (kls->user_ns_id == LISTNS_CURRENT_USER)
+		ns = to_ns_common(current_user_ns());
+	else if (kls->user_ns_id)
+		ns = lookup_ns_id(kls->user_ns_id, CLONE_NEWUSER);
+	if (!ns)
+		return -EINVAL;
+	kls->user_ns = to_user_ns(ns);
+
+	/*
+	 * Use the rbtree to find the first namespace we care about and
+	 * then use it's list entry to iterate from there.
+	 */
+	if (kls->last_ns_id) {
+		kls->first_ns = lookup_ns_owner_at(kls->last_ns_id + 1, ns);
+		if (!kls->first_ns)
+			return -ENOENT;
+		first_ns = kls->first_ns;
+	}
+
+	ret = 0;
+	head = &to_ns_common(kls->user_ns)->ns_owner;
+	kls->userns_capable = ns_capable_noaudit(kls->user_ns, CAP_SYS_ADMIN);
+
+	rcu_read_lock();
+
+	if (!first_ns)
+		first_ns = list_entry_rcu(head->next, typeof(*ns), ns_owner_entry);
+	for (ns = first_ns; &ns->ns_owner_entry != head && nr_ns_ids;
+	     ns = list_entry_rcu(ns->ns_owner_entry.next, typeof(*ns), ns_owner_entry)) {
+		struct ns_common *valid __free(ns_put);
+
+		valid = legitimize_ns(kls, ns);
+		if (!valid)
+			continue;
+
+		rcu_read_unlock();
+
+		if (put_user(valid->ns_id, ns_ids + ret))
+			return -EINVAL;
+		nr_ns_ids--;
+		ret++;
+
+		rcu_read_lock();
+	}
+
+	rcu_read_unlock();
+	return ret;
+}
+
+/*
+ * Lookup a namespace with id >= ns_id in either the unified tree or a type-specific tree.
+ * Returns the namespace with the smallest id that is >= ns_id.
+ */
+static struct ns_common *lookup_ns_id_at(u64 ns_id, int ns_type)
+{
+	struct ns_common *ret = NULL;
+	struct ns_tree *ns_tree = NULL;
+	struct rb_node *node;
+
+	if (ns_type) {
+		ns_tree = ns_tree_from_type(ns_type);
+		if (!ns_tree)
+			return NULL;
+	}
+
+	read_seqlock_excl(&ns_tree_lock);
+	if (ns_tree)
+		node = ns_tree->ns_tree.rb_node;
+	else
+		node = ns_unified_tree.rb_node;
+
+	while (node) {
+		struct ns_common *ns;
+
+		if (ns_type)
+			ns = node_to_ns(node);
+		else
+			ns = node_to_ns_unified(node);
+
+		if (ns_id <= ns->ns_id) {
+			if (ns_type)
+				ret = node_to_ns(node);
+			else
+				ret = node_to_ns_unified(node);
+			if (ns_id == ns->ns_id)
+				break;
+			node = node->rb_left;
+		} else {
+			node = node->rb_right;
+		}
+	}
+
+	if (ret)
+		ret = ns_get_unless_inactive(ret);
+	read_sequnlock_excl(&ns_tree_lock);
+	return ret;
+}
+
+static inline struct ns_common *first_ns_common(const struct list_head *head,
+						struct ns_tree *ns_tree)
+{
+	if (ns_tree)
+		return list_entry_rcu(head->next, struct ns_common, ns_list_node);
+	return list_entry_rcu(head->next, struct ns_common, ns_unified_list_node);
+}
+
+static inline struct ns_common *next_ns_common(struct ns_common *ns,
+					       struct ns_tree *ns_tree)
+{
+	if (ns_tree)
+		return list_entry_rcu(ns->ns_list_node.next, struct ns_common, ns_list_node);
+	return list_entry_rcu(ns->ns_unified_list_node.next, struct ns_common, ns_unified_list_node);
+}
+
+static inline bool ns_common_is_head(struct ns_common *ns,
+				     const struct list_head *head,
+				     struct ns_tree *ns_tree)
+{
+	if (ns_tree)
+		return &ns->ns_list_node == head;
+	return &ns->ns_unified_list_node == head;
+}
+
+static ssize_t do_listns(struct klistns *kls)
+{
+	u64 __user *ns_ids = kls->uns_ids;
+	size_t nr_ns_ids = kls->nr_ns_ids;
+	struct ns_common *ns, *first_ns = NULL;
+	struct ns_tree *ns_tree = NULL;
+	const struct list_head *head;
+	u32 ns_type;
+	ssize_t ret;
+
+	if (hweight32(kls->ns_type) == 1)
+		ns_type = kls->ns_type;
+	else
+		ns_type = 0;
+
+	if (ns_type) {
+		ns_tree = ns_tree_from_type(ns_type);
+		if (!ns_tree)
+			return -EINVAL;
+	}
+
+	if (kls->last_ns_id) {
+		kls->first_ns = lookup_ns_id_at(kls->last_ns_id + 1, ns_type);
+		if (!kls->first_ns)
+			return -ENOENT;
+		first_ns = kls->first_ns;
+	}
+
+	ret = 0;
+	if (ns_tree)
+		head = &ns_tree->ns_list;
+	else
+		head = &ns_unified_list;
+
+	rcu_read_lock();
+
+	if (!first_ns)
+		first_ns = first_ns_common(head, ns_tree);
+
+	for (ns = first_ns; !ns_common_is_head(ns, head, ns_tree) && nr_ns_ids;
+	     ns = next_ns_common(ns, ns_tree)) {
+		struct ns_common *valid __free(ns_put);
+
+		valid = legitimize_ns(kls, ns);
+		if (!valid)
+			continue;
+
+		rcu_read_unlock();
+
+		if (put_user(valid->ns_id, ns_ids + ret))
+			return -EINVAL;
+
+		nr_ns_ids--;
+		ret++;
+
+		rcu_read_lock();
+	}
+
+	rcu_read_unlock();
+	return ret;
+}
+
+SYSCALL_DEFINE4(listns, const struct ns_id_req __user *, req,
+		u64 __user *, ns_ids, size_t, nr_ns_ids, unsigned int, flags)
+{
+	struct klistns klns __free(klistns_free) = {};
+	const size_t maxcount = 1000000;
+	struct ns_id_req kreq;
+	ssize_t ret;
+
+	if (flags)
+		return -EINVAL;
+
+	if (unlikely(nr_ns_ids > maxcount))
+		return -EOVERFLOW;
+
+	if (!access_ok(ns_ids, nr_ns_ids * sizeof(*ns_ids)))
+		return -EFAULT;
+
+	ret = copy_ns_id_req(req, &kreq);
+	if (ret)
+		return ret;
+
+	ret = prepare_klistns(&klns, &kreq, ns_ids, nr_ns_ids);
+	if (ret)
+		return ret;
+
+	if (kreq.user_ns_id)
+		return do_listns_userns(&klns);
+
+	return do_listns(&klns);
+}
diff --git a/kernel/pid.c b/kernel/pid.c
index 4fffec767a63..a5a63dc0a491 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -71,21 +71,16 @@ static int pid_max_max = PID_MAX_LIMIT;
  * the scheme scales to up to 4 million PIDs, runtime.
  */
 struct pid_namespace init_pid_ns = {
-	.ns.__ns_ref = REFCOUNT_INIT(2),
+	.ns = NS_COMMON_INIT(init_pid_ns, 2),
 	.idr = IDR_INIT(init_pid_ns.idr),
 	.pid_allocated = PIDNS_ADDING,
 	.level = 0,
 	.child_reaper = &init_task,
 	.user_ns = &init_user_ns,
-	.ns.inum = ns_init_inum(&init_pid_ns),
-#ifdef CONFIG_PID_NS
-	.ns.ops = &pidns_operations,
-#endif
 	.pid_max = PID_MAX_DEFAULT,
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
 	.memfd_noexec_scope = MEMFD_NOEXEC_SCOPE_EXEC,
 #endif
-	.ns.ns_type = ns_common_type(&init_pid_ns),
 };
 EXPORT_SYMBOL_GPL(init_pid_ns);
 
@@ -117,9 +112,13 @@ static void delayed_put_pid(struct rcu_head *rhp)
 void free_pid(struct pid *pid)
 {
 	int i;
+	struct pid_namespace *active_ns;
 
 	lockdep_assert_not_held(&tasklist_lock);
 
+	active_ns = pid->numbers[pid->level].ns;
+	ns_ref_active_put(active_ns);
+
 	spin_lock(&pidmap_lock);
 	for (i = 0; i <= pid->level; i++) {
 		struct upid *upid = pid->numbers + i;
@@ -283,6 +282,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 	}
 	spin_unlock(&pidmap_lock);
 	idr_preload_end();
+	ns_ref_active_get(ns);
 
 	return pid;
 
diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
index 5b6997f4dc3d..19911f88e2b8 100644
--- a/kernel/time/namespace.c
+++ b/kernel/time/namespace.c
@@ -478,11 +478,8 @@ const struct proc_ns_operations timens_for_children_operations = {
 };
 
 struct time_namespace init_time_ns = {
-	.ns.ns_type	= ns_common_type(&init_time_ns),
-	.ns.__ns_ref	= REFCOUNT_INIT(3),
+	.ns		= NS_COMMON_INIT(init_time_ns, 3),
 	.user_ns	= &init_user_ns,
-	.ns.inum	= ns_init_inum(&init_time_ns),
-	.ns.ops		= &timens_operations,
 	.frozen_offsets	= true,
 };
 
diff --git a/kernel/user.c b/kernel/user.c
index 0163665914c9..4b3132e786d9 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -35,6 +35,7 @@ EXPORT_SYMBOL_GPL(init_binfmt_misc);
  * and 1 for... ?
  */
 struct user_namespace init_user_ns = {
+	.ns = NS_COMMON_INIT(init_user_ns, 3),
 	.uid_map = {
 		{
 			.extent[0] = {
@@ -65,14 +66,8 @@ struct user_namespace init_user_ns = {
 			.nr_extents = 1,
 		},
 	},
-	.ns.ns_type = ns_common_type(&init_user_ns),
-	.ns.__ns_ref = REFCOUNT_INIT(3),
 	.owner = GLOBAL_ROOT_UID,
 	.group = GLOBAL_ROOT_GID,
-	.ns.inum = ns_init_inum(&init_user_ns),
-#ifdef CONFIG_USER_NS
-	.ns.ops = &userns_operations,
-#endif
 	.flags = USERNS_INIT_FLAGS,
 #ifdef CONFIG_KEYS
 	.keyring_name_list = LIST_HEAD_INIT(init_user_ns.keyring_name_list),
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index b0e0f22d7b21..83cbec4afcb3 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -439,7 +439,7 @@ static __net_init int setup_net(struct net *net)
 	LIST_HEAD(net_exit_list);
 	int error = 0;
 
-	net->net_cookie = ns_tree_gen_id(&net->ns);
+	net->net_cookie = ns_tree_gen_id(net);
 
 	list_for_each_entry(ops, &pernet_list, list) {
 		error = ops_init(ops, net);
diff --git a/scripts/syscall.tbl b/scripts/syscall.tbl
index d1ae5e92c615..e74868be513c 100644
--- a/scripts/syscall.tbl
+++ b/scripts/syscall.tbl
@@ -410,3 +410,4 @@
 467	common	open_tree_attr			sys_open_tree_attr
 468	common	file_getattr			sys_file_getattr
 469	common	file_setattr			sys_file_setattr
+470	common	listns				sys_listns
diff --git a/tools/include/uapi/linux/nsfs.h b/tools/include/uapi/linux/nsfs.h
index 33c9b578b3b2..a25e38d1c874 100644
--- a/tools/include/uapi/linux/nsfs.h
+++ b/tools/include/uapi/linux/nsfs.h
@@ -53,6 +53,76 @@ enum init_ns_ino {
 	TIME_NS_INIT_INO	= 0xEFFFFFFAU,
 	NET_NS_INIT_INO		= 0xEFFFFFF9U,
 	MNT_NS_INIT_INO		= 0xEFFFFFF8U,
+#ifdef __KERNEL__
+	MNT_NS_ANON_INO		= 0xEFFFFFF7U,
+#endif
 };
 
+struct nsfs_file_handle {
+	__u64 ns_id;
+	__u32 ns_type;
+	__u32 ns_inum;
+};
+
+#define NSFS_FILE_HANDLE_SIZE_VER0 16 /* sizeof first published struct */
+#define NSFS_FILE_HANDLE_SIZE_LATEST sizeof(struct nsfs_file_handle) /* sizeof latest published struct */
+
+enum init_ns_id {
+	IPC_NS_INIT_ID		= 1ULL,
+	UTS_NS_INIT_ID		= 2ULL,
+	USER_NS_INIT_ID		= 3ULL,
+	PID_NS_INIT_ID		= 4ULL,
+	CGROUP_NS_INIT_ID	= 5ULL,
+	TIME_NS_INIT_ID		= 6ULL,
+	NET_NS_INIT_ID		= 7ULL,
+	MNT_NS_INIT_ID		= 8ULL,
+#ifdef __KERNEL__
+	NS_LAST_INIT_ID		= MNT_NS_INIT_ID,
+#endif
+};
+
+enum ns_type {
+	TIME_NS    = (1ULL << 7),  /* CLONE_NEWTIME */
+	MNT_NS     = (1ULL << 17), /* CLONE_NEWNS */
+	CGROUP_NS  = (1ULL << 25), /* CLONE_NEWCGROUP */
+	UTS_NS     = (1ULL << 26), /* CLONE_NEWUTS */
+	IPC_NS     = (1ULL << 27), /* CLONE_NEWIPC */
+	USER_NS    = (1ULL << 28), /* CLONE_NEWUSER */
+	PID_NS     = (1ULL << 29), /* CLONE_NEWPID */
+	NET_NS     = (1ULL << 30), /* CLONE_NEWNET */
+};
+
+/**
+ * struct ns_id_req - namespace ID request structure
+ * @size: size of this structure
+ * @spare: reserved for future use
+ * @filter: filter mask
+ * @ns_id: last namespace id
+ * @user_ns_id: owning user namespace ID
+ *
+ * Structure for passing namespace ID and miscellaneous parameters to
+ * statns(2) and listns(2).
+ *
+ * For statns(2) @param represents the request mask.
+ * For listns(2) @param represents the last listed mount id (or zero).
+ */
+struct ns_id_req {
+	__u32 size;
+	__u32 spare;
+	__u64 ns_id;
+	struct /* listns */ {
+		__u32 ns_type;
+		__u32 spare2;
+		__u64 user_ns_id;
+	};
+};
+
+/*
+ * Special @user_ns_id value that can be passed to listns()
+ */
+#define LISTNS_CURRENT_USER 0xffffffffffffffff /* Caller's userns */
+
+/* List of all ns_id_req versions. */
+#define NS_ID_REQ_SIZE_VER0 32 /* sizeof first published struct */
+
 #endif /* __LINUX_NSFS_H */
diff --git a/tools/testing/selftests/filesystems/utils.c b/tools/testing/selftests/filesystems/utils.c
index c43a69dffd83..a0c64f415a7f 100644
--- a/tools/testing/selftests/filesystems/utils.c
+++ b/tools/testing/selftests/filesystems/utils.c
@@ -487,7 +487,7 @@ int setup_userns(void)
 	uid_t uid = getuid();
 	gid_t gid = getgid();
 
-	ret = unshare(CLONE_NEWNS|CLONE_NEWUSER|CLONE_NEWPID);
+	ret = unshare(CLONE_NEWNS|CLONE_NEWUSER);
 	if (ret) {
 		ksft_exit_fail_msg("unsharing mountns and userns: %s\n",
 				   strerror(errno));
diff --git a/tools/testing/selftests/namespaces/.gitignore b/tools/testing/selftests/namespaces/.gitignore
index ccfb40837a73..f4d2209ca4e4 100644
--- a/tools/testing/selftests/namespaces/.gitignore
+++ b/tools/testing/selftests/namespaces/.gitignore
@@ -1,3 +1,10 @@
 nsid_test
 file_handle_test
 init_ino_test
+ns_active_ref_test
+listns_test
+listns_permissions_test
+siocgskns_test
+cred_change_test
+stress_test
+listns_pagination_bug
diff --git a/tools/testing/selftests/namespaces/Makefile b/tools/testing/selftests/namespaces/Makefile
index 5fe4b3dc07d3..01569e0abbdb 100644
--- a/tools/testing/selftests/namespaces/Makefile
+++ b/tools/testing/selftests/namespaces/Makefile
@@ -1,7 +1,25 @@
 # SPDX-License-Identifier: GPL-2.0-only
 CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
+LDLIBS += -lcap
 
-TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test
+TEST_GEN_PROGS := nsid_test \
+		  file_handle_test \
+		  init_ino_test \
+		  ns_active_ref_test \
+		  listns_test \
+		  listns_permissions_test \
+		  siocgskns_test \
+		  cred_change_test \
+		  stress_test \
+		  listns_pagination_bug
 
 include ../lib.mk
 
+$(OUTPUT)/ns_active_ref_test: ../filesystems/utils.c
+$(OUTPUT)/listns_test: ../filesystems/utils.c
+$(OUTPUT)/listns_permissions_test: ../filesystems/utils.c
+$(OUTPUT)/siocgskns_test: ../filesystems/utils.c
+$(OUTPUT)/cred_change_test: ../filesystems/utils.c
+$(OUTPUT)/stress_test: ../filesystems/utils.c
+$(OUTPUT)/listns_pagination_bug: ../filesystems/utils.c
+
diff --git a/tools/testing/selftests/namespaces/cred_change_test.c b/tools/testing/selftests/namespaces/cred_change_test.c
new file mode 100644
index 000000000000..7b4f5ad3f725
--- /dev/null
+++ b/tools/testing/selftests/namespaces/cred_change_test.c
@@ -0,0 +1,814 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/capability.h>
+#include <sys/ioctl.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <linux/nsfs.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Test credential changes and their impact on namespace active references.
+ */
+
+/*
+ * Test setuid() in a user namespace properly swaps active references.
+ * Create a user namespace with multiple UIDs mapped, then setuid() between them.
+ * Verify that the user namespace remains active throughout.
+ */
+TEST(setuid_preserves_active_refs)
+{
+	pid_t pid;
+	int status;
+	__u64 userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 child_userns_id;
+		uid_t orig_uid = getuid();
+		int setuid_count;
+
+		close(pipefd[0]);
+
+		/* Create new user namespace with multiple UIDs mapped (0-9) */
+		userns_fd = get_userns_fd(0, orig_uid, 10);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send namespace ID to parent */
+		write(pipefd[1], &child_userns_id, sizeof(child_userns_id));
+
+		/*
+		 * Perform multiple setuid() calls.
+		 * Each setuid() triggers commit_creds() which should properly
+		 * swap active references via switch_cred_namespaces().
+		 */
+		for (setuid_count = 0; setuid_count < 50; setuid_count++) {
+			uid_t target_uid = (setuid_count % 10);
+			if (setuid(target_uid) < 0) {
+				if (errno != EPERM) {
+					close(pipefd[1]);
+					exit(1);
+				}
+			}
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	if (read(pipefd[0], &userns_id, sizeof(userns_id)) != sizeof(userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace ID from child");
+	}
+	close(pipefd[0]);
+
+	TH_LOG("Child user namespace ID: %llu", (unsigned long long)userns_id);
+
+	/* Verify namespace is active while child is running */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+	ASSERT_TRUE(found);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify namespace becomes inactive after child exits */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	found = false;
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found);
+	TH_LOG("setuid() correctly preserved active references (no leak)");
+}
+
+/*
+ * Test setgid() in a user namespace properly handles active references.
+ */
+TEST(setgid_preserves_active_refs)
+{
+	pid_t pid;
+	int status;
+	__u64 userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 child_userns_id;
+		uid_t orig_uid = getuid();
+		int setgid_count;
+
+		close(pipefd[0]);
+
+		/* Create new user namespace with multiple GIDs mapped */
+		userns_fd = get_userns_fd(0, orig_uid, 10);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		write(pipefd[1], &child_userns_id, sizeof(child_userns_id));
+
+		/* Perform multiple setgid() calls */
+		for (setgid_count = 0; setgid_count < 50; setgid_count++) {
+			gid_t target_gid = (setgid_count % 10);
+			if (setgid(target_gid) < 0) {
+				if (errno != EPERM) {
+					close(pipefd[1]);
+					exit(1);
+				}
+			}
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	if (read(pipefd[0], &userns_id, sizeof(userns_id)) != sizeof(userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace ID from child");
+	}
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify namespace becomes inactive */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found);
+	TH_LOG("setgid() correctly preserved active references (no leak)");
+}
+
+/*
+ * Test setresuid() which changes real, effective, and saved UIDs.
+ * This should properly swap active references via commit_creds().
+ */
+TEST(setresuid_preserves_active_refs)
+{
+	pid_t pid;
+	int status;
+	__u64 userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 child_userns_id;
+		uid_t orig_uid = getuid();
+		int setres_count;
+
+		close(pipefd[0]);
+
+		/* Create new user namespace */
+		userns_fd = get_userns_fd(0, orig_uid, 10);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		write(pipefd[1], &child_userns_id, sizeof(child_userns_id));
+
+		/* Perform multiple setresuid() calls */
+		for (setres_count = 0; setres_count < 30; setres_count++) {
+			uid_t uid1 = (setres_count % 5);
+			uid_t uid2 = ((setres_count + 1) % 5);
+			uid_t uid3 = ((setres_count + 2) % 5);
+
+			if (setresuid(uid1, uid2, uid3) < 0) {
+				if (errno != EPERM) {
+					close(pipefd[1]);
+					exit(1);
+				}
+			}
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	if (read(pipefd[0], &userns_id, sizeof(userns_id)) != sizeof(userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace ID from child");
+	}
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify namespace becomes inactive */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found);
+	TH_LOG("setresuid() correctly preserved active references (no leak)");
+}
+
+/*
+ * Test credential changes across multiple user namespaces.
+ * Create nested user namespaces and verify active reference tracking.
