should_follow_link(): validate ->d_seq after having decided to follow

... otherwise d_is_symlink() above might have nothing to do with
the inode value we've got.

Cc: stable@vger.kernel.org # v4.2+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index e0881c0..65a0e9d 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1712,6 +1712,11 @@ static inline int should_follow_link(struct nameidata *nd, struct path *link,
                return 0;
        if (!follow)
                return 0;
+       /* make sure that d_is_symlink above matches inode */
+       if (nd->flags & LOOKUP_RCU) {
+               if (read_seqcount_retry(&link->dentry->d_seq, seq))
+                       return -ECHILD;
+       }
        return pick_link(nd, link, inode, seq);
 }