Rename is_initial_namespace() to ns_init_inum() and make it symmetrical
with the ns id variant.

Link: https://patch.msgid.link/20251110-work-namespace-nstree-fixes-v1-9-e8a9264e0fb9@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Switch the nstree management to the new combined structures.

Link: https://patch.msgid.link/20251110-work-namespace-nstree-fixes-v1-5-e8a9264e0fb9@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Add a few more assert to detect active reference count underflows.

Link: https://patch.msgid.link/20251109-namespace-6-19-fixes-v1-6-ae8a4ad5a3b3@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

The setns() system call supports:

(1) namespace file descriptors (nsfd)
(2) process file descriptors (pidfd)

When using nsfds the namespaces will remain active because they are
pinned by the vfs. However, when pidfds are used things are more
complicated.

When the target task exits and passes through exit_nsproxy_namespaces()
or is reaped and thus also passes through exit_cred_namespaces() after
the setns()'ing task has called prepare_nsset() but before the active
reference count of the set of namespaces it wants to setns() to might
have been dropped already:

  P1                                                              P2

  pid_p1 = clone(CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWNS)
                                                                  pidfd = pidfd_open(pid_p1)
                                                                  setns(pidfd, CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWNS)
                                                                  prepare_nsset()

  exit(0)
  // ns->__ns_active_ref        == 1
  // parent_ns->__ns_active_ref == 1
  -> exit_nsproxy_namespaces()
  -> exit_cred_namespaces()

  // ns_active_ref_put() will also put
  // the reference on the owner of the
  // namespace. If the only reason the
  // owning namespace was alive was
  // because it was a parent of @ns
  // it's active reference count now goes
  // to zero... --------------------------------
  //                                           |
  // ns->__ns_active_ref        == 0           |
  // parent_ns->__ns_active_ref == 0           |
                                               |                  commit_nsset()
                                               -----------------> // If setns()
                                                                  // now manages to install the namespaces
                                                                  // it will call ns_active_ref_get()
                                                                  // on them thus bumping the active reference
                                                                  // count from zero again but without also
                                                                  // taking the required reference on the owner.
                                                                  // Thus we get:
                                                                  //
                                                                  // ns->__ns_active_ref        == 1
                                                                  // parent_ns->__ns_active_ref == 0

  When later someone does ns_active_ref_put() on @ns it will underflow
  parent_ns->__ns_active_ref leading to a splat from our asserts
  thinking there are still active references when in fact the counter
  just underflowed.

So resurrect the ownership chain if necessary as well. If the caller
succeeded to grab passive references to the set of namespaces the
setns() should simply succeed even if the target task exists or gets
reaped in the meantime and thus has dropped all active references to its
namespaces.

The race is rare and can only be triggered when using pidfs to setns()
to namespaces. Also note that active reference on initial namespaces are
nops.

Since we now always handle parent references directly we can drop
ns_ref_active_get_owner() when adding a namespace to a namespace tree.
This is now all handled uniformly in the places where the new namespaces
actually become active.

Link: https://patch.msgid.link/20251109-namespace-6-19-fixes-v1-5-ae8a4ad5a3b3@kernel.org
Fixes: 3c9820d5c64a ("ns: add active reference count")
Reported-by: syzbot+1957b26299cf3ff7890c@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

There's no need to bump the active reference counts of initial
namespaces as they're always active and can simply remain at 1.

Link: https://patch.msgid.link/20251109-namespace-6-19-fixes-v1-2-ae8a4ad5a3b3@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Don't skip active reference count initialization for initial namespaces.
Doing this will break network namespace active reference counting.

Link: https://patch.msgid.link/20251109-namespace-6-19-fixes-v1-1-ae8a4ad5a3b3@kernel.org
Fixes: 3a18f809184b ("ns: add active reference count")
Signed-off-by: Christian Brauner <brauner@kernel.org>

Add 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);
}

Permission Model

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()

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-19-2e6f823ebdc0@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Allow to walk the unified namespace list completely locklessly.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-18-2e6f823ebdc0@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

The namespace tree doesn't express the ownership concept of namespace
appropriately. Maintain a list of directly owned namespaces per user
namespace. This will allow userspace and the kernel to use the listns()
system call to walk the namespace tree by owning user namespace. The
rbtree is used to find the relevant namespace entry point which allows
to continue iteration and the owner list can be used to walk the tree
completely lock free.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-16-2e6f823ebdc0@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

This will allow userspace to lookup and stat a namespace simply by its
identifier without having to know what type of namespace it is.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-13-2e6f823ebdc0@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>