[PATCH] fs: fix is_mnt_ns_file()

[Miklos Szeredi <mszeredi@redhat.com>]

Commit 1fa08aece425 ("nsfs: convert to path_from_stashed() helper") reused
nsfs dentry's d_fsdata, which no longer contains a pointer to
proc_ns_operations.

Fix the remaining use in is_mnt_ns_file().

Fixes: 1fa08aece425 ("nsfs: convert to path_from_stashed() helper")
Cc: <stable@vger.kernel.org> # v6.9
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---

Came across this while getting the mnt_ns in fsnotify_mark(), tested the
fix in that context.  I don't have a test for mainline, though.

 fs/namespace.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 23e81c2a1e3f..6eec7794f707 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2055,9 +2055,15 @@ SYSCALL_DEFINE1(oldumount, char __user *, name)
 
 static bool is_mnt_ns_file(struct dentry *dentry)
 {
+	struct ns_common *ns;
+
 	/* Is this a proxy for a mount namespace? */
-	return dentry->d_op == &ns_dentry_operations &&
-	       dentry->d_fsdata == &mntns_operations;
+	if (dentry->d_op != &ns_dentry_operations)
+		return false;
+
+	ns = d_inode(dentry)->i_private;
+
+	return ns->ops == &mntns_operations;
 }
 
 struct ns_common *from_mnt_ns(struct mnt_namespace *mnt)
-- 
2.47.0