ns: don't add init_ipc_ns and init_net without namespace support

When CONFIG_IPC_NS or CONFIG_NET_NS are turned off we still add the
initial ipc and network namespace to the namespace trees. ipc_ns_init()
calls ns_tree_add(&init_ipc_ns) whenever CONFIG_SYSVIPC is set and
setup_net() calls ns_tree_add_raw() on init_net whenever CONFIG_NET is
set.

But without CONFIG_IPC_NS and CONFIG_NET_NS these aren't namespaces
userspace can do anything with. They don't have proc_ns_operations so
there's no /proc/<pid>/ns/{ipc,net} for them, nsfs_fh_to_dentry()
refuses them with -EOPNOTSUPP, and since ns->ops is NULL
__ns_tree_add_raw() doesn't add them to the owner list of init_user_ns
either. So they end up half registered. That's also not how we treat
any of the other namespace types. Without CONFIG_UTS_NS, CONFIG_PID_NS,
CONFIG_TIME_NS or CONFIG_USER_NS the initial namespace is never added
to the tree.

listns() walks the unified tree and calls is_current_namespace() for
every namespace it finds via may_list_ns(). But is_current_namespace()
only knows about CLONE_NEWIPC and CLONE_NEWNET if CONFIG_IPC_NS and
CONFIG_NET_NS are set. Otherwise we end up in the default case and its
VFS_WARN_ON_ONCE(). Any unprivileged caller can trigger that with a
plain listns() call:

  WARNING: CPU: 0 PID: 526 at fs/nsfs.c:508 is_current_namespace+0x12f/0x440
  Call Trace:
   legitimize_ns+0x25f/0x4c0
   do_listns+0x185/0x5c0
   __do_sys_listns+0x13b/0x300

Instead of teaching is_current_namespace() about namespaces that can't
be opened, only add the initial namespaces to the tree if their
namespace type is enabled. Move the registration of init_ipc_ns into
ipc/namespace.c which is only built with CONFIG_IPC_NS. That also fixes
CONFIG_SYSVIPC=n, CONFIG_POSIX_MQUEUE=y, CONFIG_IPC_NS=y where
ipc/shm.c isn't built and init_ipc_ns was never added at all. For the
network namespace only call ns_tree_add_raw() if CONFIG_NET_NS is set.
Without CONFIG_NET_NS cleanup_net() isn't built and init_net is never
removed so there's nothing to balance.

Fixes: 74b24a582e1f ("ipc: support ns lookup")
Fixes: 195f7422298d ("net: support ns lookup")
Reported-by: kernel test robot <yi1.lai@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202610260917.6248e316-lkp@intel.com
Cc: stable@vger.kernel.org # v6.18+
Link: https://patch.msgid.link/20261030-work-nsfs-init_ns_tree-v1-1-7ec0adb47c54@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions