finish_automount() prevents a mount trap from mounting a dentry onto
itself, as this could cause a loop - repeatedly automounting.  There is
nothing intrinsically wrong with this arrangement, and the d_automount
function can easily avoid the loop.  btrfs will use it to expose subvols
in the mount table.

It may well be a problem to mount a dentry onto itself when it is
already the root of the vfsmount, so narrow the test to only check that
case.

The test on mnt_sb is redundant and has been removed.  path->mnt and
path->dentry must have the same sb, so if m->mnt_root == dentry, then
m->mnt_sb must be the same as path->mnt->mnt_sb.

Signed-off-by: NeilBrown <redacted>
---
 fs/namespace.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index ab4174a3c802..81b0f2b2e701 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c

@@ -2928,7 +2928,7 @@ int finish_automount(struct vfsmount *m, struct path *path)
 	 */
 	BUG_ON(mnt_get_count(mnt) < 2);
 
-	if (m->mnt_sb == path->mnt->mnt_sb &&
+	if (m->mnt_root == path->mnt->mnt_root &&
 	    m->mnt_root == dentry) {
 		err = -ELOOP;
 		goto discard;