vfs: new internal helper: mnt_has_parent(mnt)

vfsmounts have ->mnt_parent pointing either to a different vfsmount
or to itself; it's never NULL and termination condition in loops
traversing the tree towards root is mnt == mnt->mnt_parent.  At least
one place (see the next patch) is confused about what's going on;
let's add an explicit helper checking it right way and use it in
all places where we need it.  Not that there had been too many,
but...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
(cherry picked from commit b2dba1af3c4157040303a76d25216b1713d333d0)
CVE-2014-7970
BugLink: http://bugs.launchpad.net/bugs/1383356
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>

Change-Id: Iaa5ab510804f3b17fe71197b8919d663a416bf05

1 files changed, 1 insertions, 1 deletions

diff --git a/fs/pnode.c b/fs/pnode.c
index d42514e..f1cd958 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -36,7 +36,7 @@ static inline struct vfsmount *next_slave(struct vfsmount *p)
 static bool is_path_reachable(struct vfsmount *mnt, struct dentry *dentry,
 			 const struct path *root)
 {
-	while (mnt != root->mnt && mnt->mnt_parent != mnt) {
+	while (mnt != root->mnt && mnt_has_parent(mnt)) {
 		dentry = mnt->mnt_mountpoint;
 		mnt = mnt->mnt_parent;
 	}