[added to the 4.1 stable tree] fs/pnode.c: treat zero mnt_group_id-s as unequal

[Sasha Levin <sasha.levin@oracle.com>]

From: Maxim Patlasov <mpatlasov@virtuozzo.com>

This patch has been added to the 4.1 stable tree. If you have any
objections, please let us know.

===============

[ Upstream commit 7ae8fd0351f912b075149a1e03a017be8b903b9a ]

propagate_one(m) calculates "type" argument for copy_tree() like this:

>    if (m->mnt_group_id == last_dest->mnt_group_id) {
>        type = CL_MAKE_SHARED;
>    } else {
>        type = CL_SLAVE;
>        if (IS_MNT_SHARED(m))
>           type |= CL_MAKE_SHARED;
>   }

The "type" argument then governs clone_mnt() behavior with respect to flags
and mnt_master of new mount. When we iterate through a slave group, it is
possible that both current "m" and "last_dest" are not shared (although,
both are slaves, i.e. have non-NULL mnt_master-s). Then the comparison
above erroneously makes new mount shared and sets its mnt_master to
last_source->mnt_master. The patch fixes the problem by handling zero
mnt_group_id-s as though they are unequal.

The similar problem exists in the implementation of "else" clause above
when we have to ascend upward in the master/slave tree by calling:

>    last_source = last_source->mnt_master;
>    last_dest = last_source->mnt_parent;

proper number of times. The last step is governed by
"n->mnt_group_id != last_dest->mnt_group_id" condition that may lie if
both are zero. The patch fixes this case in the same way as the former one.

[AV: don't open-code an obvious helper...]

Signed-off-by: Maxim Patlasov <mpatlasov@virtuozzo.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
---
 fs/pnode.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/pnode.c b/fs/pnode.c
index 6367e1e..c524fdd 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -202,6 +202,11 @@ static struct mount *last_dest, *last_source, *dest_master;
 static struct mountpoint *mp;
 static struct hlist_head *list;
 
+static inline bool peers(struct mount *m1, struct mount *m2)
+{
+	return m1->mnt_group_id == m2->mnt_group_id && m1->mnt_group_id;
+}
+
 static int propagate_one(struct mount *m)
 {
 	struct mount *child;
@@ -212,7 +217,7 @@ static int propagate_one(struct mount *m)
 	/* skip if mountpoint isn't covered by it */
 	if (!is_subdir(mp->m_dentry, m->mnt.mnt_root))
 		return 0;
-	if (m->mnt_group_id == last_dest->mnt_group_id) {
+	if (peers(m, last_dest)) {
 		type = CL_MAKE_SHARED;
 	} else {
 		struct mount *n, *p;
@@ -223,7 +228,7 @@ static int propagate_one(struct mount *m)
 					last_source = last_source->mnt_master;
 					last_dest = last_source->mnt_parent;
 				}
-				if (n->mnt_group_id != last_dest->mnt_group_id) {
+				if (!peers(n, last_dest)) {
 					last_source = last_source->mnt_master;
 					last_dest = last_source->mnt_parent;
 				}
-- 
2.5.0