+ */
+TEST(cred_change_nested_userns)
+{
+	pid_t pid;
+	int status;
+	__u64 parent_userns_id, child_userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found_parent = false, found_child = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 parent_id, child_id;
+		uid_t orig_uid = getuid();
+
+		close(pipefd[0]);
+
+		/* Create first user namespace */
+		userns_fd = get_userns_fd(0, orig_uid, 1);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get first namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &parent_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Create nested user namespace */
+		userns_fd = get_userns_fd(0, 0, 1);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get nested namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send both IDs to parent */
+		write(pipefd[1], &parent_id, sizeof(parent_id));
+		write(pipefd[1], &child_id, sizeof(child_id));
+
+		/* Perform some credential changes in nested namespace */
+		setuid(0);
+		setgid(0);
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	/* Read both namespace IDs */
+	if (read(pipefd[0], &parent_userns_id, sizeof(parent_userns_id)) != sizeof(parent_userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get parent namespace ID");
+	}
+
+	if (read(pipefd[0], &child_userns_id, sizeof(child_userns_id)) != sizeof(child_userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get child namespace ID");
+	}
+	close(pipefd[0]);
+
+	TH_LOG("Parent userns: %llu, Child userns: %llu",
+	       (unsigned long long)parent_userns_id,
+	       (unsigned long long)child_userns_id);
+
+	/* Verify both namespaces are active */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == parent_userns_id)
+			found_parent = true;
+		if (ns_ids[i] == child_userns_id)
+			found_child = true;
+	}
+
+	ASSERT_TRUE(found_parent);
+	ASSERT_TRUE(found_child);
+
+	/* Wait for child */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify both namespaces become inactive */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	found_parent = false;
+	found_child = false;
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == parent_userns_id)
+			found_parent = true;
+		if (ns_ids[i] == child_userns_id)
+			found_child = true;
+	}
+
+	ASSERT_FALSE(found_parent);
+	ASSERT_FALSE(found_child);
+	TH_LOG("Nested user namespace credential changes preserved active refs (no leak)");
+}
+
+/*
+ * Test rapid credential changes don't cause refcount imbalances.
+ * This stress-tests the switch_cred_namespaces() logic.
+ */
+TEST(rapid_cred_changes_no_leak)
+{
+	pid_t pid;
+	int status;
+	__u64 userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 child_userns_id;
+		uid_t orig_uid = getuid();
+		int change_count;
+
+		close(pipefd[0]);
+
+		/* Create new user namespace with wider range of UIDs/GIDs */
+		userns_fd = get_userns_fd(0, orig_uid, 100);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		write(pipefd[1], &child_userns_id, sizeof(child_userns_id));
+
+		/*
+		 * Perform many rapid credential changes.
+		 * Mix setuid, setgid, setreuid, setregid, setresuid, setresgid.
+		 */
+		for (change_count = 0; change_count < 200; change_count++) {
+			switch (change_count % 6) {
+			case 0:
+				setuid(change_count % 50);
+				break;
+			case 1:
+				setgid(change_count % 50);
+				break;
+			case 2:
+				setreuid(change_count % 50, (change_count + 1) % 50);
+				break;
+			case 3:
+				setregid(change_count % 50, (change_count + 1) % 50);
+				break;
+			case 4:
+				setresuid(change_count % 50, (change_count + 1) % 50, (change_count + 2) % 50);
+				break;
+			case 5:
+				setresgid(change_count % 50, (change_count + 1) % 50, (change_count + 2) % 50);
+				break;
+			}
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	if (read(pipefd[0], &userns_id, sizeof(userns_id)) != sizeof(userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace ID from child");
+	}
+	close(pipefd[0]);
+
+	TH_LOG("Testing with user namespace ID: %llu", (unsigned long long)userns_id);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify namespace becomes inactive (no leaked active refs) */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found);
+	TH_LOG("200 rapid credential changes completed with no active ref leak");
+}
+
+/*
+ * Test setfsuid/setfsgid which change filesystem UID/GID.
+ * These also trigger credential changes but may have different code paths.
+ */
+TEST(setfsuid_preserves_active_refs)
+{
+	pid_t pid;
+	int status;
+	__u64 userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	ssize_t ret;
+	int i;
+	bool found = false;
+	int pipefd[2];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		int fd, userns_fd;
+		__u64 child_userns_id;
+		uid_t orig_uid = getuid();
+		int change_count;
+
+		close(pipefd[0]);
+
+		/* Create new user namespace */
+		userns_fd = get_userns_fd(0, orig_uid, 10);
+		if (userns_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+			close(userns_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(userns_fd);
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		write(pipefd[1], &child_userns_id, sizeof(child_userns_id));
+
+		/* Perform multiple setfsuid/setfsgid calls */
+		for (change_count = 0; change_count < 50; change_count++) {
+			setfsuid(change_count % 10);
+			setfsgid(change_count % 10);
+		}
+
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	if (read(pipefd[0], &userns_id, sizeof(userns_id)) != sizeof(userns_id)) {
+		close(pipefd[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace ID from child");
+	}
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Verify namespace becomes inactive */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == userns_id) {
+			found = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found);
+	TH_LOG("setfsuid/setfsgid correctly preserved active references (no leak)");
+}
+
+TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/namespaces/listns_pagination_bug.c b/tools/testing/selftests/namespaces/listns_pagination_bug.c
new file mode 100644
index 000000000000..da7d33f96397
--- /dev/null
+++ b/tools/testing/selftests/namespaces/listns_pagination_bug.c
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/socket.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Minimal test case to reproduce KASAN out-of-bounds in listns pagination.
+ *
+ * The bug occurs when:
+ * 1. Filtering by a specific namespace type (e.g., CLONE_NEWUSER)
+ * 2. Using pagination (req.ns_id != 0)
+ * 3. The lookup_ns_id_at() call in do_listns() passes ns_type=0 instead of
+ *    the filtered type, causing it to search the unified tree and potentially
+ *    return a namespace of the wrong type.
+ */
+TEST(pagination_with_type_filter)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,  /* Filter by user namespace */
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	pid_t pids[10];
+	int num_children = 10;
+	int i;
+	int sv[2];
+	__u64 first_batch[3];
+	ssize_t ret;
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	/* Create children with user namespaces */
+	for (i = 0; i < num_children; i++) {
+		pids[i] = fork();
+		ASSERT_GE(pids[i], 0);
+
+		if (pids[i] == 0) {
+			char c;
+			close(sv[0]);
+
+			if (setup_userns() < 0) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Signal parent we're ready */
+			if (write(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Wait for parent signal to exit */
+			if (read(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			close(sv[1]);
+			exit(0);
+		}
+	}
+
+	close(sv[1]);
+
+	/* Wait for all children to signal ready */
+	for (i = 0; i < num_children; i++) {
+		char c;
+		if (read(sv[0], &c, 1) != 1) {
+			close(sv[0]);
+			for (int j = 0; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	/* First batch - this should work */
+	ret = sys_listns(&req, first_batch, 3, 0);
+	if (ret < 0) {
+		if (errno == ENOSYS) {
+			close(sv[0]);
+			for (i = 0; i < num_children; i++)
+				kill(pids[i], SIGKILL);
+			for (i = 0; i < num_children; i++)
+				waitpid(pids[i], NULL, 0);
+			SKIP(return, "listns() not supported");
+		}
+		ASSERT_GE(ret, 0);
+	}
+
+	TH_LOG("First batch returned %zd entries", ret);
+
+	if (ret == 3) {
+		__u64 second_batch[3];
+
+		/* Second batch - pagination triggers the bug */
+		req.ns_id = first_batch[2];  /* Continue from last ID */
+		ret = sys_listns(&req, second_batch, 3, 0);
+
+		TH_LOG("Second batch returned %zd entries", ret);
+		ASSERT_GE(ret, 0);
+	}
+
+	/* Signal all children to exit */
+	for (i = 0; i < num_children; i++) {
+		char c = 'X';
+		if (write(sv[0], &c, 1) != 1) {
+			close(sv[0]);
+			for (int j = i; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	close(sv[0]);
+
+	/* Cleanup */
+	for (i = 0; i < num_children; i++) {
+		int status;
+		waitpid(pids[i], &status, 0);
+	}
+}
+
+TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/namespaces/listns_permissions_test.c b/tools/testing/selftests/namespaces/listns_permissions_test.c
new file mode 100644
index 000000000000..82d818751a5f
--- /dev/null
+++ b/tools/testing/selftests/namespaces/listns_permissions_test.c
@@ -0,0 +1,759 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <linux/nsfs.h>
+#include <sys/capability.h>
+#include <sys/ioctl.h>
+#include <sys/prctl.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Test that unprivileged users can only see namespaces they're currently in.
+ * Create a namespace, drop privileges, verify we can only see our own namespaces.
+ */
+TEST(listns_unprivileged_current_only)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[100];
+	ssize_t ret;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool found_ours;
+	int unexpected_count;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 our_netns_id;
+		bool found_ours;
+		int unexpected_count;
+
+		close(pipefd[0]);
+
+		/* Create user namespace to be unprivileged */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Create a network namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get our network namespace ID */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &our_netns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Now we're unprivileged - list all network namespaces */
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* We should only see our own network namespace */
+		found_ours = false;
+		unexpected_count = 0;
+
+		for (ssize_t i = 0; i < ret; i++) {
+			if (ns_ids[i] == our_netns_id) {
+				found_ours = true;
+			} else {
+				/* This is either init_net (which we can see) or unexpected */
+				unexpected_count++;
+			}
+		}
+
+		/* Send results to parent */
+		write(pipefd[1], &found_ours, sizeof(found_ours));
+		write(pipefd[1], &unexpected_count, sizeof(unexpected_count));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	found_ours = false;
+	unexpected_count = 0;
+	read(pipefd[0], &found_ours, sizeof(found_ours));
+	read(pipefd[0], &unexpected_count, sizeof(unexpected_count));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Child should have seen its own namespace */
+	ASSERT_TRUE(found_ours);
+
+	TH_LOG("Unprivileged child saw its own namespace, plus %d others (likely init_net)",
+			unexpected_count);
+}
+
+/*
+ * Test that users with CAP_SYS_ADMIN in a user namespace can see
+ * all namespaces owned by that user namespace.
+ */
+TEST(listns_cap_sys_admin_in_userns)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,  /* All types */
+		.spare2 = 0,
+		.user_ns_id = 0,  /* Will be set to our created user namespace */
+	};
+	__u64 ns_ids[100];
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool success;
+	ssize_t count;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 userns_id;
+		ssize_t ret;
+		int min_expected;
+		bool success;
+
+		close(pipefd[0]);
+
+		/* Create user namespace - we'll have CAP_SYS_ADMIN in it */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get the user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Create several namespaces owned by this user namespace */
+		unshare(CLONE_NEWNET);
+		unshare(CLONE_NEWUTS);
+		unshare(CLONE_NEWIPC);
+
+		/* List namespaces owned by our user namespace */
+		req.user_ns_id = userns_id;
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/*
+		 * We have CAP_SYS_ADMIN in this user namespace,
+		 * so we should see all namespaces owned by it.
+		 * That includes: net, uts, ipc, and the user namespace itself.
+		 */
+		min_expected = 4;
+		success = (ret >= min_expected);
+
+		write(pipefd[1], &success, sizeof(success));
+		write(pipefd[1], &ret, sizeof(ret));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	success = false;
+	count = 0;
+	read(pipefd[0], &success, sizeof(success));
+	read(pipefd[0], &count, sizeof(count));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_TRUE(success);
+	TH_LOG("User with CAP_SYS_ADMIN saw %zd namespaces owned by their user namespace",
+			count);
+}
+
+/*
+ * Test that users cannot see namespaces from unrelated user namespaces.
+ * Create two sibling user namespaces, verify they can't see each other's
+ * owned namespaces.
+ */
+TEST(listns_cannot_see_sibling_userns_namespaces)
+{
+	int pipefd[2];
+	pid_t pid1, pid2;
+	int status;
+	__u64 netns_a_id;
+	int pipefd2[2];
+	bool found_sibling_netns;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	/* Fork first child - creates user namespace A */
+	pid1 = fork();
+	ASSERT_GE(pid1, 0);
+
+	if (pid1 == 0) {
+		int fd;
+		__u64 netns_a_id;
+		char buf;
+
+		close(pipefd[0]);
+
+		/* Create user namespace A */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Create network namespace owned by user namespace A */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get network namespace ID */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &netns_a_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send namespace ID to parent */
+		write(pipefd[1], &netns_a_id, sizeof(netns_a_id));
+
+		/* Keep alive for sibling to check */
+		read(pipefd[1], &buf, 1);
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent reads namespace A ID */
+	close(pipefd[1]);
+	netns_a_id = 0;
+	read(pipefd[0], &netns_a_id, sizeof(netns_a_id));
+
+	TH_LOG("User namespace A created network namespace with ID %llu",
+	       (unsigned long long)netns_a_id);
+
+	/* Fork second child - creates user namespace B */
+	ASSERT_EQ(pipe(pipefd2), 0);
+
+	pid2 = fork();
+	ASSERT_GE(pid2, 0);
+
+	if (pid2 == 0) {
+		struct ns_id_req req = {
+			.size = sizeof(req),
+			.spare = 0,
+			.ns_id = 0,
+			.ns_type = CLONE_NEWNET,
+			.spare2 = 0,
+			.user_ns_id = 0,
+		};
+		__u64 ns_ids[100];
+		ssize_t ret;
+		bool found_sibling_netns;
+
+		close(pipefd[0]);
+		close(pipefd2[0]);
+
+		/* Create user namespace B (sibling to A) */
+		if (setup_userns() < 0) {
+			close(pipefd2[1]);
+			exit(1);
+		}
+
+		/* Try to list all network namespaces */
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+		found_sibling_netns = false;
+		if (ret > 0) {
+			for (ssize_t i = 0; i < ret; i++) {
+				if (ns_ids[i] == netns_a_id) {
+					found_sibling_netns = true;
+					break;
+				}
+			}
+		}
+
+		/* We should NOT see the sibling's network namespace */
+		write(pipefd2[1], &found_sibling_netns, sizeof(found_sibling_netns));
+		close(pipefd2[1]);
+		exit(0);
+	}
+
+	/* Parent reads result from second child */
+	close(pipefd2[1]);
+	found_sibling_netns = false;
+	read(pipefd2[0], &found_sibling_netns, sizeof(found_sibling_netns));
+	close(pipefd2[0]);
+
+	/* Signal first child to exit */
+	close(pipefd[0]);
+
+	/* Wait for both children */
+	waitpid(pid2, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+
+	waitpid(pid1, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+
+	/* Second child should NOT have seen first child's namespace */
+	ASSERT_FALSE(found_sibling_netns);
+	TH_LOG("User namespace B correctly could not see sibling namespace A's network namespace");
+}
+
+/*
+ * Test permission checking with LISTNS_CURRENT_USER.
+ * Verify that listing with LISTNS_CURRENT_USER respects permissions.
+ */
+TEST(listns_current_user_permissions)
+{
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool success;
+	ssize_t count;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		struct ns_id_req req = {
+			.size = sizeof(req),
+			.spare = 0,
+			.ns_id = 0,
+			.ns_type = 0,
+			.spare2 = 0,
+			.user_ns_id = LISTNS_CURRENT_USER,
+		};
+		__u64 ns_ids[100];
+		ssize_t ret;
+		bool success;
+
+		close(pipefd[0]);
+
+		/* Create user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Create some namespaces owned by this user namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (unshare(CLONE_NEWUTS) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* List with LISTNS_CURRENT_USER - should see our owned namespaces */
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+		success = (ret >= 3);  /* At least user, net, uts */
+		write(pipefd[1], &success, sizeof(success));
+		write(pipefd[1], &ret, sizeof(ret));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	success = false;
+	count = 0;
+	read(pipefd[0], &success, sizeof(success));
+	read(pipefd[0], &count, sizeof(count));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_TRUE(success);
+	TH_LOG("LISTNS_CURRENT_USER returned %zd namespaces", count);
+}
+
+/*
+ * Test that CAP_SYS_ADMIN in parent user namespace allows seeing
+ * child user namespace's owned namespaces.
+ */
+TEST(listns_parent_userns_cap_sys_admin)
+{
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool found_child_userns;
+	ssize_t count;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 parent_userns_id;
+		__u64 child_userns_id;
+		struct ns_id_req req;
+		__u64 ns_ids[100];
+		ssize_t ret;
+		bool found_child_userns;
+
+		close(pipefd[0]);
+
+		/* Create parent user namespace - we have CAP_SYS_ADMIN in it */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get parent user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &parent_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Create child user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get child user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Create namespaces owned by child user namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* List namespaces owned by parent user namespace */
+		req.size = sizeof(req);
+		req.spare = 0;
+		req.ns_id = 0;
+		req.ns_type = 0;
+		req.spare2 = 0;
+		req.user_ns_id = parent_userns_id;
+
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+		/* Should see child user namespace in the list */
+		found_child_userns = false;
+		if (ret > 0) {
+			for (ssize_t i = 0; i < ret; i++) {
+				if (ns_ids[i] == child_userns_id) {
+					found_child_userns = true;
+					break;
+				}
+			}
+		}
+
+		write(pipefd[1], &found_child_userns, sizeof(found_child_userns));
+		write(pipefd[1], &ret, sizeof(ret));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	found_child_userns = false;
+	count = 0;
+	read(pipefd[0], &found_child_userns, sizeof(found_child_userns));
+	read(pipefd[0], &count, sizeof(count));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_TRUE(found_child_userns);
+	TH_LOG("Process with CAP_SYS_ADMIN in parent user namespace saw child user namespace (total: %zd)",
+			count);
+}
+
+/*
+ * Test that we can see user namespaces we have CAP_SYS_ADMIN inside of.
+ * This is different from seeing namespaces owned by a user namespace.
+ */
+TEST(listns_cap_sys_admin_inside_userns)
+{
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool found_ours;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 our_userns_id;
+		struct ns_id_req req;
+		__u64 ns_ids[100];
+		ssize_t ret;
+		bool found_ours;
+
+		close(pipefd[0]);
+
+		/* Create user namespace - we have CAP_SYS_ADMIN inside it */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get our user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &our_userns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* List all user namespaces globally */
+		req.size = sizeof(req);
+		req.spare = 0;
+		req.ns_id = 0;
+		req.ns_type = CLONE_NEWUSER;
+		req.spare2 = 0;
+		req.user_ns_id = 0;
+
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+		/* We should be able to see our own user namespace */
+		found_ours = false;
+		if (ret > 0) {
+			for (ssize_t i = 0; i < ret; i++) {
+				if (ns_ids[i] == our_userns_id) {
+					found_ours = true;
+					break;
+				}
+			}
+		}
+
+		write(pipefd[1], &found_ours, sizeof(found_ours));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	found_ours = false;
+	read(pipefd[0], &found_ours, sizeof(found_ours));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_TRUE(found_ours);
+	TH_LOG("Process can see user namespace it has CAP_SYS_ADMIN inside of");
+}
+
+/*
+ * Test that dropping CAP_SYS_ADMIN restricts what we can see.
+ */
+TEST(listns_drop_cap_sys_admin)
+{
+	cap_t caps;
+	cap_value_t cap_list[1] = { CAP_SYS_ADMIN };
+
+	/* This test needs to start with CAP_SYS_ADMIN */
+	caps = cap_get_proc();
+	if (!caps) {
+		SKIP(return, "Cannot get capabilities");
+	}
+
+	cap_flag_value_t cap_val;
+	if (cap_get_flag(caps, CAP_SYS_ADMIN, CAP_EFFECTIVE, &cap_val) < 0) {
+		cap_free(caps);
+		SKIP(return, "Cannot check CAP_SYS_ADMIN");
+	}
+
+	if (cap_val != CAP_SET) {
+		cap_free(caps);
+		SKIP(return, "Test needs CAP_SYS_ADMIN to start");
+	}
+	cap_free(caps);
+
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool correct;
+	ssize_t count_before, count_after;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		struct ns_id_req req = {
+			.size = sizeof(req),
+			.spare = 0,
+			.ns_id = 0,
+			.ns_type = CLONE_NEWNET,
+			.spare2 = 0,
+			.user_ns_id = LISTNS_CURRENT_USER,
+		};
+		__u64 ns_ids_before[100];
+		ssize_t count_before;
+		__u64 ns_ids_after[100];
+		ssize_t count_after;
+		bool correct;
+
+		close(pipefd[0]);
+
+		/* Create user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Count namespaces with CAP_SYS_ADMIN */
+		count_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+
+		/* Drop CAP_SYS_ADMIN */
+		caps = cap_get_proc();
+		if (caps) {
+			cap_set_flag(caps, CAP_EFFECTIVE, 1, cap_list, CAP_CLEAR);
+			cap_set_flag(caps, CAP_PERMITTED, 1, cap_list, CAP_CLEAR);
+			cap_set_proc(caps);
+			cap_free(caps);
+		}
+
+		/* Ensure we can't regain the capability */
+		prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+
+		/* Count namespaces without CAP_SYS_ADMIN */
+		count_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+
+		/* Without CAP_SYS_ADMIN, we should see same or fewer namespaces */
+		correct = (count_after <= count_before);
+
+		write(pipefd[1], &correct, sizeof(correct));
+		write(pipefd[1], &count_before, sizeof(count_before));
+		write(pipefd[1], &count_after, sizeof(count_after));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	correct = false;
+	count_before = 0;
+	count_after = 0;
+	read(pipefd[0], &correct, sizeof(correct));
+	read(pipefd[0], &count_before, sizeof(count_before));
+	read(pipefd[0], &count_after, sizeof(count_after));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_TRUE(correct);
+	TH_LOG("With CAP_SYS_ADMIN: %zd namespaces, without: %zd namespaces",
+			count_before, count_after);
+}
+
+TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/namespaces/listns_test.c b/tools/testing/selftests/namespaces/listns_test.c
new file mode 100644
index 000000000000..8a95789d6a87
--- /dev/null
+++ b/tools/testing/selftests/namespaces/listns_test.c
@@ -0,0 +1,679 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <linux/nsfs.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Test basic listns() functionality with the unified namespace tree.
+ * List all active namespaces globally.
+ */
+TEST(listns_basic_unified)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,  /* All types */
+		.spare2 = 0,
+		.user_ns_id = 0,  /* Global listing */
+	};
+	__u64 ns_ids[100];
+	ssize_t ret;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+
+	/* Should find at least the initial namespaces */
+	ASSERT_GT(ret, 0);
+	TH_LOG("Found %zd active namespaces", ret);
+
+	/* Verify all returned IDs are non-zero */
+	for (ssize_t i = 0; i < ret; i++) {
+		ASSERT_NE(ns_ids[i], 0);
+		TH_LOG("  [%zd] ns_id: %llu", i, (unsigned long long)ns_ids[i]);
+	}
+}
+
+/*
+ * Test listns() with type filtering.
+ * List only network namespaces.
+ */
+TEST(listns_filter_by_type)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET,  /* Only network namespaces */
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[100];
+	ssize_t ret;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	ASSERT_GE(ret, 0);
+
+	/* Should find at least init_net */
+	ASSERT_GT(ret, 0);
+	TH_LOG("Found %zd active network namespaces", ret);
+
+	/* Verify we can open each namespace and it's actually a network namespace */
+	for (ssize_t i = 0; i < ret && i < 5; i++) {
+		struct nsfs_file_handle nsfh = {
+			.ns_id = ns_ids[i],
+			.ns_type = CLONE_NEWNET,
+			.ns_inum = 0,
+		};
+		struct file_handle *fh;
+		int fd;
+
+		fh = (struct file_handle *)malloc(sizeof(*fh) + sizeof(nsfh));
+		ASSERT_NE(fh, NULL);
+		fh->handle_bytes = sizeof(nsfh);
+		fh->handle_type = 0;
+		memcpy(fh->f_handle, &nsfh, sizeof(nsfh));
+
+		fd = open_by_handle_at(-10003, fh, O_RDONLY);
+		free(fh);
+
+		if (fd >= 0) {
+			int ns_type;
+			/* Verify it's a network namespace via ioctl */
+			ns_type = ioctl(fd, NS_GET_NSTYPE);
+			if (ns_type >= 0) {
+				ASSERT_EQ(ns_type, CLONE_NEWNET);
+			}
+			close(fd);
+		}
+	}
+}
+
+/*
+ * Test listns() pagination.
+ * List namespaces in batches.
+ */
+TEST(listns_pagination)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 batch1[2], batch2[2];
+	ssize_t ret1, ret2;
+
+	/* Get first batch */
+	ret1 = sys_listns(&req, batch1, ARRAY_SIZE(batch1), 0);
+	if (ret1 < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	ASSERT_GE(ret1, 0);
+
+	if (ret1 == 0)
+		SKIP(return, "No namespaces found");
+
+	TH_LOG("First batch: %zd namespaces", ret1);
+
+	/* Get second batch using last ID from first batch */
+	if (ret1 == ARRAY_SIZE(batch1)) {
+		req.ns_id = batch1[ret1 - 1];
+		ret2 = sys_listns(&req, batch2, ARRAY_SIZE(batch2), 0);
+		ASSERT_GE(ret2, 0);
+
+		TH_LOG("Second batch: %zd namespaces (after ns_id=%llu)",
+		       ret2, (unsigned long long)req.ns_id);
+
+		/* If we got more results, verify IDs are monotonically increasing */
+		if (ret2 > 0) {
+			ASSERT_GT(batch2[0], batch1[ret1 - 1]);
+			TH_LOG("Pagination working: %llu > %llu",
+			       (unsigned long long)batch2[0],
+			       (unsigned long long)batch1[ret1 - 1]);
+		}
+	} else {
+		TH_LOG("All namespaces fit in first batch");
+	}
+}
+
+/*
+ * Test listns() with LISTNS_CURRENT_USER.
+ * List namespaces owned by current user namespace.
+ */
+TEST(listns_current_user)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = LISTNS_CURRENT_USER,
+	};
+	__u64 ns_ids[100];
+	ssize_t ret;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	ASSERT_GE(ret, 0);
+
+	/* Should find at least the initial namespaces if we're in init_user_ns */
+	TH_LOG("Found %zd namespaces owned by current user namespace", ret);
+
+	for (ssize_t i = 0; i < ret; i++)
+		TH_LOG("  [%zd] ns_id: %llu", i, (unsigned long long)ns_ids[i]);
+}
+
+/*
+ * Test that listns() only returns active namespaces.
+ * Create a namespace, let it become inactive, verify it's not listed.
+ */
+TEST(listns_only_active)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[100], ns_ids_after[100];
+	ssize_t ret_before, ret_after;
+	int pipefd[2];
+	pid_t pid;
+	__u64 new_ns_id = 0;
+	int status;
+
+	/* Get initial list */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	ASSERT_GE(ret_before, 0);
+
+	TH_LOG("Before: %zd active network namespaces", ret_before);
+
+	/* Create a new namespace in a child process and get its ID */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 ns_id;
+
+		close(pipefd[0]);
+
+		/* Create new network namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get its ID */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &ns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send ID to parent */
+		write(pipefd[1], &ns_id, sizeof(ns_id));
+		close(pipefd[1]);
+
+		/* Keep namespace active briefly */
+		usleep(100000);
+		exit(0);
+	}
+
+	/* Parent reads the new namespace ID */
+	{
+		int bytes;
+
+		close(pipefd[1]);
+		bytes = read(pipefd[0], &new_ns_id, sizeof(new_ns_id));
+		close(pipefd[0]);
+
+		if (bytes == sizeof(new_ns_id)) {
+			__u64 ns_ids_during[100];
+			int ret_during;
+
+			TH_LOG("Child created namespace with ID %llu", (unsigned long long)new_ns_id);
+
+			/* List namespaces while child is still alive - should see new one */
+			ret_during = sys_listns(&req, ns_ids_during, ARRAY_SIZE(ns_ids_during), 0);
+			ASSERT_GE(ret_during, 0);
+			TH_LOG("During: %d active network namespaces", ret_during);
+
+			/* Should have more namespaces than before */
+			ASSERT_GE(ret_during, ret_before);
+		}
+	}
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+
+	/* Give time for namespace to become inactive */
+	usleep(100000);
+
+	/* List namespaces after child exits - should not see new one */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+	TH_LOG("After: %zd active network namespaces", ret_after);
+
+	/* Verify the new namespace ID is not in the after list */
+	if (new_ns_id != 0) {
+		bool found = false;
+
+		for (ssize_t i = 0; i < ret_after; i++) {
+			if (ns_ids_after[i] == new_ns_id) {
+				found = true;
+				break;
+			}
+		}
+		ASSERT_FALSE(found);
+	}
+}
+
+/*
+ * Test listns() with specific user namespace ID.
+ * Create a user namespace and list namespaces it owns.
+ */
+TEST(listns_specific_userns)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = 0,  /* Will be filled with created userns ID */
+	};
+	__u64 ns_ids[100];
+	int sv[2];
+	pid_t pid;
+	int status;
+	__u64 user_ns_id = 0;
+	int bytes;
+	ssize_t ret;
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 ns_id;
+		char buf;
+
+		close(sv[0]);
+
+		/* Create new user namespace */
+		if (setup_userns() < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		/* Get user namespace ID */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &ns_id) < 0) {
+			close(fd);
+			close(sv[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send ID to parent */
+		if (write(sv[1], &ns_id, sizeof(ns_id)) != sizeof(ns_id)) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		/* Create some namespaces owned by this user namespace */
+		unshare(CLONE_NEWNET);
+		unshare(CLONE_NEWUTS);
+
+		/* Wait for parent signal */
+		if (read(sv[1], &buf, 1) != 1) {
+			close(sv[1]);
+			exit(1);
+		}
+		close(sv[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(sv[1]);
+	bytes = read(sv[0], &user_ns_id, sizeof(user_ns_id));
+
+	if (bytes != sizeof(user_ns_id)) {
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get user namespace ID from child");
+	}
+
+	TH_LOG("Child created user namespace with ID %llu", (unsigned long long)user_ns_id);
+
+	/* List namespaces owned by this user namespace */
+	req.user_ns_id = user_ns_id;
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+	if (ret < 0) {
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		if (errno == ENOSYS) {
+			SKIP(return, "listns() not supported");
+		}
+		ASSERT_GE(ret, 0);
+	}
+
+	TH_LOG("Found %zd namespaces owned by user namespace %llu", ret,
+	       (unsigned long long)user_ns_id);
+
+	/* Should find at least the network and UTS namespaces we created */
+	if (ret > 0) {
+		for (ssize_t i = 0; i < ret && i < 10; i++)
+			TH_LOG("  [%zd] ns_id: %llu", i, (unsigned long long)ns_ids[i]);
+	}
+
+	/* Signal child to exit */
+	if (write(sv[0], "X", 1) != 1) {
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		ASSERT_TRUE(false);
+	}
+	close(sv[0]);
+	waitpid(pid, &status, 0);
+}
+
+/*
+ * Test listns() with multiple namespace types filter.
+ */
+TEST(listns_multiple_types)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET | CLONE_NEWUTS,  /* Network and UTS */
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[100];
+	ssize_t ret;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	ASSERT_GE(ret, 0);
+
+	TH_LOG("Found %zd active network/UTS namespaces", ret);
+
+	for (ssize_t i = 0; i < ret; i++)
+		TH_LOG("  [%zd] ns_id: %llu", i, (unsigned long long)ns_ids[i]);
+}
+
+/*
+ * Test that hierarchical active reference propagation keeps parent
+ * user namespaces visible in listns().
+ */
+TEST(listns_hierarchical_visibility)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 parent_ns_id = 0, child_ns_id = 0;
+	int sv[2];
+	pid_t pid;
+	int status;
+	int bytes;
+	__u64 ns_ids[100];
+	ssize_t ret;
+	bool found_parent, found_child;
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		char buf;
+
+		close(sv[0]);
+
+		/* Create parent user namespace */
+		if (setup_userns() < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &parent_ns_id) < 0) {
+			close(fd);
+			close(sv[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Create child user namespace */
+		if (setup_userns() < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &child_ns_id) < 0) {
+			close(fd);
+			close(sv[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Send both IDs to parent */
+		if (write(sv[1], &parent_ns_id, sizeof(parent_ns_id)) != sizeof(parent_ns_id)) {
+			close(sv[1]);
+			exit(1);
+		}
+		if (write(sv[1], &child_ns_id, sizeof(child_ns_id)) != sizeof(child_ns_id)) {
+			close(sv[1]);
+			exit(1);
+		}
+
+		/* Wait for parent signal */
+		if (read(sv[1], &buf, 1) != 1) {
+			close(sv[1]);
+			exit(1);
+		}
+		close(sv[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(sv[1]);
+
+	/* Read both namespace IDs */
+	bytes = read(sv[0], &parent_ns_id, sizeof(parent_ns_id));
+	bytes += read(sv[0], &child_ns_id, sizeof(child_ns_id));
+
+	if (bytes != (int)(2 * sizeof(__u64))) {
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to get namespace IDs from child");
+	}
+
+	TH_LOG("Parent user namespace ID: %llu", (unsigned long long)parent_ns_id);
+	TH_LOG("Child user namespace ID: %llu", (unsigned long long)child_ns_id);
+
+	/* List all user namespaces */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+
+	if (ret < 0 && errno == ENOSYS) {
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "listns() not supported");
+	}
+
+	ASSERT_GE(ret, 0);
+	TH_LOG("Found %zd active user namespaces", ret);
+
+	/* Both parent and child should be visible (active due to child process) */
+	found_parent = false;
+	found_child = false;
+	for (ssize_t i = 0; i < ret; i++) {
+		if (ns_ids[i] == parent_ns_id)
+			found_parent = true;
+		if (ns_ids[i] == child_ns_id)
+			found_child = true;
+	}
+
+	TH_LOG("Parent namespace %s, child namespace %s",
+	       found_parent ? "found" : "NOT FOUND",
+	       found_child ? "found" : "NOT FOUND");
+
+	ASSERT_TRUE(found_child);
+	/* With hierarchical propagation, parent should also be active */
+	ASSERT_TRUE(found_parent);
+
+	/* Signal child to exit */
+	if (write(sv[0], "X", 1) != 1) {
+		close(sv[0]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		ASSERT_TRUE(false);
+	}
+	close(sv[0]);
+	waitpid(pid, &status, 0);
+}
+
+/*
+ * Test error cases for listns().
+ */
+TEST(listns_error_cases)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[10];
+	int ret;
+
+	/* Test with invalid flags */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0xFFFF);
+	if (errno == ENOSYS) {
+		/* listns() not supported, skip this check */
+	} else {
+		ASSERT_LT(ret, 0);
+		ASSERT_EQ(errno, EINVAL);
+	}
+
+	/* Test with NULL ns_ids array */
+	ret = sys_listns(&req, NULL, 10, 0);
+	ASSERT_LT(ret, 0);
+
+	/* Test with invalid spare field */
+	req.spare = 1;
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (errno == ENOSYS) {
+		/* listns() not supported, skip this check */
+	} else {
+		ASSERT_LT(ret, 0);
+		ASSERT_EQ(errno, EINVAL);
+	}
+	req.spare = 0;
+
+	/* Test with huge nr_ns_ids */
+	ret = sys_listns(&req, ns_ids, 2000000, 0);
+	if (errno == ENOSYS) {
+		/* listns() not supported, skip this check */
+	} else {
+		ASSERT_LT(ret, 0);
+	}
+}
+
+TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/namespaces/ns_active_ref_test.c b/tools/testing/selftests/namespaces/ns_active_ref_test.c
new file mode 100644
index 000000000000..093268f0efaa
--- /dev/null
+++ b/tools/testing/selftests/namespaces/ns_active_ref_test.c
@@ -0,0 +1,2672 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <linux/nsfs.h>
+#include <sys/mount.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+#include <pthread.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+#ifndef FD_NSFS_ROOT
+#define FD_NSFS_ROOT -10003 /* Root of the nsfs filesystem */
+#endif
+
+#ifndef FILEID_NSFS
+#define FILEID_NSFS 0xf1
+#endif
+
+/*
+ * Test that initial namespaces can be reopened via file handle.
+ * Initial namespaces should have active ref count of 1 from boot.
+ */
+TEST(init_ns_always_active)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd1, fd2;
+	struct stat st1, st2;
+
+	handle = malloc(sizeof(*handle) + MAX_HANDLE_SZ);
+	ASSERT_NE(handle, NULL);
+
+	/* Open initial network namespace */
+	fd1 = open("/proc/1/ns/net", O_RDONLY);
+	ASSERT_GE(fd1, 0);
+
+	/* Get file handle for initial namespace */
+	handle->handle_bytes = MAX_HANDLE_SZ;
+	ret = name_to_handle_at(fd1, "", handle, &mount_id, AT_EMPTY_PATH);
+	if (ret < 0 && errno == EOPNOTSUPP) {
+		SKIP(free(handle); close(fd1);
+		     return, "nsfs doesn't support file handles");
+	}
+	ASSERT_EQ(ret, 0);
+
+	/* Close the namespace fd */
+	close(fd1);
+
+	/* Try to reopen via file handle - should succeed since init ns is always active */
+	fd2 = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (fd2 < 0 && (errno == EINVAL || errno == EOPNOTSUPP)) {
+		SKIP(free(handle);
+		     return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+	}
+	ASSERT_GE(fd2, 0);
+
+	/* Verify we opened the same namespace */
+	fd1 = open("/proc/1/ns/net", O_RDONLY);
+	ASSERT_GE(fd1, 0);
+	ASSERT_EQ(fstat(fd1, &st1), 0);
+	ASSERT_EQ(fstat(fd2, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+
+	close(fd1);
+	close(fd2);
+	free(handle);
+}
+
+/*
+ * Test namespace lifecycle: create a namespace in a child process,
+ * get a file handle while it's active, then try to reopen after
+ * the process exits (namespace becomes inactive).
+ */
+TEST(ns_inactive_after_exit)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+
+	/* Create pipe for passing file handle from child */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* Create new network namespace */
+		ret = unshare(CLONE_NEWNET);
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Open our new namespace */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get file handle for the namespace */
+		handle = (struct file_handle *)buf;
+		handle->handle_bytes = MAX_HANDLE_SZ;
+		ret = name_to_handle_at(fd, "", handle, &mount_id, AT_EMPTY_PATH);
+		close(fd);
+
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Send handle to parent */
+		write(pipefd[1], buf, sizeof(*handle) + handle->handle_bytes);
+		close(pipefd[1]);
+
+		/* Exit - namespace should become inactive */
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	/* Read file handle from child */
+	ret = read(pipefd[0], buf, sizeof(buf));
+	close(pipefd[0]);
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_GT(ret, 0);
+	handle = (struct file_handle *)buf;
+
+	/* Try to reopen namespace - should fail with ENOENT since it's inactive */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(fd, 0);
+	/* Should fail with ENOENT (namespace inactive) or ESTALE */
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
+/*
+ * Test that a namespace remains active while a process is using it,
+ * even after the creating process exits.
+ */
+TEST(ns_active_with_multiple_processes)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int pipefd[2];
+	int syncpipe[2];
+	pid_t pid1, pid2;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+	char sync_byte;
+
+	/* Create pipes for communication */
+	ASSERT_EQ(pipe(pipefd), 0);
+	ASSERT_EQ(pipe(syncpipe), 0);
+
+	pid1 = fork();
+	ASSERT_GE(pid1, 0);
+
+	if (pid1 == 0) {
+		/* First child - creates namespace */
+		close(pipefd[0]);
+		close(syncpipe[1]);
+
+		/* Create new network namespace */
+		ret = unshare(CLONE_NEWNET);
+		if (ret < 0) {
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		/* Open and get handle */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		handle = (struct file_handle *)buf;
+		handle->handle_bytes = MAX_HANDLE_SZ;
+		ret = name_to_handle_at(fd, "", handle, &mount_id, AT_EMPTY_PATH);
+		close(fd);
+
+		if (ret < 0) {
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		/* Send handle to parent */
+		write(pipefd[1], buf, sizeof(*handle) + handle->handle_bytes);
+		close(pipefd[1]);
+
+		/* Wait for signal before exiting */
+		read(syncpipe[0], &sync_byte, 1);
+		close(syncpipe[0]);
+		exit(0);
+	}
+
+	/* Parent reads handle */
+	close(pipefd[1]);
+	ret = read(pipefd[0], buf, sizeof(buf));
+	close(pipefd[0]);
+	ASSERT_GT(ret, 0);
+
+	handle = (struct file_handle *)buf;
+
+	/* Create second child that will keep namespace active */
+	pid2 = fork();
+	ASSERT_GE(pid2, 0);
+
+	if (pid2 == 0) {
+		/* Second child - reopens the namespace */
+		close(syncpipe[0]);
+		close(syncpipe[1]);
+
+		/* Open the namespace via handle */
+		fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+		if (fd < 0) {
+			exit(1);
+		}
+
+		/* Join the namespace */
+		ret = setns(fd, CLONE_NEWNET);
+		close(fd);
+		if (ret < 0) {
+			exit(1);
+		}
+
+		/* Sleep to keep namespace active */
+		sleep(1);
+		exit(0);
+	}
+
+	/* Let second child enter the namespace */
+	usleep(100000); /* 100ms */
+
+	/* Signal first child to exit */
+	close(syncpipe[0]);
+	sync_byte = 'X';
+	write(syncpipe[1], &sync_byte, 1);
+	close(syncpipe[1]);
+
+	/* Wait for first child */
+	waitpid(pid1, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+
+	/* Namespace should still be active because second child is using it */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_GE(fd, 0);
+	close(fd);
+
+	/* Wait for second child */
+	waitpid(pid2, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+}
+
+/*
+ * Test user namespace active ref tracking via credential lifecycle
+ */
+TEST(userns_active_ref_lifecycle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* Create new user namespace */
+		ret = unshare(CLONE_NEWUSER);
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Set up uid/gid mappings */
+		int uid_map_fd = open("/proc/self/uid_map", O_WRONLY);
+		int gid_map_fd = open("/proc/self/gid_map", O_WRONLY);
+		int setgroups_fd = open("/proc/self/setgroups", O_WRONLY);
+
+		if (uid_map_fd >= 0 && gid_map_fd >= 0 && setgroups_fd >= 0) {
+			write(setgroups_fd, "deny", 4);
+			close(setgroups_fd);
+
+			char mapping[64];
+			snprintf(mapping, sizeof(mapping), "0 %d 1", getuid());
+			write(uid_map_fd, mapping, strlen(mapping));
+			close(uid_map_fd);
+
+			snprintf(mapping, sizeof(mapping), "0 %d 1", getgid());
+			write(gid_map_fd, mapping, strlen(mapping));
+			close(gid_map_fd);
+		}
+
+		/* Get file handle */
+		fd = open("/proc/self/ns/user", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		handle = (struct file_handle *)buf;
+		handle->handle_bytes = MAX_HANDLE_SZ;
+		ret = name_to_handle_at(fd, "", handle, &mount_id, AT_EMPTY_PATH);
+		close(fd);
+
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Send handle to parent */
+		write(pipefd[1], buf, sizeof(*handle) + handle->handle_bytes);
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+	ret = read(pipefd[0], buf, sizeof(buf));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_GT(ret, 0);
+	handle = (struct file_handle *)buf;
+
+	/* Namespace should be inactive after all tasks exit */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(fd, 0);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
+/*
+ * Test PID namespace active ref tracking
+ */
+TEST(pidns_active_ref_lifecycle)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* Create new PID namespace */
+		ret = unshare(CLONE_NEWPID);
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Fork to actually enter the PID namespace */
+		pid_t child = fork();
+		if (child < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (child == 0) {
+			/* Grandchild - in new PID namespace */
+			fd = open("/proc/self/ns/pid", O_RDONLY);
+			if (fd < 0) {
+				exit(1);
+			}
+
+			handle = (struct file_handle *)buf;
+			handle->handle_bytes = MAX_HANDLE_SZ;
+			ret = name_to_handle_at(fd, "", handle, &mount_id, AT_EMPTY_PATH);
+			close(fd);
+
+			if (ret < 0) {
+				exit(1);
+			}
+
+			/* Send handle to grandparent */
+			write(pipefd[1], buf, sizeof(*handle) + handle->handle_bytes);
+			close(pipefd[1]);
+			exit(0);
+		}
+
+		/* Wait for grandchild */
+		waitpid(child, NULL, 0);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+	ret = read(pipefd[0], buf, sizeof(buf));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_GT(ret, 0);
+	handle = (struct file_handle *)buf;
+
+	/* Namespace should be inactive after all processes exit */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(fd, 0);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
+/*
+ * Test that an open file descriptor keeps a namespace active.
+ * Even after the creating process exits, the namespace should remain
+ * active as long as an fd is held open.
