Re: [PATCH 5/7] overlay filesystem (inode.c bad error path)

From: Erez Zadok
Date: Fri May 20 2011 - 01:55:25 EST

On May 20, 2011, at 1:39 AM, Erez Zadok wrote:

> Miklos,
> I tried your overlayfs.v9 git repo w/ racer, using two separate ext3 filesystems (one for lowerdir and another for upperdir). I got the WARN_ON in ovl_permission to trigger within about 10 minutes of testing. Looking at the code, I see a problem in returning w/o cleaning up an dput-ing the alias dentry. Simple patch enclosed below.
> Cheers,
> Erez.

Niklos, I forgot to mention that I had to apply a small fix to a VFS bug in fs/namei.c:1362, where a mix of symlinks and renames, using racer, triggers an BUG_ON at the VFS layer (very reproducible). Without this fix, racer oopses in the VFS well before it gets to trigger overlayfs bugs. It's a pity 2.6.39 was released with this very reproducible *VFS* level bug (doesn't anyone run racer+fsx+ltp before releasing a new kernel?)

Anyway, here's the small patch here. Clearly viro/hch need to review this "fix" b/c I'm not sure it's really the right one


VFS: move BUG_ON test for symlink nd->depth after current->link_count test

This solves a bug in nested_symlink (which was rewritten from
do_follow_link), and follows the order of depth tests that existed before.
The bug triggers a BUG_ON in fs/namei.c:1346, when running racer with
symlink and rename ops.

Signed-off-by: Erez Zadok <ezk@xxxxxxxxxxxxx>
diff --git a/fs/namei.c b/fs/namei.c
index 017c3fa..7a93387 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1343,12 +1343,12 @@ static inline int nested_symlink(struct path *path, struct nameidata *nd)
int res;

- BUG_ON(nd->depth >= MAX_NESTED_LINKS);
if (unlikely(current->link_count >= MAX_NESTED_LINKS)) {
path_put_conditional(path, nd);
return -ELOOP;
+ BUG_ON(nd->depth >= MAX_NESTED_LINKS);


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