[Devel] [PATCH RHEL7 COMMIT] ms/fs/pnode.c: treat zero mnt_group_id-s as unequal

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-327.4.5.vz7.11.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-327.4.5.vz7.11.4
------>
commit daea6fcefb376aee535e2a4b8e14a634820e353f
Author: Maxim Patlasov <mpatlasov at virtuozzo.com>
Date:   Wed Feb 17 12:28:18 2016 +0400

    ms/fs/pnode.c: treat zero mnt_group_id-s as unequal
    
    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.
    
    Mainstream thread: https://lkml.org/lkml/2016/2/16/779
    Reproducer: https://lkml.org/lkml/2016/2/17/47
    
    https://jira.sw.ru/browse/PSBM-43834
    
    Signed-off-by: Maxim Patlasov <mpatlasov at virtuozzo.com>
    Acked-by: Andrei Vagin <avagin at virtuozzo.com>
---
 fs/pnode.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/pnode.c b/fs/pnode.c
index 30d9a54..74f10c0 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -212,7 +212,7 @@ 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 (m->mnt_group_id && m->mnt_group_id == last_dest->mnt_group_id) {
 		type = CL_MAKE_SHARED;
 	} else {
 		struct mount *n, *p;
@@ -223,7 +223,9 @@ 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 (n->mnt_group_id != last_dest->mnt_group_id ||
+				    (!n->mnt_group_id &&
+				     !last_dest->mnt_group_id)) {
 					last_source = last_source->mnt_master;
 					last_dest = last_source->mnt_parent;
 				}