+ */
+TEST(ns_fd_keeps_active)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int nsfd;
+	int pipe_child_ready[2];
+	int pipe_parent_ready[2];
+	pid_t pid;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+	char sync_byte;
+	char proc_path[64];
+
+	ASSERT_EQ(pipe(pipe_child_ready), 0);
+	ASSERT_EQ(pipe(pipe_parent_ready), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipe_child_ready[0]);
+		close(pipe_parent_ready[1]);
+
+		TH_LOG("Child: creating new network namespace");
+
+		/* Create new network namespace */
+		ret = unshare(CLONE_NEWNET);
+		if (ret < 0) {
+			TH_LOG("Child: unshare(CLONE_NEWNET) failed: %s", strerror(errno));
+			close(pipe_child_ready[1]);
+			close(pipe_parent_ready[0]);
+			exit(1);
+		}
+
+		TH_LOG("Child: network namespace created successfully");
+
+		/* Get file handle for the namespace */
+		nsfd = open("/proc/self/ns/net", O_RDONLY);
+		if (nsfd < 0) {
+			TH_LOG("Child: failed to open /proc/self/ns/net: %s", strerror(errno));
+			close(pipe_child_ready[1]);
+			close(pipe_parent_ready[0]);
+			exit(1);
+		}
+
+		TH_LOG("Child: opened namespace fd %d", nsfd);
+
+		handle = (struct file_handle *)buf;
+		handle->handle_bytes = MAX_HANDLE_SZ;
+		ret = name_to_handle_at(nsfd, "", handle, &mount_id, AT_EMPTY_PATH);
+		close(nsfd);
+
+		if (ret < 0) {
+			TH_LOG("Child: name_to_handle_at failed: %s", strerror(errno));
+			close(pipe_child_ready[1]);
+			close(pipe_parent_ready[0]);
+			exit(1);
+		}
+
+		TH_LOG("Child: got file handle (bytes=%u)", handle->handle_bytes);
+
+		/* Send file handle to parent */
+		ret = write(pipe_child_ready[1], buf, sizeof(*handle) + handle->handle_bytes);
+		TH_LOG("Child: sent %d bytes of file handle to parent", ret);
+		close(pipe_child_ready[1]);
+
+		/* Wait for parent to open the fd */
+		TH_LOG("Child: waiting for parent to open fd");
+		ret = read(pipe_parent_ready[0], &sync_byte, 1);
+		close(pipe_parent_ready[0]);
+
+		TH_LOG("Child: parent signaled (read %d bytes), exiting now", ret);
+		/* Exit - namespace should stay active because parent holds fd */
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipe_child_ready[1]);
+	close(pipe_parent_ready[0]);
+
+	TH_LOG("Parent: reading file handle from child");
+
+	/* Read file handle from child */
+	ret = read(pipe_child_ready[0], buf, sizeof(buf));
+	close(pipe_child_ready[0]);
+	ASSERT_GT(ret, 0);
+	handle = (struct file_handle *)buf;
+
+	TH_LOG("Parent: received %d bytes, handle size=%u", ret, handle->handle_bytes);
+
+	/* Open the child's namespace while it's still alive */
+	snprintf(proc_path, sizeof(proc_path), "/proc/%d/ns/net", pid);
+	TH_LOG("Parent: opening child's namespace at %s", proc_path);
+	nsfd = open(proc_path, O_RDONLY);
+	if (nsfd < 0) {
+		TH_LOG("Parent: failed to open %s: %s", proc_path, strerror(errno));
+		close(pipe_parent_ready[1]);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open child's namespace");
+	}
+
+	TH_LOG("Parent: opened child's namespace, got fd %d", nsfd);
+
+	/* Signal child that we have the fd */
+	sync_byte = 'G';
+	write(pipe_parent_ready[1], &sync_byte, 1);
+	close(pipe_parent_ready[1]);
+	TH_LOG("Parent: signaled child that we have the fd");
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	TH_LOG("Child exited, parent holds fd %d to namespace", nsfd);
+
+	/*
+	 * Namespace should still be ACTIVE because we hold an fd.
+	 * We should be able to reopen it via file handle.
+	 */
+	TH_LOG("Attempting to reopen namespace via file handle (should succeed - fd held)");
+	int fd2 = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_GE(fd2, 0);
+
+	TH_LOG("Successfully reopened namespace via file handle, got fd %d", fd2);
+
+	/* Verify it's the same namespace */
+	struct stat st1, st2;
+	ASSERT_EQ(fstat(nsfd, &st1), 0);
+	ASSERT_EQ(fstat(fd2, &st2), 0);
+	TH_LOG("Namespace inodes: nsfd=%lu, fd2=%lu", st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	close(fd2);
+
+	/* Now close the fd - namespace should become inactive */
+	TH_LOG("Closing fd %d - namespace should become inactive", nsfd);
+	close(nsfd);
+
+	/* Now reopening should fail - namespace is inactive */
+	TH_LOG("Attempting to reopen namespace via file handle (should fail - inactive)");
+	fd2 = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(fd2, 0);
+	/* Should fail with ENOENT (inactive) or ESTALE (gone) */
+	TH_LOG("Reopen failed as expected: %s (errno=%d)", strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
+/*
+ * Test hierarchical active reference propagation.
+ * When a child namespace is active, its owning user namespace should also
+ * be active automatically due to hierarchical active reference propagation.
+ * This ensures parents are always reachable when children are active.
+ */
+TEST(ns_parent_always_reachable)
+{
+	struct file_handle *parent_handle, *child_handle;
+	int ret;
+	int child_nsfd;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 parent_id, child_id;
+	char parent_buf[sizeof(*parent_handle) + MAX_HANDLE_SZ];
+	char child_buf[sizeof(*child_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		TH_LOG("Child: creating parent user namespace and setting up mappings");
+
+		/* Create parent user namespace with mappings */
+		ret = setup_userns();
+		if (ret < 0) {
+			TH_LOG("Child: setup_userns() for parent failed: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: parent user namespace created, now uid=%d gid=%d", getuid(), getgid());
+
+		/* Get namespace ID for parent user namespace */
+		int parent_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (parent_fd < 0) {
+			TH_LOG("Child: failed to open parent /proc/self/ns/user: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: opened parent userns fd %d", parent_fd);
+
+		if (ioctl(parent_fd, NS_GET_ID, &parent_id) < 0) {
+			TH_LOG("Child: NS_GET_ID for parent failed: %s", strerror(errno));
+			close(parent_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(parent_fd);
+
+		TH_LOG("Child: got parent namespace ID %llu", (unsigned long long)parent_id);
+
+		/* Create child user namespace within parent */
+		TH_LOG("Child: creating nested child user namespace");
+		ret = setup_userns();
+		if (ret < 0) {
+			TH_LOG("Child: setup_userns() for child failed: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: nested child user namespace created, uid=%d gid=%d", getuid(), getgid());
+
+		/* Get namespace ID for child user namespace */
+		int child_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (child_fd < 0) {
+			TH_LOG("Child: failed to open child /proc/self/ns/user: %s", strerror(errno));
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		TH_LOG("Child: opened child userns fd %d", child_fd);
+
+		if (ioctl(child_fd, NS_GET_ID, &child_id) < 0) {
+			TH_LOG("Child: NS_GET_ID for child failed: %s", strerror(errno));
+			close(child_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(child_fd);
+
+		TH_LOG("Child: got child namespace ID %llu", (unsigned long long)child_id);
+
+		/* Send both namespace IDs to parent */
+		TH_LOG("Child: sending both namespace IDs to parent");
+		write(pipefd[1], &parent_id, sizeof(parent_id));
+		write(pipefd[1], &child_id, sizeof(child_id));
+		close(pipefd[1]);
+
+		TH_LOG("Child: exiting - parent userns should become inactive");
+		/* Exit - parent user namespace should become inactive */
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+
+	TH_LOG("Parent: reading both namespace IDs from child");
+
+	/* Read both namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &parent_id, sizeof(parent_id));
+	if (ret != sizeof(parent_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read parent namespace ID from child");
+	}
+
+	ret = read(pipefd[0], &child_id, sizeof(child_id));
+	close(pipefd[0]);
+	if (ret != sizeof(child_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read child namespace ID from child");
+	}
+
+	TH_LOG("Parent: received parent_id=%llu, child_id=%llu",
+	       (unsigned long long)parent_id, (unsigned long long)child_id);
+
+	/* Construct file handles from namespace IDs */
+	parent_handle = (struct file_handle *)parent_buf;
+	parent_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	parent_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *parent_fh = (struct nsfs_file_handle *)parent_handle->f_handle;
+	parent_fh->ns_id = parent_id;
+	parent_fh->ns_type = 0;
+	parent_fh->ns_inum = 0;
+
+	child_handle = (struct file_handle *)child_buf;
+	child_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	child_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *child_fh = (struct nsfs_file_handle *)child_handle->f_handle;
+	child_fh->ns_id = child_id;
+	child_fh->ns_type = 0;
+	child_fh->ns_inum = 0;
+
+	TH_LOG("Parent: opening child namespace BEFORE child exits");
+
+	/* Open child namespace while child is still alive to keep it active */
+	child_nsfd = open_by_handle_at(FD_NSFS_ROOT, child_handle, O_RDONLY);
+	if (child_nsfd < 0) {
+		TH_LOG("Failed to open child namespace: %s (errno=%d)", strerror(errno), errno);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open child namespace");
+	}
+
+	TH_LOG("Opened child namespace fd %d", child_nsfd);
+
+	/* Now wait for child to exit */
+	TH_LOG("Parent: waiting for child to exit");
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	TH_LOG("Child process exited, parent holds fd to child namespace");
+
+	/*
+	 * With hierarchical active reference propagation:
+	 * Since the child namespace is active (parent process holds fd),
+	 * the parent user namespace should ALSO be active automatically.
+	 * This is because when we took an active reference on the child,
+	 * it propagated up to the owning user namespace.
+	 */
+	TH_LOG("Attempting to reopen parent namespace (should SUCCEED - hierarchical propagation)");
+	int parent_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_GE(parent_fd, 0);
+
+	TH_LOG("SUCCESS: Parent namespace is active (fd=%d) due to active child", parent_fd);
+
+	/* Verify we can also get parent via NS_GET_USERNS */
+	TH_LOG("Verifying NS_GET_USERNS also works");
+	int parent_fd2 = ioctl(child_nsfd, NS_GET_USERNS);
+	if (parent_fd2 < 0) {
+		close(parent_fd);
+		close(child_nsfd);
+		TH_LOG("NS_GET_USERNS failed: %s (errno=%d)", strerror(errno), errno);
+		SKIP(return, "NS_GET_USERNS not supported or failed");
+	}
+
+	TH_LOG("NS_GET_USERNS succeeded, got parent fd %d", parent_fd2);
+
+	/* Verify both methods give us the same namespace */
+	struct stat st1, st2;
+	ASSERT_EQ(fstat(parent_fd, &st1), 0);
+	ASSERT_EQ(fstat(parent_fd2, &st2), 0);
+	TH_LOG("Parent namespace inodes: parent_fd=%lu, parent_fd2=%lu", st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+
+	/*
+	 * Close child fd - parent should remain active because we still
+	 * hold direct references to it (parent_fd and parent_fd2).
+	 */
+	TH_LOG("Closing child fd - parent should remain active (direct refs held)");
+	close(child_nsfd);
+
+	/* Parent should still be openable */
+	TH_LOG("Verifying parent still active via file handle");
+	int parent_fd3 = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_GE(parent_fd3, 0);
+	close(parent_fd3);
+
+	TH_LOG("Closing all fds to parent namespace");
+	close(parent_fd);
+	close(parent_fd2);
+
+	/* Both should now be inactive */
+	TH_LOG("Attempting to reopen parent (should fail - inactive, no refs)");
+	parent_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_LT(parent_fd, 0);
+	TH_LOG("Parent inactive as expected: %s (errno=%d)", strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
+/*
+ * Test that bind mounts keep namespaces in the tree even when inactive
+ */
+TEST(ns_bind_mount_keeps_in_tree)
+{
+	struct file_handle *handle;
+	int mount_id;
+	int ret;
+	int fd;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+	char tmpfile[] = "/tmp/ns-test-XXXXXX";
+	int tmpfd;
+
+	/* Create temporary file for bind mount */
+	tmpfd = mkstemp(tmpfile);
+	if (tmpfd < 0) {
+		SKIP(return, "Cannot create temporary file");
+	}
+	close(tmpfd);
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+
+		/* Unshare mount namespace and make mounts private to avoid propagation */
+		ret = unshare(CLONE_NEWNS);
+		if (ret < 0) {
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+		ret = mount(NULL, "/", NULL, MS_PRIVATE | MS_REC, NULL);
+		if (ret < 0) {
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+
+		/* Create new network namespace */
+		ret = unshare(CLONE_NEWNET);
+		if (ret < 0) {
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+
+		/* Bind mount the namespace */
+		ret = mount("/proc/self/ns/net", tmpfile, NULL, MS_BIND, NULL);
+		if (ret < 0) {
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+
+		/* Get file handle */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			umount(tmpfile);
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+
+		handle = (struct file_handle *)buf;
+		handle->handle_bytes = MAX_HANDLE_SZ;
+		ret = name_to_handle_at(fd, "", handle, &mount_id, AT_EMPTY_PATH);
+		close(fd);
+
+		if (ret < 0) {
+			umount(tmpfile);
+			close(pipefd[1]);
+			unlink(tmpfile);
+			exit(1);
+		}
+
+		/* Send handle to parent */
+		write(pipefd[1], buf, sizeof(*handle) + handle->handle_bytes);
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+	ret = read(pipefd[0], buf, sizeof(buf));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	ASSERT_GT(ret, 0);
+	handle = (struct file_handle *)buf;
+
+	/*
+	 * Namespace should be inactive but still in tree due to bind mount.
+	 * Reopening should fail with ENOENT (inactive) not ESTALE (not in tree).
+	 */
+	fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(fd, 0);
+	/* Should be ENOENT (inactive) since bind mount keeps it in tree */
+	if (errno != ENOENT && errno != ESTALE) {
+		TH_LOG("Unexpected error: %d", errno);
+	}
+
+	/* Cleanup */
+	umount(tmpfile);
+	unlink(tmpfile);
+}
+
+/*
+ * Test multi-level hierarchy (3+ levels deep).
+ * Grandparent → Parent → Child
+ * When child is active, both parent AND grandparent should be active.
+ */
+TEST(ns_multilevel_hierarchy)
+{
+	struct file_handle *gp_handle, *p_handle, *c_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 gp_id, p_id, c_id;
+	char gp_buf[sizeof(*gp_handle) + MAX_HANDLE_SZ];
+	char p_buf[sizeof(*p_handle) + MAX_HANDLE_SZ];
+	char c_buf[sizeof(*c_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		/* Create grandparent user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int gp_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (gp_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(gp_fd, NS_GET_ID, &gp_id) < 0) {
+			close(gp_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(gp_fd);
+
+		/* Create parent user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int p_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (p_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(p_fd, NS_GET_ID, &p_id) < 0) {
+			close(p_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(p_fd);
+
+		/* Create child user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int c_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (c_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(c_fd, NS_GET_ID, &c_id) < 0) {
+			close(c_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(c_fd);
+
+		/* Send all three namespace IDs */
+		write(pipefd[1], &gp_id, sizeof(gp_id));
+		write(pipefd[1], &p_id, sizeof(p_id));
+		write(pipefd[1], &c_id, sizeof(c_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &gp_id, sizeof(gp_id));
+	if (ret != sizeof(gp_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read grandparent namespace ID from child");
+	}
+
+	ret = read(pipefd[0], &p_id, sizeof(p_id));
+	if (ret != sizeof(p_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read parent namespace ID from child");
+	}
+
+	ret = read(pipefd[0], &c_id, sizeof(c_id));
+	close(pipefd[0]);
+	if (ret != sizeof(c_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read child namespace ID from child");
+	}
+
+	/* Construct file handles from namespace IDs */
+	gp_handle = (struct file_handle *)gp_buf;
+	gp_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	gp_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *gp_fh = (struct nsfs_file_handle *)gp_handle->f_handle;
+	gp_fh->ns_id = gp_id;
+	gp_fh->ns_type = 0;
+	gp_fh->ns_inum = 0;
+
+	p_handle = (struct file_handle *)p_buf;
+	p_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	p_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *p_fh = (struct nsfs_file_handle *)p_handle->f_handle;
+	p_fh->ns_id = p_id;
+	p_fh->ns_type = 0;
+	p_fh->ns_inum = 0;
+
+	c_handle = (struct file_handle *)c_buf;
+	c_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	c_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *c_fh = (struct nsfs_file_handle *)c_handle->f_handle;
+	c_fh->ns_id = c_id;
+	c_fh->ns_type = 0;
+	c_fh->ns_inum = 0;
+
+	/* Open child before process exits */
+	int c_fd = open_by_handle_at(FD_NSFS_ROOT, c_handle, O_RDONLY);
+	if (c_fd < 0) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open child namespace");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/*
+	 * With 3-level hierarchy and child active:
+	 * - Child is active (we hold fd)
+	 * - Parent should be active (propagated from child)
+	 * - Grandparent should be active (propagated from parent)
+	 */
+	TH_LOG("Testing parent active when child is active");
+	int p_fd = open_by_handle_at(FD_NSFS_ROOT, p_handle, O_RDONLY);
+	ASSERT_GE(p_fd, 0);
+
+	TH_LOG("Testing grandparent active when child is active");
+	int gp_fd = open_by_handle_at(FD_NSFS_ROOT, gp_handle, O_RDONLY);
+	ASSERT_GE(gp_fd, 0);
+
+	close(c_fd);
+	close(p_fd);
+	close(gp_fd);
+}
+
+/*
+ * Test multiple children sharing same parent.
+ * Parent should stay active as long as ANY child is active.
+ */
+TEST(ns_multiple_children_same_parent)
+{
+	struct file_handle *p_handle, *c1_handle, *c2_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 p_id, c1_id, c2_id;
+	char p_buf[sizeof(*p_handle) + MAX_HANDLE_SZ];
+	char c1_buf[sizeof(*c1_handle) + MAX_HANDLE_SZ];
+	char c2_buf[sizeof(*c2_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		/* Create parent user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int p_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (p_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(p_fd, NS_GET_ID, &p_id) < 0) {
+			close(p_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(p_fd);
+
+		/* Create first child user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int c1_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (c1_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(c1_fd, NS_GET_ID, &c1_id) < 0) {
+			close(c1_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(c1_fd);
+
+		/* Return to parent user namespace and create second child */
+		/* We can't actually do this easily, so let's create a sibling namespace
+		 * by creating a network namespace instead */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int c2_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (c2_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(c2_fd, NS_GET_ID, &c2_id) < 0) {
+			close(c2_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(c2_fd);
+
+		/* Send all namespace IDs */
+		write(pipefd[1], &p_id, sizeof(p_id));
+		write(pipefd[1], &c1_id, sizeof(c1_id));
+		write(pipefd[1], &c2_id, sizeof(c2_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &p_id, sizeof(p_id));
+	if (ret != sizeof(p_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read parent namespace ID");
+	}
+
+	ret = read(pipefd[0], &c1_id, sizeof(c1_id));
+	if (ret != sizeof(c1_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read first child namespace ID");
+	}
+
+	ret = read(pipefd[0], &c2_id, sizeof(c2_id));
+	close(pipefd[0]);
+	if (ret != sizeof(c2_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read second child namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	p_handle = (struct file_handle *)p_buf;
+	p_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	p_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *p_fh = (struct nsfs_file_handle *)p_handle->f_handle;
+	p_fh->ns_id = p_id;
+	p_fh->ns_type = 0;
+	p_fh->ns_inum = 0;
+
+	c1_handle = (struct file_handle *)c1_buf;
+	c1_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	c1_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *c1_fh = (struct nsfs_file_handle *)c1_handle->f_handle;
+	c1_fh->ns_id = c1_id;
+	c1_fh->ns_type = 0;
+	c1_fh->ns_inum = 0;
+
+	c2_handle = (struct file_handle *)c2_buf;
+	c2_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	c2_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *c2_fh = (struct nsfs_file_handle *)c2_handle->f_handle;
+	c2_fh->ns_id = c2_id;
+	c2_fh->ns_type = 0;
+	c2_fh->ns_inum = 0;
+
+	/* Open both children before process exits */
+	int c1_fd = open_by_handle_at(FD_NSFS_ROOT, c1_handle, O_RDONLY);
+	int c2_fd = open_by_handle_at(FD_NSFS_ROOT, c2_handle, O_RDONLY);
+
+	if (c1_fd < 0 || c2_fd < 0) {
+		if (c1_fd >= 0) close(c1_fd);
+		if (c2_fd >= 0) close(c2_fd);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open child namespaces");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Parent should be active (both children active) */
+	TH_LOG("Both children active - parent should be active");
+	int p_fd = open_by_handle_at(FD_NSFS_ROOT, p_handle, O_RDONLY);
+	ASSERT_GE(p_fd, 0);
+	close(p_fd);
+
+	/* Close first child - parent should STILL be active */
+	TH_LOG("Closing first child - parent should still be active");
+	close(c1_fd);
+	p_fd = open_by_handle_at(FD_NSFS_ROOT, p_handle, O_RDONLY);
+	ASSERT_GE(p_fd, 0);
+	close(p_fd);
+
+	/* Close second child - NOW parent should become inactive */
+	TH_LOG("Closing second child - parent should become inactive");
+	close(c2_fd);
+	p_fd = open_by_handle_at(FD_NSFS_ROOT, p_handle, O_RDONLY);
+	ASSERT_LT(p_fd, 0);
+}
+
+/*
+ * Test that different namespace types with same owner all contribute
+ * active references to the owning user namespace.
