[PATCH] mnt: fix __detach_mounts infinite loop

[Benjamin Coddington <bcodding@redhat.com>]

Since commit ff17fa561a04 ("d_invalidate(): unhash immediately")
immediately unhashes the dentry, we'll never return the mountpoint in
lookup_mountpoint(), which can lead to an unbreakable loop in
d_invalidate().

I have reports of NFS clients getting into this condition after the server
removes an export of an existing mount created through follow_automount(),
but I suspect there are various other ways to produce this problem if we
hunt down users of d_invalidate().  For example, it is possible to get into
this state by using XFS' d_invalidate() call in xfs_vn_unlink():

truncate -s 100m img{1,2}

mkfs.xfs -q -n version=ci img1
mkfs.xfs -q -n version=ci img2

mkdir -p /mnt/xfs
mount img1 /mnt/xfs

mkdir /mnt/xfs/sub1
mount img2 /mnt/xfs/sub1

cat > /mnt/xfs/sub1/foo &
umount -l /mnt/xfs/sub1
mount img2 /mnt/xfs/sub1

mount --make-private /mnt/xfs

mkdir /mnt/xfs/sub2
mount --move /mnt/xfs/sub1 /mnt/xfs/sub2
rmdir /mnt/xfs/sub1

Fix this by moving the check for an unlinked dentry out of the
detach_mounts() path.

Fixes: ff17fa561a04 ("d_invalidate(): unhash immediately")
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
---
 fs/namespace.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index bd2f4c68506a..79cf26d9866a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -780,9 +780,6 @@ static struct mountpoint *lookup_mountpoint(struct dentry *dentry)
 
 	hlist_for_each_entry(mp, chain, m_hash) {
 		if (mp->m_dentry == dentry) {
-			/* might be worth a WARN_ON() */
-			if (d_unlinked(dentry))
-				return ERR_PTR(-ENOENT);
 			mp->m_count++;
 			return mp;
 		}
@@ -795,7 +792,11 @@ static struct mountpoint *get_mountpoint(struct dentry *dentry)
 	struct mountpoint *mp, *new = NULL;
 	int ret;
 
+
 	if (d_mountpoint(dentry)) {
+		/* might be worth a WARN_ON() */
+		if (d_unlinked(dentry))
+			return ERR_PTR(-ENOENT);
 mountpoint:
 		read_seqlock_excl(&mount_lock);
 		mp = lookup_mountpoint(dentry);
-- 
2.14.3