vfs: fix BUG_ON() in fs/namei.c:1461

When Al moved the nameidata_dentry_drop_rcu_maybe() call into the
do_follow_link function in commit 844a391799c2 ("nothing in
do_follow_link() is going to see RCU"), he mistakenly left the

	BUG_ON(inode != path->dentry->d_inode);

behind.  Which would otherwise be ok, but that BUG_ON() really needs to
be _after_ dropping RCU, since the dentry isn't necessarily stable
otherwise.

So complete the code movement in that commit, and move the BUG_ON() into
do_follow_link() too.  This means that we need to pass in 'inode' as an
argument (just for this one use), but that's a small thing.  And
eventually we may be confident enough in our path lookup that we can
just remove the BUG_ON() and the unnecessary inode argument.

Reported-and-tested-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 5 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 9e701e28a329..0087cf9c2c6b 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -795,7 +795,7 @@ __do_follow_link(const struct path *link, struct nameidata *nd, void **p)
  * Without that kind of total limit, nasty chains of consecutive
  * symlinks can cause almost arbitrarily long lookups. 
  */
-static inline int do_follow_link(struct path *path, struct nameidata *nd)
+static inline int do_follow_link(struct inode *inode, struct path *path, struct nameidata *nd)
 {
         void *cookie;
         int err = -ELOOP;
@@ -803,6 +803,7 @@ static inline int do_follow_link(struct path *path, struct nameidata *nd)
         /* We drop rcu-walk here */
         if (nameidata_dentry_drop_rcu_maybe(nd, path->dentry))
                 return -ECHILD;
+        BUG_ON(inode != path->dentry->d_inode);
 
         if (current->link_count >= MAX_NESTED_LINKS)
                 goto loop;
@@ -1413,8 +1414,7 @@ exec_again:
                         goto out_dput;
 
                 if (inode->i_op->follow_link) {
-                        BUG_ON(inode != next.dentry->d_inode);
-                        err = do_follow_link(&next, nd);
+                        err = do_follow_link(inode, &next, nd);
                         if (err)
                                 goto return_err;
                         nd->inode = nd->path.dentry->d_inode;
@@ -1458,8 +1458,7 @@ last_component:
                         break;
                 if (inode && unlikely(inode->i_op->follow_link) &&
                     (lookup_flags & LOOKUP_FOLLOW)) {
-                        BUG_ON(inode != next.dentry->d_inode);
-                        err = do_follow_link(&next, nd);
+                        err = do_follow_link(inode, &next, nd);
                         if (err)
                                 goto return_err;
                         nd->inode = nd->path.dentry->d_inode;