+ */
+TEST(ns_different_types_same_owner)
+{
+	struct file_handle *u_handle, *n_handle, *ut_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 u_id, n_id, ut_id;
+	char u_buf[sizeof(*u_handle) + MAX_HANDLE_SZ];
+	char n_buf[sizeof(*n_handle) + MAX_HANDLE_SZ];
+	char ut_buf[sizeof(*ut_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		/* Create user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int u_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (u_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(u_fd, NS_GET_ID, &u_id) < 0) {
+			close(u_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(u_fd);
+
+		/* Create network namespace (owned by user namespace) */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int n_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (n_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(n_fd, NS_GET_ID, &n_id) < 0) {
+			close(n_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(n_fd);
+
+		/* Create UTS namespace (also owned by user namespace) */
+		if (unshare(CLONE_NEWUTS) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ut_fd = open("/proc/self/ns/uts", O_RDONLY);
+		if (ut_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ut_fd, NS_GET_ID, &ut_id) < 0) {
+			close(ut_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ut_fd);
+
+		/* Send all namespace IDs */
+		write(pipefd[1], &u_id, sizeof(u_id));
+		write(pipefd[1], &n_id, sizeof(n_id));
+		write(pipefd[1], &ut_id, sizeof(ut_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &u_id, sizeof(u_id));
+	if (ret != sizeof(u_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user namespace ID");
+	}
+
+	ret = read(pipefd[0], &n_id, sizeof(n_id));
+	if (ret != sizeof(n_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read network namespace ID");
+	}
+
+	ret = read(pipefd[0], &ut_id, sizeof(ut_id));
+	close(pipefd[0]);
+	if (ret != sizeof(ut_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read UTS namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	u_handle = (struct file_handle *)u_buf;
+	u_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	u_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *u_fh = (struct nsfs_file_handle *)u_handle->f_handle;
+	u_fh->ns_id = u_id;
+	u_fh->ns_type = 0;
+	u_fh->ns_inum = 0;
+
+	n_handle = (struct file_handle *)n_buf;
+	n_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	n_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *n_fh = (struct nsfs_file_handle *)n_handle->f_handle;
+	n_fh->ns_id = n_id;
+	n_fh->ns_type = 0;
+	n_fh->ns_inum = 0;
+
+	ut_handle = (struct file_handle *)ut_buf;
+	ut_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	ut_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ut_fh = (struct nsfs_file_handle *)ut_handle->f_handle;
+	ut_fh->ns_id = ut_id;
+	ut_fh->ns_type = 0;
+	ut_fh->ns_inum = 0;
+
+	/* Open both non-user namespaces before process exits */
+	int n_fd = open_by_handle_at(FD_NSFS_ROOT, n_handle, O_RDONLY);
+	int ut_fd = open_by_handle_at(FD_NSFS_ROOT, ut_handle, O_RDONLY);
+
+	if (n_fd < 0 || ut_fd < 0) {
+		if (n_fd >= 0) close(n_fd);
+		if (ut_fd >= 0) close(ut_fd);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open namespaces");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/*
+	 * Both network and UTS namespaces are active.
+	 * User namespace should be active (gets 2 active refs).
+	 */
+	TH_LOG("Both net and uts active - user namespace should be active");
+	int u_fd = open_by_handle_at(FD_NSFS_ROOT, u_handle, O_RDONLY);
+	ASSERT_GE(u_fd, 0);
+	close(u_fd);
+
+	/* Close network namespace - user namespace should STILL be active */
+	TH_LOG("Closing network ns - user ns should still be active (uts still active)");
+	close(n_fd);
+	u_fd = open_by_handle_at(FD_NSFS_ROOT, u_handle, O_RDONLY);
+	ASSERT_GE(u_fd, 0);
+	close(u_fd);
+
+	/* Close UTS namespace - user namespace should become inactive */
+	TH_LOG("Closing uts ns - user ns should become inactive");
+	close(ut_fd);
+	u_fd = open_by_handle_at(FD_NSFS_ROOT, u_handle, O_RDONLY);
+	ASSERT_LT(u_fd, 0);
+}
+
+/*
+ * Test hierarchical propagation with deep namespace hierarchy.
+ * Create: init_user_ns -> user_A -> user_B -> net_ns
+ * When net_ns is active, both user_A and user_B should be active.
+ * This verifies the conditional recursion in __ns_ref_active_put() works.
+ */
+TEST(ns_deep_hierarchy_propagation)
+{
+	struct file_handle *ua_handle, *ub_handle, *net_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 ua_id, ub_id, net_id;
+	char ua_buf[sizeof(*ua_handle) + MAX_HANDLE_SZ];
+	char ub_buf[sizeof(*ub_handle) + MAX_HANDLE_SZ];
+	char net_buf[sizeof(*net_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		/* Create user_A -> user_B -> net hierarchy */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ua_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (ua_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ua_fd, NS_GET_ID, &ua_id) < 0) {
+			close(ua_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ua_fd);
+
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ub_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (ub_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ub_fd, NS_GET_ID, &ub_id) < 0) {
+			close(ub_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ub_fd);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int net_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (net_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(net_fd, NS_GET_ID, &net_id) < 0) {
+			close(net_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(net_fd);
+
+		/* Send all three namespace IDs */
+		write(pipefd[1], &ua_id, sizeof(ua_id));
+		write(pipefd[1], &ub_id, sizeof(ub_id));
+		write(pipefd[1], &net_id, sizeof(net_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &ua_id, sizeof(ua_id));
+	if (ret != sizeof(ua_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user_A namespace ID");
+	}
+
+	ret = read(pipefd[0], &ub_id, sizeof(ub_id));
+	if (ret != sizeof(ub_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user_B namespace ID");
+	}
+
+	ret = read(pipefd[0], &net_id, sizeof(net_id));
+	close(pipefd[0]);
+	if (ret != sizeof(net_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read network namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	ua_handle = (struct file_handle *)ua_buf;
+	ua_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	ua_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ua_fh = (struct nsfs_file_handle *)ua_handle->f_handle;
+	ua_fh->ns_id = ua_id;
+	ua_fh->ns_type = 0;
+	ua_fh->ns_inum = 0;
+
+	ub_handle = (struct file_handle *)ub_buf;
+	ub_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	ub_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ub_fh = (struct nsfs_file_handle *)ub_handle->f_handle;
+	ub_fh->ns_id = ub_id;
+	ub_fh->ns_type = 0;
+	ub_fh->ns_inum = 0;
+
+	net_handle = (struct file_handle *)net_buf;
+	net_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	net_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *net_fh = (struct nsfs_file_handle *)net_handle->f_handle;
+	net_fh->ns_id = net_id;
+	net_fh->ns_type = 0;
+	net_fh->ns_inum = 0;
+
+	/* Open net_ns before child exits to keep it active */
+	int net_fd = open_by_handle_at(FD_NSFS_ROOT, net_handle, O_RDONLY);
+	if (net_fd < 0) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open network namespace");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* With net_ns active, both user_A and user_B should be active */
+	TH_LOG("Testing user_B active (net_ns active causes propagation)");
+	int ub_fd = open_by_handle_at(FD_NSFS_ROOT, ub_handle, O_RDONLY);
+	ASSERT_GE(ub_fd, 0);
+
+	TH_LOG("Testing user_A active (propagated through user_B)");
+	int ua_fd = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_GE(ua_fd, 0);
+
+	/* Close net_ns - user_B should stay active (we hold direct ref) */
+	TH_LOG("Closing net_ns, user_B should remain active (direct ref held)");
+	close(net_fd);
+	int ub_fd2 = open_by_handle_at(FD_NSFS_ROOT, ub_handle, O_RDONLY);
+	ASSERT_GE(ub_fd2, 0);
+	close(ub_fd2);
+
+	/* Close user_B - user_A should stay active (we hold direct ref) */
+	TH_LOG("Closing user_B, user_A should remain active (direct ref held)");
+	close(ub_fd);
+	int ua_fd2 = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_GE(ua_fd2, 0);
+	close(ua_fd2);
+
+	/* Close user_A - everything should become inactive */
+	TH_LOG("Closing user_A, all should become inactive");
+	close(ua_fd);
+
+	/* All should now be inactive */
+	ua_fd = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_LT(ua_fd, 0);
+}
+
+/*
+ * Test that parent stays active as long as ANY child is active.
+ * Create parent user namespace with two child net namespaces.
+ * Parent should remain active until BOTH children are inactive.
+ */
+TEST(ns_parent_multiple_children_refcount)
+{
+	struct file_handle *parent_handle, *net1_handle, *net2_handle;
+	int ret, pipefd[2], syncpipe[2];
+	pid_t pid;
+	int status;
+	__u64 p_id, n1_id, n2_id;
+	char p_buf[sizeof(*parent_handle) + MAX_HANDLE_SZ];
+	char n1_buf[sizeof(*net1_handle) + MAX_HANDLE_SZ];
+	char n2_buf[sizeof(*net2_handle) + MAX_HANDLE_SZ];
+	char sync_byte;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	ASSERT_EQ(pipe(syncpipe), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+		close(syncpipe[1]);
+
+		/* Create parent user namespace */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int p_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (p_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(p_fd, NS_GET_ID, &p_id) < 0) {
+			close(p_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(p_fd);
+
+		/* Create first network namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		int n1_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (n1_fd < 0) {
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+		if (ioctl(n1_fd, NS_GET_ID, &n1_id) < 0) {
+			close(n1_fd);
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+		/* Keep n1_fd open so first namespace stays active */
+
+		/* Create second network namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(n1_fd);
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		int n2_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (n2_fd < 0) {
+			close(n1_fd);
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+		if (ioctl(n2_fd, NS_GET_ID, &n2_id) < 0) {
+			close(n1_fd);
+			close(n2_fd);
+			close(pipefd[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+		/* Keep both n1_fd and n2_fd open */
+
+		/* Send all namespace IDs */
+		write(pipefd[1], &p_id, sizeof(p_id));
+		write(pipefd[1], &n1_id, sizeof(n1_id));
+		write(pipefd[1], &n2_id, sizeof(n2_id));
+		close(pipefd[1]);
+
+		/* Wait for parent to signal before exiting */
+		read(syncpipe[0], &sync_byte, 1);
+		close(syncpipe[0]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+	close(syncpipe[0]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &p_id, sizeof(p_id));
+	if (ret != sizeof(p_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read parent namespace ID");
+	}
+
+	ret = read(pipefd[0], &n1_id, sizeof(n1_id));
+	if (ret != sizeof(n1_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read first network namespace ID");
+	}
+
+	ret = read(pipefd[0], &n2_id, sizeof(n2_id));
+	close(pipefd[0]);
+	if (ret != sizeof(n2_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read second network namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	parent_handle = (struct file_handle *)p_buf;
+	parent_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	parent_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *p_fh = (struct nsfs_file_handle *)parent_handle->f_handle;
+	p_fh->ns_id = p_id;
+	p_fh->ns_type = 0;
+	p_fh->ns_inum = 0;
+
+	net1_handle = (struct file_handle *)n1_buf;
+	net1_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	net1_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *n1_fh = (struct nsfs_file_handle *)net1_handle->f_handle;
+	n1_fh->ns_id = n1_id;
+	n1_fh->ns_type = 0;
+	n1_fh->ns_inum = 0;
+
+	net2_handle = (struct file_handle *)n2_buf;
+	net2_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	net2_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *n2_fh = (struct nsfs_file_handle *)net2_handle->f_handle;
+	n2_fh->ns_id = n2_id;
+	n2_fh->ns_type = 0;
+	n2_fh->ns_inum = 0;
+
+	/* Open both net namespaces while child is still alive */
+	int n1_fd = open_by_handle_at(FD_NSFS_ROOT, net1_handle, O_RDONLY);
+	int n2_fd = open_by_handle_at(FD_NSFS_ROOT, net2_handle, O_RDONLY);
+	if (n1_fd < 0 || n2_fd < 0) {
+		if (n1_fd >= 0) close(n1_fd);
+		if (n2_fd >= 0) close(n2_fd);
+		sync_byte = 'G';
+		write(syncpipe[1], &sync_byte, 1);
+		close(syncpipe[1]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open net namespaces");
+	}
+
+	/* Signal child that we have opened the namespaces */
+	sync_byte = 'G';
+	write(syncpipe[1], &sync_byte, 1);
+	close(syncpipe[1]);
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Parent should be active (has 2 active children) */
+	TH_LOG("Both net namespaces active - parent should be active");
+	int p_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_GE(p_fd, 0);
+	close(p_fd);
+
+	/* Close first net namespace - parent should STILL be active */
+	TH_LOG("Closing first net ns - parent should still be active");
+	close(n1_fd);
+	p_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_GE(p_fd, 0);
+	close(p_fd);
+
+	/* Close second net namespace - parent should become inactive */
+	TH_LOG("Closing second net ns - parent should become inactive");
+	close(n2_fd);
+	p_fd = open_by_handle_at(FD_NSFS_ROOT, parent_handle, O_RDONLY);
+	ASSERT_LT(p_fd, 0);
+}
+
+/*
+ * Test that user namespace as a child also propagates correctly.
+ * Create user_A -> user_B, verify when user_B is active that user_A
+ * is also active. This is different from non-user namespace children.
+ */
+TEST(ns_userns_child_propagation)
+{
+	struct file_handle *ua_handle, *ub_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 ua_id, ub_id;
+	char ua_buf[sizeof(*ua_handle) + MAX_HANDLE_SZ];
+	char ub_buf[sizeof(*ub_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		/* Create user_A */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ua_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (ua_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ua_fd, NS_GET_ID, &ua_id) < 0) {
+			close(ua_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ua_fd);
+
+		/* Create user_B (child of user_A) */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ub_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (ub_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ub_fd, NS_GET_ID, &ub_id) < 0) {
+			close(ub_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ub_fd);
+
+		/* Send both namespace IDs */
+		write(pipefd[1], &ua_id, sizeof(ua_id));
+		write(pipefd[1], &ub_id, sizeof(ub_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read both namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &ua_id, sizeof(ua_id));
+	if (ret != sizeof(ua_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user_A namespace ID");
+	}
+
+	ret = read(pipefd[0], &ub_id, sizeof(ub_id));
+	close(pipefd[0]);
+	if (ret != sizeof(ub_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user_B namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	ua_handle = (struct file_handle *)ua_buf;
+	ua_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	ua_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ua_fh = (struct nsfs_file_handle *)ua_handle->f_handle;
+	ua_fh->ns_id = ua_id;
+	ua_fh->ns_type = 0;
+	ua_fh->ns_inum = 0;
+
+	ub_handle = (struct file_handle *)ub_buf;
+	ub_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	ub_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ub_fh = (struct nsfs_file_handle *)ub_handle->f_handle;
+	ub_fh->ns_id = ub_id;
+	ub_fh->ns_type = 0;
+	ub_fh->ns_inum = 0;
+
+	/* Open user_B before child exits */
+	int ub_fd = open_by_handle_at(FD_NSFS_ROOT, ub_handle, O_RDONLY);
+	if (ub_fd < 0) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open user_B");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* With user_B active, user_A should also be active */
+	TH_LOG("Testing user_A active when child user_B is active");
+	int ua_fd = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_GE(ua_fd, 0);
+
+	/* Close user_B */
+	TH_LOG("Closing user_B");
+	close(ub_fd);
+
+	/* user_A should remain active (we hold direct ref) */
+	int ua_fd2 = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_GE(ua_fd2, 0);
+	close(ua_fd2);
+
+	/* Close user_A - should become inactive */
+	TH_LOG("Closing user_A - should become inactive");
+	close(ua_fd);
+
+	ua_fd = open_by_handle_at(FD_NSFS_ROOT, ua_handle, O_RDONLY);
+	ASSERT_LT(ua_fd, 0);
+}
+
+/*
+ * Test different namespace types (net, uts, ipc) all contributing
+ * active references to the same owning user namespace.
+ */
+TEST(ns_mixed_types_same_owner)
+{
+	struct file_handle *user_handle, *net_handle, *uts_handle;
+	int ret, pipefd[2];
+	pid_t pid;
+	int status;
+	__u64 u_id, n_id, ut_id;
+	char u_buf[sizeof(*user_handle) + MAX_HANDLE_SZ];
+	char n_buf[sizeof(*net_handle) + MAX_HANDLE_SZ];
+	char ut_buf[sizeof(*uts_handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(pipefd[0]);
+
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int u_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (u_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(u_fd, NS_GET_ID, &u_id) < 0) {
+			close(u_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(u_fd);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int n_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (n_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(n_fd, NS_GET_ID, &n_id) < 0) {
+			close(n_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(n_fd);
+
+		if (unshare(CLONE_NEWUTS) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		int ut_fd = open("/proc/self/ns/uts", O_RDONLY);
+		if (ut_fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+		if (ioctl(ut_fd, NS_GET_ID, &ut_id) < 0) {
+			close(ut_fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(ut_fd);
+
+		/* Send all namespace IDs */
+		write(pipefd[1], &u_id, sizeof(u_id));
+		write(pipefd[1], &n_id, sizeof(n_id));
+		write(pipefd[1], &ut_id, sizeof(ut_id));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	close(pipefd[1]);
+
+	/* Read all three namespace IDs - fixed size, no parsing needed */
+	ret = read(pipefd[0], &u_id, sizeof(u_id));
+	if (ret != sizeof(u_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user namespace ID");
+	}
+
+	ret = read(pipefd[0], &n_id, sizeof(n_id));
+	if (ret != sizeof(n_id)) {
+		close(pipefd[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read network namespace ID");
+	}
+
+	ret = read(pipefd[0], &ut_id, sizeof(ut_id));
+	close(pipefd[0]);
+	if (ret != sizeof(ut_id)) {
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read UTS namespace ID");
+	}
+
+	/* Construct file handles from namespace IDs */
+	user_handle = (struct file_handle *)u_buf;
+	user_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	user_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *u_fh = (struct nsfs_file_handle *)user_handle->f_handle;
+	u_fh->ns_id = u_id;
+	u_fh->ns_type = 0;
+	u_fh->ns_inum = 0;
+
+	net_handle = (struct file_handle *)n_buf;
+	net_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	net_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *n_fh = (struct nsfs_file_handle *)net_handle->f_handle;
+	n_fh->ns_id = n_id;
+	n_fh->ns_type = 0;
+	n_fh->ns_inum = 0;
+
+	uts_handle = (struct file_handle *)ut_buf;
+	uts_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	uts_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *ut_fh = (struct nsfs_file_handle *)uts_handle->f_handle;
+	ut_fh->ns_id = ut_id;
+	ut_fh->ns_type = 0;
+	ut_fh->ns_inum = 0;
+
+	/* Open both non-user namespaces */
+	int n_fd = open_by_handle_at(FD_NSFS_ROOT, net_handle, O_RDONLY);
+	int ut_fd = open_by_handle_at(FD_NSFS_ROOT, uts_handle, O_RDONLY);
+	if (n_fd < 0 || ut_fd < 0) {
+		if (n_fd >= 0) close(n_fd);
+		if (ut_fd >= 0) close(ut_fd);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to open namespaces");
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* User namespace should be active (2 active children) */
+	TH_LOG("Both net and uts active - user ns should be active");
+	int u_fd = open_by_handle_at(FD_NSFS_ROOT, user_handle, O_RDONLY);
+	ASSERT_GE(u_fd, 0);
+	close(u_fd);
+
+	/* Close net - user ns should STILL be active (uts still active) */
+	TH_LOG("Closing net - user ns should still be active");
+	close(n_fd);
+	u_fd = open_by_handle_at(FD_NSFS_ROOT, user_handle, O_RDONLY);
+	ASSERT_GE(u_fd, 0);
+	close(u_fd);
+
+	/* Close uts - user ns should become inactive */
+	TH_LOG("Closing uts - user ns should become inactive");
+	close(ut_fd);
+	u_fd = open_by_handle_at(FD_NSFS_ROOT, user_handle, O_RDONLY);
+	ASSERT_LT(u_fd, 0);
+}
+
+/* Thread test helpers and structures */
+struct thread_ns_info {
+	__u64 ns_id;
+	int pipefd;
+	int syncfd_read;
+	int syncfd_write;
+	int exit_code;
+};
+
+static void *thread_create_namespace(void *arg)
+{
+	struct thread_ns_info *info = (struct thread_ns_info *)arg;
+	int ret;
+
+	/* Create new network namespace */
+	ret = unshare(CLONE_NEWNET);
+	if (ret < 0) {
+		info->exit_code = 1;
+		return NULL;
+	}
+
+	/* Get namespace ID */
+	int fd = open("/proc/thread-self/ns/net", O_RDONLY);
+	if (fd < 0) {
+		info->exit_code = 2;
+		return NULL;
+	}
+
+	ret = ioctl(fd, NS_GET_ID, &info->ns_id);
+	close(fd);
+	if (ret < 0) {
+		info->exit_code = 3;
+		return NULL;
+	}
+
+	/* Send namespace ID to main thread */
+	if (write(info->pipefd, &info->ns_id, sizeof(info->ns_id)) != sizeof(info->ns_id)) {
+		info->exit_code = 4;
+		return NULL;
+	}
+
+	/* Wait for signal to exit */
+	char sync_byte;
+	if (read(info->syncfd_read, &sync_byte, 1) != 1) {
+		info->exit_code = 5;
+		return NULL;
+	}
+
+	info->exit_code = 0;
+	return NULL;
+}
+
+/*
+ * Test that namespace becomes inactive after thread exits.
+ * This verifies active reference counting works with threads, not just processes.
+ */
+TEST(thread_ns_inactive_after_exit)
+{
+	pthread_t thread;
+	struct thread_ns_info info;
+	struct file_handle *handle;
+	int pipefd[2];
+	int syncpipe[2];
+	int ret;
+	char sync_byte;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	ASSERT_EQ(pipe(syncpipe), 0);
+
+	info.pipefd = pipefd[1];
+	info.syncfd_read = syncpipe[0];
+	info.syncfd_write = -1;
+	info.exit_code = -1;
+
+	/* Create thread that will create a namespace */
+	ret = pthread_create(&thread, NULL, thread_create_namespace, &info);
+	ASSERT_EQ(ret, 0);
+
+	/* Read namespace ID from thread */
+	__u64 ns_id;
+	ret = read(pipefd[0], &ns_id, sizeof(ns_id));
+	if (ret != sizeof(ns_id)) {
+		sync_byte = 'X';
+		write(syncpipe[1], &sync_byte, 1);
+		pthread_join(thread, NULL);
+		close(pipefd[0]);
+		close(pipefd[1]);
+		close(syncpipe[0]);
+		close(syncpipe[1]);
+		SKIP(return, "Failed to read namespace ID from thread");
+	}
+
+	TH_LOG("Thread created namespace with ID %llu", (unsigned long long)ns_id);
+
+	/* Construct file handle */
+	handle = (struct file_handle *)buf;
+	handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *fh = (struct nsfs_file_handle *)handle->f_handle;
+	fh->ns_id = ns_id;
+	fh->ns_type = 0;
+	fh->ns_inum = 0;
+
+	/* Namespace should be active while thread is alive */
+	TH_LOG("Attempting to open namespace while thread is alive (should succeed)");
+	int nsfd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_GE(nsfd, 0);
+	close(nsfd);
+
+	/* Signal thread to exit */
+	TH_LOG("Signaling thread to exit");
+	sync_byte = 'X';
+	ASSERT_EQ(write(syncpipe[1], &sync_byte, 1), 1);
+	close(syncpipe[1]);
+
+	/* Wait for thread to exit */
+	ASSERT_EQ(pthread_join(thread, NULL), 0);
+	close(pipefd[0]);
+	close(pipefd[1]);
+	close(syncpipe[0]);
+
+	if (info.exit_code != 0)
+		SKIP(return, "Thread failed to create namespace");
+
+	TH_LOG("Thread exited, namespace should be inactive");
+
+	/* Namespace should now be inactive */
+	nsfd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(nsfd, 0);
+	/* Should fail with ENOENT (inactive) or ESTALE (gone) */
+	TH_LOG("Namespace inactive as expected: %s (errno=%d)", strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
+/*
+ * Test that a namespace remains active while a thread holds an fd to it.
