linux-kernel - Re: [PATCH] reiserfs: Expand i_mutex to enclose lookup_one

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Message-ID: <20090503085236.GT8633@ZenIV.linux.org.uk>
Date:	Sun, 3 May 2009 09:52:36 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Jeff Mahoney <jeffm@...e.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ReiserFS Mailing List <reiserfs-devel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@....linux.org.uk>,
	Alexander Beregalov <a.beregalov@...il.com>,
	David <david@...olicited.net>
Subject: Re: [PATCH] reiserfs: Expand i_mutex to enclose lookup_one_len

On Fri, May 01, 2009 at 12:11:12PM -0400, Jeff Mahoney wrote:
>  2.6.30-rc3 introduced some sanity checks in the VFS code to avoid NFS
>  bugs by ensuring that lookup_one_len is always called under i_mutex.
> 
>  This patch expands the i_mutex locking to enclose lookup_one_len. This was
>  always required, but not not enforced in the reiserfs code since it
>  does locking around the xattr interactions with the xattr_sem.
> 
>  This is obvious enough, but it survived an overnight 50 thread ACL test.

It's not enough, unfortunately ;-/  It deals with the warning, but it
leaves an actual hole in there.

Look: what happens if we mount it r/o without that directory and then
remount r/w?  We get dentry for privroot, hash it (negative at that point),
then do actual mkdir, unlock root and modify the ->d_compare() of root
to reject lookups on that sucker.  Too late - in the meanwhile lookups
might very well come and find privroot in dcache.

BTW, the way ->d_compare() is done in there is rather dumb -
	if (q1 == &priv_root->d_name)
		return -ENOENT;
	...
would do just as well.  Why don't we do that lookup *once* (on ->get_sb(),
before anything can come and race with us), and then just keep negative
dentry if the directory hadn't been around?  And set d_compare() for root
immediately after that lookup...

I've applied your patch as-is, and unless you have objections to the
variant above I'll do that as incremental.  Comments?
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