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

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Message-Id: <B0729E4C-D581-4EAB-A175-A6B8B34692B5@fsl.cs.sunysb.edu>
Date:	Fri, 20 May 2011 01:55:12 -0400
From:	Erez Zadok <ezk@....cs.sunysb.edu>
To:	Miklos Szeredi <miklos@...redi.hu>, viro@...IV.linux.org.uk
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, apw@...onical.com, nbd@...nwrt.org,
	neilb@...e.de, hramrach@...trum.cz, jordipujolp@...il.com,
	mszeredi@...e.cz, Erez Zadok <ezk@....cs.sunysb.edu>
Subject: Re: [PATCH 5/7] overlay filesystem (inode.c bad error path)

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

Cheers,
Erez.

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@...sunysb.edu>
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);
 		path_put(&nd->path);
 		return -ELOOP;
 	}
+	BUG_ON(nd->depth >= MAX_NESTED_LINKS);

 	nd->depth++;
 	current->link_count++;

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