+ * Even after the thread exits, the namespace should remain active as long as
+ * another thread holds a file descriptor to it.
+ */
+TEST(thread_ns_fd_keeps_active)
+{
+	pthread_t thread;
+	struct thread_ns_info info;
+	struct file_handle *handle;
+	int pipefd[2];
+	int syncpipe[2];
+	int ret;
+	char sync_byte;
+	char buf[sizeof(*handle) + MAX_HANDLE_SZ];
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	ASSERT_EQ(pipe(syncpipe), 0);
+
+	info.pipefd = pipefd[1];
+	info.syncfd_read = syncpipe[0];
+	info.syncfd_write = -1;
+	info.exit_code = -1;
+
+	/* Create thread that will create a namespace */
+	ret = pthread_create(&thread, NULL, thread_create_namespace, &info);
+	ASSERT_EQ(ret, 0);
+
+	/* Read namespace ID from thread */
+	__u64 ns_id;
+	ret = read(pipefd[0], &ns_id, sizeof(ns_id));
+	if (ret != sizeof(ns_id)) {
+		sync_byte = 'X';
+		write(syncpipe[1], &sync_byte, 1);
+		pthread_join(thread, NULL);
+		close(pipefd[0]);
+		close(pipefd[1]);
+		close(syncpipe[0]);
+		close(syncpipe[1]);
+		SKIP(return, "Failed to read namespace ID from thread");
+	}
+
+	TH_LOG("Thread created namespace with ID %llu", (unsigned long long)ns_id);
+
+	/* Construct file handle */
+	handle = (struct file_handle *)buf;
+	handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *fh = (struct nsfs_file_handle *)handle->f_handle;
+	fh->ns_id = ns_id;
+	fh->ns_type = 0;
+	fh->ns_inum = 0;
+
+	/* Open namespace while thread is alive */
+	TH_LOG("Opening namespace while thread is alive");
+	int nsfd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_GE(nsfd, 0);
+
+	/* Signal thread to exit */
+	TH_LOG("Signaling thread to exit");
+	sync_byte = 'X';
+	write(syncpipe[1], &sync_byte, 1);
+	close(syncpipe[1]);
+
+	/* Wait for thread to exit */
+	pthread_join(thread, NULL);
+	close(pipefd[0]);
+	close(pipefd[1]);
+	close(syncpipe[0]);
+
+	if (info.exit_code != 0) {
+		close(nsfd);
+		SKIP(return, "Thread failed to create namespace");
+	}
+
+	TH_LOG("Thread exited, but main thread holds fd - namespace should remain active");
+
+	/* Namespace should still be active because we hold an fd */
+	int nsfd2 = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_GE(nsfd2, 0);
+
+	/* Verify it's the same namespace */
+	struct stat st1, st2;
+	ASSERT_EQ(fstat(nsfd, &st1), 0);
+	ASSERT_EQ(fstat(nsfd2, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	close(nsfd2);
+
+	TH_LOG("Closing fd - namespace should become inactive");
+	close(nsfd);
+
+	/* Now namespace should be inactive */
+	nsfd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(nsfd, 0);
+	/* Should fail with ENOENT (inactive) or ESTALE (gone) */
+	TH_LOG("Namespace inactive as expected: %s (errno=%d)", strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+}
+
+/* Structure for thread data in subprocess */
+struct thread_sleep_data {
+	int syncfd_read;
+};
+
+static void *thread_sleep_and_wait(void *arg)
+{
+	struct thread_sleep_data *data = (struct thread_sleep_data *)arg;
+	char sync_byte;
+
+	/* Wait for signal to exit - read will unblock when pipe is closed */
+	(void)read(data->syncfd_read, &sync_byte, 1);
+	return NULL;
+}
+
+/*
+ * Test that namespaces become inactive after subprocess with multiple threads exits.
+ * Create a subprocess that unshares user and network namespaces, then creates two
+ * threads that share those namespaces. Verify that after all threads and subprocess
+ * exit, the namespaces are no longer listed by listns() and cannot be opened by
+ * open_by_handle_at().
+ */
+TEST(thread_subprocess_ns_inactive_after_all_exit)
+{
+	int pipefd[2];
+	int sv[2];
+	pid_t pid;
+	int status;
+	__u64 user_id, net_id;
+	struct file_handle *user_handle, *net_handle;
+	char user_buf[sizeof(*user_handle) + MAX_HANDLE_SZ];
+	char net_buf[sizeof(*net_handle) + MAX_HANDLE_SZ];
+	char sync_byte;
+	int ret;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child process */
+		close(pipefd[0]);
+		close(sv[0]);
+
+		/* Create user namespace with mappings */
+		if (setup_userns() < 0) {
+			fprintf(stderr, "Child: setup_userns() failed: %s\n", strerror(errno));
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+		fprintf(stderr, "Child: setup_userns() succeeded\n");
+
+		/* Get user namespace ID */
+		int user_fd = open("/proc/self/ns/user", O_RDONLY);
+		if (user_fd < 0) {
+			fprintf(stderr, "Child: open(/proc/self/ns/user) failed: %s\n", strerror(errno));
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+
+		if (ioctl(user_fd, NS_GET_ID, &user_id) < 0) {
+			fprintf(stderr, "Child: ioctl(NS_GET_ID) for user ns failed: %s\n", strerror(errno));
+			close(user_fd);
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+		close(user_fd);
+		fprintf(stderr, "Child: user ns ID = %llu\n", (unsigned long long)user_id);
+
+		/* Unshare network namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			fprintf(stderr, "Child: unshare(CLONE_NEWNET) failed: %s\n", strerror(errno));
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+		fprintf(stderr, "Child: unshare(CLONE_NEWNET) succeeded\n");
+
+		/* Get network namespace ID */
+		int net_fd = open("/proc/self/ns/net", O_RDONLY);
+		if (net_fd < 0) {
+			fprintf(stderr, "Child: open(/proc/self/ns/net) failed: %s\n", strerror(errno));
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+
+		if (ioctl(net_fd, NS_GET_ID, &net_id) < 0) {
+			fprintf(stderr, "Child: ioctl(NS_GET_ID) for net ns failed: %s\n", strerror(errno));
+			close(net_fd);
+			close(pipefd[1]);
+			close(sv[1]);
+			exit(1);
+		}
+		close(net_fd);
+		fprintf(stderr, "Child: net ns ID = %llu\n", (unsigned long long)net_id);
+
+		/* Send namespace IDs to parent */
+		if (write(pipefd[1], &user_id, sizeof(user_id)) != sizeof(user_id)) {
+			fprintf(stderr, "Child: write(user_id) failed: %s\n", strerror(errno));
+			exit(1);
+		}
+		if (write(pipefd[1], &net_id, sizeof(net_id)) != sizeof(net_id)) {
+			fprintf(stderr, "Child: write(net_id) failed: %s\n", strerror(errno));
+			exit(1);
+		}
+		close(pipefd[1]);
+		fprintf(stderr, "Child: sent namespace IDs to parent\n");
+
+		/* Create two threads that share the namespaces */
+		pthread_t thread1, thread2;
+		struct thread_sleep_data data;
+		data.syncfd_read = sv[1];
+
+		int ret_thread = pthread_create(&thread1, NULL, thread_sleep_and_wait, &data);
+		if (ret_thread != 0) {
+			fprintf(stderr, "Child: pthread_create(thread1) failed: %s\n", strerror(ret_thread));
+			close(sv[1]);
+			exit(1);
+		}
+		fprintf(stderr, "Child: created thread1\n");
+
+		ret_thread = pthread_create(&thread2, NULL, thread_sleep_and_wait, &data);
+		if (ret_thread != 0) {
+			fprintf(stderr, "Child: pthread_create(thread2) failed: %s\n", strerror(ret_thread));
+			close(sv[1]);
+			pthread_cancel(thread1);
+			exit(1);
+		}
+		fprintf(stderr, "Child: created thread2\n");
+
+		/* Wait for threads to complete - they will unblock when parent writes */
+		fprintf(stderr, "Child: waiting for threads to exit\n");
+		pthread_join(thread1, NULL);
+		fprintf(stderr, "Child: thread1 exited\n");
+		pthread_join(thread2, NULL);
+		fprintf(stderr, "Child: thread2 exited\n");
+
+		close(sv[1]);
+
+		/* Exit - namespaces should become inactive */
+		fprintf(stderr, "Child: all threads joined, exiting with success\n");
+		exit(0);
+	}
+
+	/* Parent process */
+	close(pipefd[1]);
+	close(sv[1]);
+
+	TH_LOG("Parent: waiting to read namespace IDs from child");
+
+	/* Read namespace IDs from child */
+	ret = read(pipefd[0], &user_id, sizeof(user_id));
+	if (ret != sizeof(user_id)) {
+		TH_LOG("Parent: failed to read user_id, ret=%d, errno=%s", ret, strerror(errno));
+		close(pipefd[0]);
+		sync_byte = 'X';
+		(void)write(sv[0], &sync_byte, 1);
+		close(sv[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read user namespace ID from child");
+	}
+
+	ret = read(pipefd[0], &net_id, sizeof(net_id));
+	close(pipefd[0]);
+	if (ret != sizeof(net_id)) {
+		TH_LOG("Parent: failed to read net_id, ret=%d, errno=%s", ret, strerror(errno));
+		sync_byte = 'X';
+		(void)write(sv[0], &sync_byte, 1);
+		close(sv[0]);
+		waitpid(pid, NULL, 0);
+		SKIP(return, "Failed to read network namespace ID from child");
+	}
+
+	TH_LOG("Child created user ns %llu and net ns %llu with 2 threads",
+	       (unsigned long long)user_id, (unsigned long long)net_id);
+
+	/* Construct file handles */
+	user_handle = (struct file_handle *)user_buf;
+	user_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	user_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *user_fh = (struct nsfs_file_handle *)user_handle->f_handle;
+	user_fh->ns_id = user_id;
+	user_fh->ns_type = 0;
+	user_fh->ns_inum = 0;
+
+	net_handle = (struct file_handle *)net_buf;
+	net_handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	net_handle->handle_type = FILEID_NSFS;
+	struct nsfs_file_handle *net_fh = (struct nsfs_file_handle *)net_handle->f_handle;
+	net_fh->ns_id = net_id;
+	net_fh->ns_type = 0;
+	net_fh->ns_inum = 0;
+
+	/* Verify namespaces are active while subprocess and threads are alive */
+	TH_LOG("Verifying namespaces are active while subprocess with threads is running");
+	int user_fd = open_by_handle_at(FD_NSFS_ROOT, user_handle, O_RDONLY);
+	ASSERT_GE(user_fd, 0);
+
+	int net_fd = open_by_handle_at(FD_NSFS_ROOT, net_handle, O_RDONLY);
+	ASSERT_GE(net_fd, 0);
+
+	close(user_fd);
+	close(net_fd);
+
+	/* Also verify they appear in listns() */
+	TH_LOG("Verifying namespaces appear in listns() while active");
+	struct ns_id_req req = {
+		.size = sizeof(struct ns_id_req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	int nr_ids = sys_listns(&req, ns_ids, 256, 0);
+	if (nr_ids < 0) {
+		TH_LOG("listns() not available, skipping listns verification");
+	} else {
+		/* Check if user_id is in the list */
+		int found_user = 0;
+		for (int i = 0; i < nr_ids; i++) {
+			if (ns_ids[i] == user_id) {
+				found_user = 1;
+				break;
+			}
+		}
+		ASSERT_TRUE(found_user);
+		TH_LOG("User namespace found in listns() as expected");
+
+		/* Check network namespace */
+		req.ns_type = CLONE_NEWNET;
+		nr_ids = sys_listns(&req, ns_ids, 256, 0);
+		if (nr_ids >= 0) {
+			int found_net = 0;
+			for (int i = 0; i < nr_ids; i++) {
+				if (ns_ids[i] == net_id) {
+					found_net = 1;
+					break;
+				}
+			}
+			ASSERT_TRUE(found_net);
+			TH_LOG("Network namespace found in listns() as expected");
+		}
+	}
+
+	/* Signal threads to exit */
+	TH_LOG("Signaling threads to exit");
+	sync_byte = 'X';
+	/* Write two bytes - one for each thread */
+	ASSERT_EQ(write(sv[0], &sync_byte, 1), 1);
+	ASSERT_EQ(write(sv[0], &sync_byte, 1), 1);
+	close(sv[0]);
+
+	/* Wait for child process to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	if (WEXITSTATUS(status) != 0) {
+		TH_LOG("Child process failed with exit code %d", WEXITSTATUS(status));
+		SKIP(return, "Child process failed");
+	}
+
+	TH_LOG("Subprocess and all threads have exited successfully");
+
+	/* Verify namespaces are now inactive - open_by_handle_at should fail */
+	TH_LOG("Verifying namespaces are inactive after subprocess and threads exit");
+	user_fd = open_by_handle_at(FD_NSFS_ROOT, user_handle, O_RDONLY);
+	ASSERT_LT(user_fd, 0);
+	TH_LOG("User namespace inactive as expected: %s (errno=%d)",
+	       strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+
+	net_fd = open_by_handle_at(FD_NSFS_ROOT, net_handle, O_RDONLY);
+	ASSERT_LT(net_fd, 0);
+	TH_LOG("Network namespace inactive as expected: %s (errno=%d)",
+	       strerror(errno), errno);
+	ASSERT_TRUE(errno == ENOENT || errno == ESTALE);
+
+	/* Verify namespaces do NOT appear in listns() */
+	TH_LOG("Verifying namespaces do NOT appear in listns() when inactive");
+	memset(&req, 0, sizeof(req));
+	req.size = sizeof(struct ns_id_req);
+	req.ns_type = CLONE_NEWUSER;
+	nr_ids = sys_listns(&req, ns_ids, 256, 0);
+	if (nr_ids >= 0) {
+		int found_user = 0;
+		for (int i = 0; i < nr_ids; i++) {
+			if (ns_ids[i] == user_id) {
+				found_user = 1;
+				break;
+			}
+		}
+		ASSERT_FALSE(found_user);
+		TH_LOG("User namespace correctly not listed in listns()");
+
+		/* Check network namespace */
+		req.ns_type = CLONE_NEWNET;
+		nr_ids = sys_listns(&req, ns_ids, 256, 0);
+		if (nr_ids >= 0) {
+			int found_net = 0;
+			for (int i = 0; i < nr_ids; i++) {
+				if (ns_ids[i] == net_id) {
+					found_net = 1;
+					break;
+				}
+			}
+			ASSERT_FALSE(found_net);
+			TH_LOG("Network namespace correctly not listed in listns()");
+		}
+	}
+}
+
+TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
new file mode 100644
index 000000000000..ba689a22d82f
--- /dev/null
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -0,0 +1,1824 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <linux/if.h>
+#include <linux/sockios.h>
+#include <linux/nsfs.h>
+#include <arpa/inet.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+#ifndef SIOCGSKNS
+#define SIOCGSKNS 0x894C
+#endif
+
+#ifndef FD_NSFS_ROOT
+#define FD_NSFS_ROOT -10003
+#endif
+
+#ifndef FILEID_NSFS
+#define FILEID_NSFS 0xf1
+#endif
+
+/*
+ * Test basic SIOCGSKNS functionality.
+ * Create a socket and verify SIOCGSKNS returns the correct network namespace.
+ */
+TEST(siocgskns_basic)
+{
+	int sock_fd, netns_fd, current_netns_fd;
+	struct stat st1, st2;
+
+	/* Create a TCP socket */
+	sock_fd = socket(AF_INET, SOCK_STREAM, 0);
+	ASSERT_GE(sock_fd, 0);
+
+	/* Use SIOCGSKNS to get network namespace */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Get current network namespace */
+	current_netns_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(current_netns_fd, 0);
+
+	/* Verify they match */
+	ASSERT_EQ(fstat(netns_fd, &st1), 0);
+	ASSERT_EQ(fstat(current_netns_fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+
+	close(sock_fd);
+	close(netns_fd);
+	close(current_netns_fd);
+}
+
+/*
+ * Test that socket file descriptors keep network namespaces active.
+ * Create a network namespace, create a socket in it, then exit the namespace.
+ * The namespace should remain active while the socket FD is held.
+ */
+TEST(siocgskns_keeps_netns_active)
+{
+	int sock_fd, netns_fd, test_fd;
+	int ipc_sockets[2];
+	pid_t pid;
+	int status;
+	struct stat st;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child: create new netns and socket */
+		close(ipc_sockets[0]);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			TH_LOG("unshare(CLONE_NEWNET) failed: %s", strerror(errno));
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Create a socket in the new network namespace */
+		sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+		if (sock_fd < 0) {
+			TH_LOG("socket() failed: %s", strerror(errno));
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Send socket FD to parent via SCM_RIGHTS */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent: receive socket FD */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	ASSERT_EQ(cmsg->cmsg_type, SCM_RIGHTS);
+
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	ASSERT_EQ(fstat(netns_fd, &st), 0);
+
+	/*
+	 * Namespace should still be active because socket FD keeps it alive.
+	 * Try to access it via /proc/self/fd/<fd>.
+	 */
+	char path[64];
+	snprintf(path, sizeof(path), "/proc/self/fd/%d", netns_fd);
+	test_fd = open(path, O_RDONLY);
+	ASSERT_GE(test_fd, 0);
+	close(test_fd);
+	close(netns_fd);
+
+	/* Close socket - namespace should become inactive */
+	close(sock_fd);
+
+	/* Try SIOCGSKNS again - should fail since socket is closed */
+	ASSERT_LT(ioctl(sock_fd, SIOCGSKNS), 0);
+}
+
+/*
+ * Test SIOCGSKNS with different socket types (TCP, UDP, RAW).
+ */
+TEST(siocgskns_socket_types)
+{
+	int sock_tcp, sock_udp, sock_raw;
+	int netns_tcp, netns_udp, netns_raw;
+	struct stat st_tcp, st_udp, st_raw;
+
+	/* TCP socket */
+	sock_tcp = socket(AF_INET, SOCK_STREAM, 0);
+	ASSERT_GE(sock_tcp, 0);
+
+	/* UDP socket */
+	sock_udp = socket(AF_INET, SOCK_DGRAM, 0);
+	ASSERT_GE(sock_udp, 0);
+
+	/* RAW socket (may require privileges) */
+	sock_raw = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
+	if (sock_raw < 0 && (errno == EPERM || errno == EACCES)) {
+		sock_raw = -1; /* Skip raw socket test */
+	}
+
+	/* Test SIOCGSKNS on TCP */
+	netns_tcp = ioctl(sock_tcp, SIOCGSKNS);
+	if (netns_tcp < 0) {
+		close(sock_tcp);
+		close(sock_udp);
+		if (sock_raw >= 0) close(sock_raw);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_tcp, 0);
+	}
+
+	/* Test SIOCGSKNS on UDP */
+	netns_udp = ioctl(sock_udp, SIOCGSKNS);
+	ASSERT_GE(netns_udp, 0);
+
+	/* Test SIOCGSKNS on RAW (if available) */
+	if (sock_raw >= 0) {
+		netns_raw = ioctl(sock_raw, SIOCGSKNS);
+		ASSERT_GE(netns_raw, 0);
+	}
+
+	/* Verify all return the same network namespace */
+	ASSERT_EQ(fstat(netns_tcp, &st_tcp), 0);
+	ASSERT_EQ(fstat(netns_udp, &st_udp), 0);
+	ASSERT_EQ(st_tcp.st_ino, st_udp.st_ino);
+
+	if (sock_raw >= 0) {
+		ASSERT_EQ(fstat(netns_raw, &st_raw), 0);
+		ASSERT_EQ(st_tcp.st_ino, st_raw.st_ino);
+		close(netns_raw);
+		close(sock_raw);
+	}
+
+	close(netns_tcp);
+	close(netns_udp);
+	close(sock_tcp);
+	close(sock_udp);
+}
+
+/*
+ * Test SIOCGSKNS across setns.
+ * Create a socket in netns A, switch to netns B, verify SIOCGSKNS still
+ * returns netns A.
+ */
+TEST(siocgskns_across_setns)
+{
+	int sock_fd, netns_a_fd, netns_b_fd, result_fd;
+	struct stat st_a;
+
+	/* Get current netns (A) */
+	netns_a_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(netns_a_fd, 0);
+	ASSERT_EQ(fstat(netns_a_fd, &st_a), 0);
+
+	/* Create socket in netns A */
+	sock_fd = socket(AF_INET, SOCK_STREAM, 0);
+	ASSERT_GE(sock_fd, 0);
+
+	/* Create new netns (B) */
+	ASSERT_EQ(unshare(CLONE_NEWNET), 0);
+
+	netns_b_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(netns_b_fd, 0);
+
+	/* Get netns from socket created in A */
+	result_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (result_fd < 0) {
+		close(sock_fd);
+		setns(netns_a_fd, CLONE_NEWNET);
+		close(netns_a_fd);
+		close(netns_b_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(result_fd, 0);
+	}
+
+	/* Verify it still points to netns A */
+	struct stat st_result_stat;
+	ASSERT_EQ(fstat(result_fd, &st_result_stat), 0);
+	ASSERT_EQ(st_a.st_ino, st_result_stat.st_ino);
+
+	close(result_fd);
+	close(sock_fd);
+	close(netns_b_fd);
+
+	/* Restore original netns */
+	ASSERT_EQ(setns(netns_a_fd, CLONE_NEWNET), 0);
+	close(netns_a_fd);
+}
+
+/*
+ * Test SIOCGSKNS fails on non-socket file descriptors.
+ */
+TEST(siocgskns_non_socket)
+{
+	int fd;
+	int pipefd[2];
+
+	/* Test on regular file */
+	fd = open("/dev/null", O_RDONLY);
+	ASSERT_GE(fd, 0);
+
+	ASSERT_LT(ioctl(fd, SIOCGSKNS), 0);
+	ASSERT_TRUE(errno == ENOTTY || errno == EINVAL);
+	close(fd);
+
+	/* Test on pipe */
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	ASSERT_LT(ioctl(pipefd[0], SIOCGSKNS), 0);
+	ASSERT_TRUE(errno == ENOTTY || errno == EINVAL);
+
+	close(pipefd[0]);
+	close(pipefd[1]);
+}
+
+/*
+ * Test multiple sockets keep the same network namespace active.
+ * Create multiple sockets, verify closing some doesn't affect others.
