fs: add assert for move_mount()

After we've attached a detached mount tree the anonymous mount namespace
must be empty. Add an assert and make this assumption explicit.

Link: https://lore.kernel.org/r/20250221-brauner-open_tree-v1-3-dbcfcb98c676@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/namespace.c b/fs/namespace.c
index 8ca2873..c9670a1 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3423,10 +3423,13 @@ static int do_move_mount(struct path *old_path, struct path *new_path,
 out:
        unlock_mount(mp);
        if (!err) {
-               if (attached)
+               if (attached) {
                        mntput_no_expire(parent);
-               else
+               } else {
+                       /* Make sure we notice when we leak mounts. */
+                       VFS_WARN_ON_ONCE(!mnt_ns_empty(ns));
                        free_mnt_ns(ns);
+               }
        }
        return err;
 }