+ */
+TEST(siocgskns_multiple_sockets)
+{
+	int socks[5];
+	int netns_fds[5];
+	int i;
+	struct stat st;
+	ino_t netns_ino;
+
+	/* Create new network namespace */
+	ASSERT_EQ(unshare(CLONE_NEWNET), 0);
+
+	/* Create multiple sockets */
+	for (i = 0; i < 5; i++) {
+		socks[i] = socket(AF_INET, SOCK_STREAM, 0);
+		ASSERT_GE(socks[i], 0);
+	}
+
+	/* Get netns from all sockets */
+	for (i = 0; i < 5; i++) {
+		netns_fds[i] = ioctl(socks[i], SIOCGSKNS);
+		if (netns_fds[i] < 0) {
+			int j;
+			for (j = 0; j <= i; j++) {
+				close(socks[j]);
+				if (j < i && netns_fds[j] >= 0)
+					close(netns_fds[j]);
+			}
+			if (errno == ENOTTY || errno == EINVAL)
+				SKIP(return, "SIOCGSKNS not supported");
+			ASSERT_GE(netns_fds[i], 0);
+		}
+	}
+
+	/* Verify all point to same netns */
+	ASSERT_EQ(fstat(netns_fds[0], &st), 0);
+	netns_ino = st.st_ino;
+
+	for (i = 1; i < 5; i++) {
+		ASSERT_EQ(fstat(netns_fds[i], &st), 0);
+		ASSERT_EQ(st.st_ino, netns_ino);
+	}
+
+	/* Close some sockets */
+	for (i = 0; i < 3; i++) {
+		close(socks[i]);
+	}
+
+	/* Remaining netns FDs should still be valid */
+	for (i = 3; i < 5; i++) {
+		char path[64];
+		snprintf(path, sizeof(path), "/proc/self/fd/%d", netns_fds[i]);
+		int test_fd = open(path, O_RDONLY);
+		ASSERT_GE(test_fd, 0);
+		close(test_fd);
+	}
+
+	/* Cleanup */
+	for (i = 0; i < 5; i++) {
+		if (i >= 3)
+			close(socks[i]);
+		close(netns_fds[i]);
+	}
+}
+
+/*
+ * Test socket keeps netns active after creating process exits.
+ * Verify that as long as the socket FD exists, the namespace remains active.
+ */
+TEST(siocgskns_netns_lifecycle)
+{
+	int sock_fd, netns_fd;
+	int ipc_sockets[2];
+	int syncpipe[2];
+	pid_t pid;
+	int status;
+	char sync_byte;
+	struct stat st;
+	ino_t netns_ino;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	ASSERT_EQ(pipe(syncpipe), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child */
+		close(ipc_sockets[0]);
+		close(syncpipe[1]);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(ipc_sockets[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		sock_fd = socket(AF_INET, SOCK_STREAM, 0);
+		if (sock_fd < 0) {
+			close(ipc_sockets[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		/* Send socket to parent */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			close(syncpipe[0]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+
+		/* Wait for parent signal */
+		read(syncpipe[0], &sync_byte, 1);
+		close(syncpipe[0]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(ipc_sockets[1]);
+	close(syncpipe[0]);
+
+	/* Receive socket FD */
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Get netns from socket while child is alive */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		sync_byte = 'G';
+		write(syncpipe[1], &sync_byte, 1);
+		close(syncpipe[1]);
+		close(sock_fd);
+		waitpid(pid, NULL, 0);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+	ASSERT_EQ(fstat(netns_fd, &st), 0);
+	netns_ino = st.st_ino;
+
+	/* Signal child to exit */
+	sync_byte = 'G';
+	write(syncpipe[1], &sync_byte, 1);
+	close(syncpipe[1]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+
+	/*
+	 * Socket FD should still keep namespace active even after
+	 * the creating process exited.
+	 */
+	int test_fd = ioctl(sock_fd, SIOCGSKNS);
+	ASSERT_GE(test_fd, 0);
+
+	struct stat st_test;
+	ASSERT_EQ(fstat(test_fd, &st_test), 0);
+	ASSERT_EQ(st_test.st_ino, netns_ino);
+
+	close(test_fd);
+	close(netns_fd);
+
+	/* Close socket - namespace should become inactive */
+	close(sock_fd);
+}
+
+/*
+ * Test IPv6 sockets also work with SIOCGSKNS.
+ */
+TEST(siocgskns_ipv6)
+{
+	int sock_fd, netns_fd, current_netns_fd;
+	struct stat st1, st2;
+
+	/* Create an IPv6 TCP socket */
+	sock_fd = socket(AF_INET6, SOCK_STREAM, 0);
+	ASSERT_GE(sock_fd, 0);
+
+	/* Use SIOCGSKNS */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Verify it matches current namespace */
+	current_netns_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(current_netns_fd, 0);
+
+	ASSERT_EQ(fstat(netns_fd, &st1), 0);
+	ASSERT_EQ(fstat(current_netns_fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+
+	close(sock_fd);
+	close(netns_fd);
+	close(current_netns_fd);
+}
+
+/*
+ * Test that socket-kept netns appears in listns() output.
+ * Verify that a network namespace kept alive by a socket FD appears in
+ * listns() output even after the creating process exits, and that it
+ * disappears when the socket is closed.
+ */
+TEST(siocgskns_listns_visibility)
+{
+	int sock_fd, netns_fd, owner_fd;
+	int ipc_sockets[2];
+	pid_t pid;
+	int status;
+	__u64 netns_id, owner_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	int ret, i;
+	bool found_netns = false;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child: create new netns and socket */
+		close(ipc_sockets[0]);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+		if (sock_fd < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Send socket FD to parent via SCM_RIGHTS */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent: receive socket FD */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Get namespace ID */
+	ret = ioctl(netns_fd, NS_GET_ID, &netns_id);
+	if (ret < 0) {
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_ID not supported");
+		ASSERT_EQ(ret, 0);
+	}
+
+	/* Get owner user namespace */
+	owner_fd = ioctl(netns_fd, NS_GET_USERNS);
+	if (owner_fd < 0) {
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_USERNS not supported");
+		ASSERT_GE(owner_fd, 0);
+	}
+
+	/* Get owner namespace ID */
+	ret = ioctl(owner_fd, NS_GET_ID, &owner_id);
+	if (ret < 0) {
+		close(owner_fd);
+		close(sock_fd);
+		close(netns_fd);
+		ASSERT_EQ(ret, 0);
+	}
+	close(owner_fd);
+
+	/* Namespace should appear in listns() output */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s", strerror(errno));
+		ASSERT_GE(ret, 0);
+	}
+
+	/* Search for our network namespace in the list */
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id) {
+			found_netns = true;
+			break;
+		}
+	}
+
+	ASSERT_TRUE(found_netns);
+	TH_LOG("Found netns %llu in listns() output (kept alive by socket)", netns_id);
+
+	/* Now verify with owner filtering */
+	req.user_ns_id = owner_id;
+	found_netns = false;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id) {
+			found_netns = true;
+			break;
+		}
+	}
+
+	ASSERT_TRUE(found_netns);
+	TH_LOG("Found netns %llu owned by userns %llu", netns_id, owner_id);
+
+	/* Close socket - namespace should become inactive and disappear from listns() */
+	close(sock_fd);
+	close(netns_fd);
+
+	/* Verify it's no longer in listns() output */
+	req.user_ns_id = 0;
+	found_netns = false;
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id) {
+			found_netns = true;
+			break;
+		}
+	}
+
+	ASSERT_FALSE(found_netns);
+	TH_LOG("Netns %llu correctly disappeared from listns() after socket closed", netns_id);
+}
+
+/*
+ * Test that socket-kept netns can be reopened via file handle.
+ * Verify that a network namespace kept alive by a socket FD can be
+ * reopened using file handles even after the creating process exits.
+ */
+TEST(siocgskns_file_handle)
+{
+	int sock_fd, netns_fd, reopened_fd;
+	int ipc_sockets[2];
+	pid_t pid;
+	int status;
+	struct stat st1, st2;
+	ino_t netns_ino;
+	__u64 netns_id;
+	struct file_handle *handle;
+	struct nsfs_file_handle *nsfs_fh;
+	int ret;
+
+	/* Allocate file_handle structure for nsfs */
+	handle = malloc(sizeof(struct file_handle) + sizeof(struct nsfs_file_handle));
+	ASSERT_NE(handle, NULL);
+	handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	handle->handle_type = FILEID_NSFS;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child: create new netns and socket */
+		close(ipc_sockets[0]);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+		if (sock_fd < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Send socket FD to parent via SCM_RIGHTS */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent: receive socket FD */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	ASSERT_EQ(fstat(netns_fd, &st1), 0);
+	netns_ino = st1.st_ino;
+
+	/* Get namespace ID */
+	ret = ioctl(netns_fd, NS_GET_ID, &netns_id);
+	if (ret < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_ID not supported");
+		ASSERT_EQ(ret, 0);
+	}
+
+	/* Construct file handle from namespace ID */
+	nsfs_fh = (struct nsfs_file_handle *)handle->f_handle;
+	nsfs_fh->ns_id = netns_id;
+	nsfs_fh->ns_type = 0;  /* Type field not needed for reopening */
+	nsfs_fh->ns_inum = 0;  /* Inum field not needed for reopening */
+
+	TH_LOG("Constructed file handle for netns %lu (id=%llu)", netns_ino, netns_id);
+
+	/* Reopen namespace using file handle (while socket still keeps it alive) */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == EOPNOTSUPP || errno == ENOSYS || errno == EBADF)
+			SKIP(return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+		TH_LOG("open_by_handle_at failed: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	/* Verify it's the same namespace */
+	ASSERT_EQ(fstat(reopened_fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	TH_LOG("Successfully reopened netns %lu via file handle", netns_ino);
+
+	close(reopened_fd);
+
+	/* Close the netns FD */
+	close(netns_fd);
+
+	/* Try to reopen via file handle - should fail since namespace is now inactive */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(reopened_fd, 0);
+	TH_LOG("Correctly failed to reopen inactive netns: %s", strerror(errno));
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Reopen namespace using file handle (while socket still keeps it alive) */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == EOPNOTSUPP || errno == ENOSYS || errno == EBADF)
+			SKIP(return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+		TH_LOG("open_by_handle_at failed: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	/* Verify it's the same namespace */
+	ASSERT_EQ(fstat(reopened_fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	TH_LOG("Successfully reopened netns %lu via file handle", netns_ino);
+
+	/* Close socket - namespace should become inactive */
+	close(sock_fd);
+	free(handle);
+}
+
+/*
+ * Test combined listns() and file handle operations with socket-kept netns.
+ * Create a netns, keep it alive with a socket, verify it appears in listns(),
+ * then reopen it via file handle obtained from listns() entry.
+ */
+TEST(siocgskns_listns_and_file_handle)
+{
+	int sock_fd, netns_fd, userns_fd, reopened_fd;
+	int ipc_sockets[2];
+	pid_t pid;
+	int status;
+	struct stat st;
+	ino_t netns_ino;
+	__u64 netns_id, userns_id;
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET | CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	int ret, i;
+	bool found_netns = false, found_userns = false;
+	struct file_handle *handle;
+	struct nsfs_file_handle *nsfs_fh;
+
+	/* Allocate file_handle structure for nsfs */
+	handle = malloc(sizeof(struct file_handle) + sizeof(struct nsfs_file_handle));
+	ASSERT_NE(handle, NULL);
+	handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	handle->handle_type = FILEID_NSFS;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child: create new userns and netns with socket */
+		close(ipc_sockets[0]);
+
+		if (setup_userns() < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+		if (sock_fd < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Send socket FD to parent via SCM_RIGHTS */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent: receive socket FD */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	ASSERT_EQ(fstat(netns_fd, &st), 0);
+	netns_ino = st.st_ino;
+
+	/* Get namespace ID */
+	ret = ioctl(netns_fd, NS_GET_ID, &netns_id);
+	if (ret < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_ID not supported");
+		ASSERT_EQ(ret, 0);
+	}
+
+	/* Get owner user namespace */
+	userns_fd = ioctl(netns_fd, NS_GET_USERNS);
+	if (userns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_USERNS not supported");
+		ASSERT_GE(userns_fd, 0);
+	}
+
+	/* Get owner namespace ID */
+	ret = ioctl(userns_fd, NS_GET_ID, &userns_id);
+	if (ret < 0) {
+		close(userns_fd);
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		ASSERT_EQ(ret, 0);
+	}
+	close(userns_fd);
+
+	TH_LOG("Testing netns %lu (id=%llu) owned by userns id=%llu", netns_ino, netns_id, userns_id);
+
+	/* Verify namespace appears in listns() */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s", strerror(errno));
+		ASSERT_GE(ret, 0);
+	}
+
+	found_netns = false;
+	found_userns = false;
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id)
+			found_netns = true;
+		if (ns_ids[i] == userns_id)
+			found_userns = true;
+	}
+	ASSERT_TRUE(found_netns);
+	ASSERT_TRUE(found_userns);
+	TH_LOG("Found netns %llu in listns() output", netns_id);
+
+	/* Construct file handle from namespace ID */
+	nsfs_fh = (struct nsfs_file_handle *)handle->f_handle;
+	nsfs_fh->ns_id = netns_id;
+	nsfs_fh->ns_type = 0;
+	nsfs_fh->ns_inum = 0;
+
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == EOPNOTSUPP || errno == ENOSYS || errno == EBADF)
+			SKIP(return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+		TH_LOG("open_by_handle_at failed: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	struct stat reopened_st;
+	ASSERT_EQ(fstat(reopened_fd, &reopened_st), 0);
+	ASSERT_EQ(reopened_st.st_ino, netns_ino);
+
+	TH_LOG("Successfully reopened netns %lu via file handle (socket-kept)", netns_ino);
+
+	close(reopened_fd);
+	close(netns_fd);
+
+	/* Try to reopen via file handle - should fail since namespace is now inactive */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	ASSERT_LT(reopened_fd, 0);
+	TH_LOG("Correctly failed to reopen inactive netns: %s", strerror(errno));
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Verify namespace appears in listns() */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s", strerror(errno));
+		ASSERT_GE(ret, 0);
+	}
+
+	found_netns = false;
+	found_userns = false;
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id)
+			found_netns = true;
+		if (ns_ids[i] == userns_id)
+			found_userns = true;
+	}
+	ASSERT_TRUE(found_netns);
+	ASSERT_TRUE(found_userns);
+	TH_LOG("Found netns %llu in listns() output", netns_id);
+
+	close(netns_fd);
+
+	/* Verify namespace appears in listns() */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s", strerror(errno));
+		ASSERT_GE(ret, 0);
+	}
+
+	found_netns = false;
+	found_userns = false;
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_id)
+			found_netns = true;
+		if (ns_ids[i] == userns_id)
+			found_userns = true;
+	}
+	ASSERT_FALSE(found_netns);
+	ASSERT_FALSE(found_userns);
+	TH_LOG("Netns %llu correctly disappeared from listns() after socket closed", netns_id);
+
+	close(sock_fd);
+	free(handle);
+}
+
+/*
+ * Test multi-level namespace resurrection across three user namespace levels.
+ *
+ * This test creates a complex namespace hierarchy with three levels of user
+ * namespaces and a network namespace at the deepest level. It verifies that
+ * the resurrection semantics work correctly when SIOCGSKNS is called on a
+ * socket from an inactive namespace tree, and that listns() and
+ * open_by_handle_at() correctly respect visibility rules.
+ *
+ * Hierarchy after child processes exit (all with 0 active refcount):
+ *
+ *          net_L3A (0)                <- Level 3 network namespace
+ *              |
+ *              +
+ *          userns_L3 (0)              <- Level 3 user namespace
+ *              |
+ *              +
+ *          userns_L2 (0)              <- Level 2 user namespace
+ *              |
+ *              +
+ *          userns_L1 (0)              <- Level 1 user namespace
+ *              |
+ *              x
+ *          init_user_ns
+ *
+ * The test verifies:
+ * 1. SIOCGSKNS on a socket from inactive net_L3A resurrects the entire chain
+ * 2. After resurrection, all namespaces are visible in listns()
+ * 3. Resurrected namespaces can be reopened via file handles
+ * 4. Closing the netns FD cascades down: the entire ownership chain
+ *    (userns_L3 -> userns_L2 -> userns_L1) becomes inactive again
+ * 5. Inactive namespaces disappear from listns() and cannot be reopened
+ * 6. Calling SIOCGSKNS again on the same socket resurrects the tree again
+ * 7. After second resurrection, namespaces are visible and can be reopened
+ */
+TEST(siocgskns_multilevel_resurrection)
+{
+	int ipc_sockets[2];
+	pid_t pid_l1, pid_l2, pid_l3;
+	int status;
+
+	/* Namespace file descriptors to be received from child */
+	int sock_L3A_fd = -1;
+	int netns_L3A_fd = -1;
+	__u64 netns_L3A_id;
+	__u64 userns_L1_id, userns_L2_id, userns_L3_id;
+
+	/* For listns() and file handle testing */
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET | CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[256];
+	int ret, i;
+	struct file_handle *handle;
+	struct nsfs_file_handle *nsfs_fh;
+	int reopened_fd;
+
+	/* Allocate file handle for testing */
+	handle = malloc(sizeof(struct file_handle) + sizeof(struct nsfs_file_handle));
+	ASSERT_NE(handle, NULL);
+	handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	handle->handle_type = FILEID_NSFS;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	/*
+	 * Fork level 1 child that creates userns_L1
+	 */
+	pid_l1 = fork();
+	ASSERT_GE(pid_l1, 0);
+
+	if (pid_l1 == 0) {
+		/* Level 1 child */
+		int ipc_L2[2];
+		close(ipc_sockets[0]);
+
+		/* Create userns_L1 */
+		if (setup_userns() < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Create socketpair for communicating with L2 child */
+		if (socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_L2) < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/*
+		 * Fork level 2 child that creates userns_L2
+		 */
+		pid_l2 = fork();
+		if (pid_l2 < 0) {
+			close(ipc_sockets[1]);
+			close(ipc_L2[0]);
+			close(ipc_L2[1]);
+			exit(1);
+		}
+
+		if (pid_l2 == 0) {
+			/* Level 2 child */
+			int ipc_L3[2];
+			close(ipc_L2[0]);
+
+			/* Create userns_L2 (nested inside userns_L1) */
+			if (setup_userns() < 0) {
+				close(ipc_L2[1]);
+				exit(1);
+			}
+
+			/* Create socketpair for communicating with L3 child */
+			if (socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_L3) < 0) {
+				close(ipc_L2[1]);
+				exit(1);
+			}
+
+			/*
+			 * Fork level 3 child that creates userns_L3 and network namespaces
+			 */
+			pid_l3 = fork();
+			if (pid_l3 < 0) {
+				close(ipc_L2[1]);
+				close(ipc_L3[0]);
+				close(ipc_L3[1]);
+				exit(1);
+			}
+
+			if (pid_l3 == 0) {
+				/* Level 3 child - the deepest level */
+				int sock_fd;
+				close(ipc_L3[0]);
+
+				/* Create userns_L3 (nested inside userns_L2) */
+				if (setup_userns() < 0) {
+					close(ipc_L3[1]);
+					exit(1);
+				}
+
+				/* Create network namespace at level 3 */
+				if (unshare(CLONE_NEWNET) < 0) {
+					close(ipc_L3[1]);
+					exit(1);
+				}
+
+				/* Create socket in net_L3A */
+				sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+				if (sock_fd < 0) {
+					close(ipc_L3[1]);
+					exit(1);
+				}
+
+				/* Send socket FD to L2 parent */
+				struct msghdr msg = {0};
+				struct iovec iov = {0};
+				char buf[1] = {'X'};
+				char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+				iov.iov_base = buf;
+				iov.iov_len = 1;
+				msg.msg_iov = &iov;
+				msg.msg_iovlen = 1;
+				msg.msg_control = cmsg_buf;
+				msg.msg_controllen = sizeof(cmsg_buf);
+
+				struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+				cmsg->cmsg_level = SOL_SOCKET;
+				cmsg->cmsg_type = SCM_RIGHTS;
+				cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+				memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+				if (sendmsg(ipc_L3[1], &msg, 0) < 0) {
+					close(sock_fd);
+					close(ipc_L3[1]);
+					exit(1);
+				}
+
+				close(sock_fd);
+				close(ipc_L3[1]);
+				exit(0);
+			}
+
+			/* Level 2 child - receive from L3 and forward to L1 */
+			close(ipc_L3[1]);
+
+			struct msghdr msg = {0};
+			struct iovec iov = {0};
+			char buf[1];
+			char cmsg_buf[CMSG_SPACE(sizeof(int))];
+			int received_fd;
+
+			iov.iov_base = buf;
+			iov.iov_len = 1;
+			msg.msg_iov = &iov;
+			msg.msg_iovlen = 1;
+			msg.msg_control = cmsg_buf;
+			msg.msg_controllen = sizeof(cmsg_buf);
+
+			ssize_t n = recvmsg(ipc_L3[0], &msg, 0);
+			close(ipc_L3[0]);
+
+			if (n != 1) {
+				close(ipc_L2[1]);
+				waitpid(pid_l3, NULL, 0);
+				exit(1);
+			}
+
+			struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+			if (!cmsg) {
+				close(ipc_L2[1]);
+				waitpid(pid_l3, NULL, 0);
+				exit(1);
+			}
+			memcpy(&received_fd, CMSG_DATA(cmsg), sizeof(int));
+
+			/* Wait for L3 child */
+			waitpid(pid_l3, NULL, 0);
+
+			/* Forward the socket FD to L1 parent */
+			memset(&msg, 0, sizeof(msg));
+			buf[0] = 'Y';
+			iov.iov_base = buf;
+			iov.iov_len = 1;
+			msg.msg_iov = &iov;
+			msg.msg_iovlen = 1;
+			msg.msg_control = cmsg_buf;
+			msg.msg_controllen = sizeof(cmsg_buf);
+
+			cmsg = CMSG_FIRSTHDR(&msg);
+			cmsg->cmsg_level = SOL_SOCKET;
+			cmsg->cmsg_type = SCM_RIGHTS;
+			cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+			memcpy(CMSG_DATA(cmsg), &received_fd, sizeof(int));
+
+			if (sendmsg(ipc_L2[1], &msg, 0) < 0) {
+				close(received_fd);
+				close(ipc_L2[1]);
+				exit(1);
+			}
+
+			close(received_fd);
+			close(ipc_L2[1]);
+			exit(0);
+		}
+
+		/* Level 1 child - receive from L2 and forward to parent */
+		close(ipc_L2[1]);
+
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1];
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+		int received_fd;
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		ssize_t n = recvmsg(ipc_L2[0], &msg, 0);
+		close(ipc_L2[0]);
+
+		if (n != 1) {
+			close(ipc_sockets[1]);
+			waitpid(pid_l2, NULL, 0);
+			exit(1);
+		}
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		if (!cmsg) {
+			close(ipc_sockets[1]);
+			waitpid(pid_l2, NULL, 0);
+			exit(1);
+		}
+		memcpy(&received_fd, CMSG_DATA(cmsg), sizeof(int));
+
+		/* Wait for L2 child */
+		waitpid(pid_l2, NULL, 0);
+
+		/* Forward the socket FD to parent */
+		memset(&msg, 0, sizeof(msg));
+		buf[0] = 'Z';
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &received_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(received_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(received_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent - receive the socket from the deepest level */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+
+	if (n != 1) {
+		free(handle);
+		waitpid(pid_l1, NULL, 0);
+		SKIP(return, "Failed to receive socket from child");
+	}
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	if (!cmsg) {
+		free(handle);
+		waitpid(pid_l1, NULL, 0);
+		SKIP(return, "Failed to receive socket from child");
+	}
+	memcpy(&sock_L3A_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for L1 child */
+	waitpid(pid_l1, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/*
+	 * At this point, all child processes have exited. The socket itself
+	 * doesn't keep the namespace active - we need to call SIOCGSKNS which
+	 * will resurrect the entire namespace tree by taking active references.
+	 */
+
+	/* Get network namespace from socket - this resurrects the tree */
+	netns_L3A_fd = ioctl(sock_L3A_fd, SIOCGSKNS);
+	if (netns_L3A_fd < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_L3A_fd, 0);
+	}
+
+	/* Get namespace ID for net_L3A */
+	ret = ioctl(netns_L3A_fd, NS_GET_ID, &netns_L3A_id);
+	if (ret < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		close(netns_L3A_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_ID not supported");
+		ASSERT_EQ(ret, 0);
+	}
+
+	/* Get owner user namespace chain: userns_L3 -> userns_L2 -> userns_L1 */
+	int userns_L3_fd = ioctl(netns_L3A_fd, NS_GET_USERNS);
+	if (userns_L3_fd < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		close(netns_L3A_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_USERNS not supported");
+		ASSERT_GE(userns_L3_fd, 0);
+	}
+
+	ret = ioctl(userns_L3_fd, NS_GET_ID, &userns_L3_id);
+	ASSERT_EQ(ret, 0);
+
+	int userns_L2_fd = ioctl(userns_L3_fd, NS_GET_USERNS);
+	ASSERT_GE(userns_L2_fd, 0);
+	ret = ioctl(userns_L2_fd, NS_GET_ID, &userns_L2_id);
+	ASSERT_EQ(ret, 0);
+
+	int userns_L1_fd = ioctl(userns_L2_fd, NS_GET_USERNS);
+	ASSERT_GE(userns_L1_fd, 0);
+	ret = ioctl(userns_L1_fd, NS_GET_ID, &userns_L1_id);
+	ASSERT_EQ(ret, 0);
+
+	close(userns_L1_fd);
+	close(userns_L2_fd);
+	close(userns_L3_fd);
+
+	TH_LOG("Multi-level hierarchy: net_L3A (id=%llu) -> userns_L3 (id=%llu) -> userns_L2 (id=%llu) -> userns_L1 (id=%llu)",
+	       netns_L3A_id, userns_L3_id, userns_L2_id, userns_L1_id);
+
+	/*
+	 * Test 1: Verify net_L3A is visible in listns() after resurrection.
+	 * The entire ownership chain should be resurrected and visible.
+	 */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	if (ret < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		close(netns_L3A_fd);
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret, 0);
+	}
+
+	bool found_netns_L3A = false;
+	bool found_userns_L1 = false;
+	bool found_userns_L2 = false;
+	bool found_userns_L3 = false;
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_L3A_id)
+			found_netns_L3A = true;
+		if (ns_ids[i] == userns_L1_id)
+			found_userns_L1 = true;
+		if (ns_ids[i] == userns_L2_id)
+			found_userns_L2 = true;
+		if (ns_ids[i] == userns_L3_id)
+			found_userns_L3 = true;
+	}
+
+	ASSERT_TRUE(found_netns_L3A);
+	ASSERT_TRUE(found_userns_L1);
+	ASSERT_TRUE(found_userns_L2);
+	ASSERT_TRUE(found_userns_L3);
+	TH_LOG("Resurrection verified: all namespaces in hierarchy visible in listns()");
+
+	/*
+	 * Test 2: Verify net_L3A can be reopened via file handle.
+	 */
+	nsfs_fh = (struct nsfs_file_handle *)handle->f_handle;
+	nsfs_fh->ns_id = netns_L3A_id;
+	nsfs_fh->ns_type = 0;
+	nsfs_fh->ns_inum = 0;
+
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		close(netns_L3A_fd);
+		if (errno == EOPNOTSUPP || errno == ENOSYS || errno == EBADF)
+			SKIP(return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+		TH_LOG("open_by_handle_at failed: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	close(reopened_fd);
+	TH_LOG("File handle test passed: net_L3A can be reopened");
+
+	/*
+	 * Test 3: Verify that when we close the netns FD (dropping the last
+	 * active reference), the entire tree becomes inactive and disappears
+	 * from listns(). The cascade goes: net_L3A drops -> userns_L3 drops ->
+	 * userns_L2 drops -> userns_L1 drops.
+	 */
+	close(netns_L3A_fd);
+
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	found_netns_L3A = false;
+	found_userns_L1 = false;
+	found_userns_L2 = false;
+	found_userns_L3 = false;
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_L3A_id)
+			found_netns_L3A = true;
+		if (ns_ids[i] == userns_L1_id)
+			found_userns_L1 = true;
+		if (ns_ids[i] == userns_L2_id)
+			found_userns_L2 = true;
+		if (ns_ids[i] == userns_L3_id)
+			found_userns_L3 = true;
+	}
+
+	ASSERT_FALSE(found_netns_L3A);
+	ASSERT_FALSE(found_userns_L1);
+	ASSERT_FALSE(found_userns_L2);
+	ASSERT_FALSE(found_userns_L3);
+	TH_LOG("Cascade test passed: all namespaces disappeared after netns FD closed");
+
+	/*
+	 * Test 4: Verify file handle no longer works for inactive namespace.
+	 */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd >= 0) {
+		close(reopened_fd);
+		free(handle);
+		ASSERT_TRUE(false); /* Should have failed */
+	}
+	TH_LOG("Inactive namespace correctly cannot be reopened via file handle");
+
+	/*
+	 * Test 5: Verify that calling SIOCGSKNS again resurrects the tree again.
+	 * The socket is still valid, so we can call SIOCGSKNS on it to resurrect
+	 * the namespace tree once more.
+	 */
+	netns_L3A_fd = ioctl(sock_L3A_fd, SIOCGSKNS);
+	ASSERT_GE(netns_L3A_fd, 0);
+
+	TH_LOG("Called SIOCGSKNS again to resurrect the namespace tree");
+
+	/* Verify the namespace tree is resurrected and visible in listns() */
+	ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+	ASSERT_GE(ret, 0);
+
+	found_netns_L3A = false;
+	found_userns_L1 = false;
+	found_userns_L2 = false;
+	found_userns_L3 = false;
+
+	for (i = 0; i < ret; i++) {
+		if (ns_ids[i] == netns_L3A_id)
+			found_netns_L3A = true;
+		if (ns_ids[i] == userns_L1_id)
+			found_userns_L1 = true;
+		if (ns_ids[i] == userns_L2_id)
+			found_userns_L2 = true;
+		if (ns_ids[i] == userns_L3_id)
+			found_userns_L3 = true;
+	}
+
+	ASSERT_TRUE(found_netns_L3A);
+	ASSERT_TRUE(found_userns_L1);
+	ASSERT_TRUE(found_userns_L2);
+	ASSERT_TRUE(found_userns_L3);
+	TH_LOG("Second resurrection verified: all namespaces in hierarchy visible in listns() again");
+
+	/* Verify we can reopen via file handle again */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_L3A_fd);
+		close(netns_L3A_fd);
+		TH_LOG("open_by_handle_at failed after second resurrection: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	close(reopened_fd);
+	TH_LOG("File handle test passed: net_L3A can be reopened after second resurrection");
+
+	/* Final cleanup */
+	close(sock_L3A_fd);
+	close(netns_L3A_fd);
+	free(handle);
+}
+
+TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/namespaces/stress_test.c b/tools/testing/selftests/namespaces/stress_test.c
new file mode 100644
index 000000000000..dd7df7d6cb27
--- /dev/null
+++ b/tools/testing/selftests/namespaces/stress_test.c
@@ -0,0 +1,626 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <linux/nsfs.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Stress tests for namespace active reference counting.
+ *
+ * These tests validate that the active reference counting system can handle
+ * high load scenarios including rapid namespace creation/destruction, large
+ * numbers of concurrent namespaces, and various edge cases under stress.
+ */
+
+/*
+ * Test rapid creation and destruction of user namespaces.
+ * Create and destroy namespaces in quick succession to stress the
+ * active reference tracking and ensure no leaks occur.
+ */
+TEST(rapid_namespace_creation_destruction)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[256], ns_ids_after[256];
+	ssize_t ret_before, ret_after;
+	int i;
+
+	/* Get baseline count of active user namespaces */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active user namespaces", ret_before);
+
+	/* Rapidly create and destroy 100 user namespaces */
+	for (i = 0; i < 100; i++) {
+		pid_t pid = fork();
+		ASSERT_GE(pid, 0);
+
+		if (pid == 0) {
+			/* Child: create user namespace and immediately exit */
+			if (setup_userns() < 0)
+				exit(1);
+			exit(0);
+		}
+
+		/* Parent: wait for child */
+		int status;
+		waitpid(pid, &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+		ASSERT_EQ(WEXITSTATUS(status), 0);
+	}
+
+	/* Verify we're back to baseline (no leaked namespaces) */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 100 rapid create/destroy cycles: %zd active user namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test creating many concurrent namespaces.
+ * Verify that listns() correctly tracks all of them and that they all
+ * become inactive after processes exit.
+ */
+TEST(many_concurrent_namespaces)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_during[512], ns_ids_after[512];
+	ssize_t ret_before, ret_during, ret_after;
+	pid_t pids[50];
+	int num_children = 50;
+	int i;
+	int sv[2];
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active user namespaces", ret_before);
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	/* Create many children, each with their own user namespace */
+	for (i = 0; i < num_children; i++) {
+		pids[i] = fork();
+		ASSERT_GE(pids[i], 0);
+
+		if (pids[i] == 0) {
+			/* Child: create user namespace and wait for parent signal */
+			char c;
+
+			close(sv[0]);
+
+			if (setup_userns() < 0) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Signal parent we're ready */
+			if (write(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Wait for parent signal to exit */
+			if (read(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			close(sv[1]);
+			exit(0);
+		}
+	}
+
+	close(sv[1]);
+
+	/* Wait for all children to signal ready */
+	for (i = 0; i < num_children; i++) {
+		char c;
+		if (read(sv[0], &c, 1) != 1) {
+			/* If we fail to read, kill all children and exit */
+			close(sv[0]);
+			for (int j = 0; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	/* List namespaces while all children are running */
+	ret_during = sys_listns(&req, ns_ids_during, ARRAY_SIZE(ns_ids_during), 0);
+	ASSERT_GE(ret_during, 0);
+
+	TH_LOG("With %d children running: %zd active user namespaces", num_children, ret_during);
+
+	/* Should have at least num_children more namespaces than baseline */
+	ASSERT_GE(ret_during, ret_before + num_children);
+
+	/* Signal all children to exit */
+	for (i = 0; i < num_children; i++) {
+		char c = 'X';
+		if (write(sv[0], &c, 1) != 1) {
+			/* If we fail to write, kill remaining children */
+			close(sv[0]);
+			for (int j = i; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	close(sv[0]);
+
+	/* Wait for all children */
+	for (i = 0; i < num_children; i++) {
+		int status;
+		waitpid(pids[i], &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After all children exit: %zd active user namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test rapid namespace creation with different namespace types.
+ * Create multiple types of namespaces rapidly to stress the tracking system.
+ */
+TEST(rapid_mixed_namespace_creation)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,  /* All types */
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	int i;
+
+	/* Get baseline count */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active namespaces (all types)", ret_before);
+
+	/* Rapidly create and destroy namespaces with multiple types */
+	for (i = 0; i < 50; i++) {
+		pid_t pid = fork();
+		ASSERT_GE(pid, 0);
+
+		if (pid == 0) {
+			/* Child: create multiple namespace types */
+			if (setup_userns() < 0)
+				exit(1);
+
+			/* Create additional namespace types */
+			if (unshare(CLONE_NEWNET) < 0)
+				exit(1);
+			if (unshare(CLONE_NEWUTS) < 0)
+				exit(1);
+			if (unshare(CLONE_NEWIPC) < 0)
+				exit(1);
+
+			exit(0);
+		}
+
+		/* Parent: wait for child */
+		int status;
+		waitpid(pid, &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 50 rapid mixed namespace cycles: %zd active namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test nested namespace creation under stress.
+ * Create deeply nested namespace hierarchies and verify proper cleanup.
+ */
+TEST(nested_namespace_stress)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	int i;
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active user namespaces", ret_before);
+
+	/* Create 20 processes, each with nested user namespaces */
+	for (i = 0; i < 20; i++) {
+		pid_t pid = fork();
+		ASSERT_GE(pid, 0);
+
+		if (pid == 0) {
+			int userns_fd;
+			uid_t orig_uid = getuid();
+			int depth;
+
+			/* Create nested user namespaces (up to 5 levels) */
+			for (depth = 0; depth < 5; depth++) {
+				userns_fd = get_userns_fd(0, (depth == 0) ? orig_uid : 0, 1);
+				if (userns_fd < 0)
+					exit(1);
+
+				if (setns(userns_fd, CLONE_NEWUSER) < 0) {
+					close(userns_fd);
+					exit(1);
+				}
+				close(userns_fd);
+			}
+
+			exit(0);
+		}
+
+		/* Parent: wait for child */
+		int status;
+		waitpid(pid, &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 20 nested namespace hierarchies: %zd active user namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test listns() pagination under stress.
+ * Create many namespaces and verify pagination works correctly.
+ */
+TEST(listns_pagination_stress)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	pid_t pids[30];
+	int num_children = 30;
+	int i;
+	int sv[2];
+	__u64 all_ns_ids[512];
+	int total_found = 0;
+
+	ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sv), 0);
+
+	/* Create many children with user namespaces */
+	for (i = 0; i < num_children; i++) {
+		pids[i] = fork();
+		ASSERT_GE(pids[i], 0);
+
+		if (pids[i] == 0) {
+			char c;
+			close(sv[0]);
+
+			if (setup_userns() < 0) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Signal parent we're ready */
+			if (write(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			/* Wait for parent signal to exit */
+			if (read(sv[1], &c, 1) != 1) {
+				close(sv[1]);
+				exit(1);
+			}
+
+			close(sv[1]);
+			exit(0);
+		}
+	}
+
+	close(sv[1]);
+
+	/* Wait for all children to signal ready */
+	for (i = 0; i < num_children; i++) {
+		char c;
+		if (read(sv[0], &c, 1) != 1) {
+			/* If we fail to read, kill all children and exit */
+			close(sv[0]);
+			for (int j = 0; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	/* Paginate through all namespaces using small batch sizes */
+	req.ns_id = 0;
+	while (1) {
+		__u64 batch[5];  /* Small batch size to force pagination */
+		ssize_t ret;
+
+		ret = sys_listns(&req, batch, ARRAY_SIZE(batch), 0);
+		if (ret < 0) {
+			if (errno == ENOSYS) {
+				close(sv[0]);
+				for (i = 0; i < num_children; i++)
+					kill(pids[i], SIGKILL);
+				for (i = 0; i < num_children; i++)
+					waitpid(pids[i], NULL, 0);
+				SKIP(return, "listns() not supported");
+			}
+			ASSERT_GE(ret, 0);
+		}
+
+		if (ret == 0)
+			break;
+
+		/* Store results */
+		for (i = 0; i < ret && total_found < 512; i++) {
+			all_ns_ids[total_found++] = batch[i];
+		}
+
+		/* Update cursor for next batch */
+		if (ret == ARRAY_SIZE(batch))
+			req.ns_id = batch[ret - 1];
+		else
+			break;
+	}
+
+	TH_LOG("Paginated through %d user namespaces", total_found);
+
+	/* Verify no duplicates in pagination */
+	for (i = 0; i < total_found; i++) {
+		for (int j = i + 1; j < total_found; j++) {
+			if (all_ns_ids[i] == all_ns_ids[j]) {
+				TH_LOG("Found duplicate ns_id: %llu at positions %d and %d",
+				       (unsigned long long)all_ns_ids[i], i, j);
+				ASSERT_TRUE(false);
+			}
+		}
+	}
+
+	/* Signal all children to exit */
+	for (i = 0; i < num_children; i++) {
+		char c = 'X';
+		if (write(sv[0], &c, 1) != 1) {
+			close(sv[0]);
+			for (int j = i; j < num_children; j++)
+				kill(pids[j], SIGKILL);
+			for (int j = 0; j < num_children; j++)
+				waitpid(pids[j], NULL, 0);
+			ASSERT_TRUE(false);
+		}
+	}
+
+	close(sv[0]);
+
+	/* Wait for all children */
+	for (i = 0; i < num_children; i++) {
+		int status;
+		waitpid(pids[i], &status, 0);
+	}
+}
+
+/*
+ * Test concurrent namespace operations.
+ * Multiple processes creating, querying, and destroying namespaces concurrently.
+ */
+TEST(concurrent_namespace_operations)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	pid_t pids[20];
+	int num_workers = 20;
+	int i;
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active namespaces", ret_before);
+
+	/* Create worker processes that do concurrent operations */
+	for (i = 0; i < num_workers; i++) {
+		pids[i] = fork();
+		ASSERT_GE(pids[i], 0);
+
+		if (pids[i] == 0) {
+			/* Each worker: create namespaces, list them, repeat */
+			int iterations;
+
+			for (iterations = 0; iterations < 10; iterations++) {
+				int userns_fd;
+				__u64 temp_ns_ids[100];
+				ssize_t ret;
+
+				/* Create a user namespace */
+				userns_fd = get_userns_fd(0, getuid(), 1);
+				if (userns_fd < 0)
+					continue;
+
+				/* List namespaces */
+				ret = sys_listns(&req, temp_ns_ids, ARRAY_SIZE(temp_ns_ids), 0);
+				(void)ret;
+
+				close(userns_fd);
+
+				/* Small delay */
+				usleep(1000);
+			}
+
+			exit(0);
+		}
+	}
+
+	/* Wait for all workers */
+	for (i = 0; i < num_workers; i++) {
+		int status;
+		waitpid(pids[i], &status, 0);
+		ASSERT_TRUE(WIFEXITED(status));
+		ASSERT_EQ(WEXITSTATUS(status), 0);
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After concurrent operations: %zd active namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+/*
+ * Test namespace churn - continuous creation and destruction.
+ * Simulates high-churn scenarios like container orchestration.
+ */
+TEST(namespace_churn)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWUTS,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids_before[512], ns_ids_after[512];
+	ssize_t ret_before, ret_after;
+	int cycle;
+
+	/* Get baseline */
+	ret_before = sys_listns(&req, ns_ids_before, ARRAY_SIZE(ns_ids_before), 0);
+	if (ret_before < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		ASSERT_GE(ret_before, 0);
+	}
+
+	TH_LOG("Baseline: %zd active namespaces", ret_before);
+
+	/* Simulate churn: batches of namespaces created and destroyed */
+	for (cycle = 0; cycle < 10; cycle++) {
+		pid_t batch_pids[10];
+		int i;
+
+		/* Create batch */
+		for (i = 0; i < 10; i++) {
+			batch_pids[i] = fork();
+			ASSERT_GE(batch_pids[i], 0);
+
+			if (batch_pids[i] == 0) {
+				/* Create multiple namespace types */
+				if (setup_userns() < 0)
+					exit(1);
+				if (unshare(CLONE_NEWNET) < 0)
+					exit(1);
+				if (unshare(CLONE_NEWUTS) < 0)
+					exit(1);
+
+				/* Keep namespaces alive briefly */
+				usleep(10000);
+				exit(0);
+			}
+		}
+
+		/* Wait for batch to complete */
+		for (i = 0; i < 10; i++) {
+			int status;
+			waitpid(batch_pids[i], &status, 0);
+		}
+	}
+
+	/* Verify we're back to baseline */
+	ret_after = sys_listns(&req, ns_ids_after, ARRAY_SIZE(ns_ids_after), 0);
+	ASSERT_GE(ret_after, 0);
+
+	TH_LOG("After 10 churn cycles (100 namespace sets): %zd active namespaces", ret_after);
+	ASSERT_EQ(ret_before, ret_after);
+}
+
+TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/namespaces/wrappers.h b/tools/testing/selftests/namespaces/wrappers.h
new file mode 100644
index 000000000000..9741a64a5b1d
--- /dev/null
+++ b/tools/testing/selftests/namespaces/wrappers.h
@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/nsfs.h>
+#include <linux/types.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#ifndef __SELFTESTS_NAMESPACES_WRAPPERS_H__
+#define __SELFTESTS_NAMESPACES_WRAPPERS_H__
+
+#ifndef __NR_listns
+	#if defined __alpha__
+		#define __NR_listns 580
+	#elif defined _MIPS_SIM
+		#if _MIPS_SIM == _MIPS_SIM_ABI32	/* o32 */
+			#define __NR_listns 4470
+		#endif
+		#if _MIPS_SIM == _MIPS_SIM_NABI32	/* n32 */
+			#define __NR_listns 6470
+		#endif
+		#if _MIPS_SIM == _MIPS_SIM_ABI64	/* n64 */
+			#define __NR_listns 5470
+		#endif
+	#else
+		#define __NR_listns 470
+	#endif
+#endif
+
+static inline int sys_listns(const struct ns_id_req *req, __u64 *ns_ids,
+			     size_t nr_ns_ids, unsigned int flags)
+{
+	return syscall(__NR_listns, req, ns_ids, nr_ns_ids, flags);
+}
+
+#endif /* __SELFTESTS_NAMESPACES_WRAPPERS_H__ */