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

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Date:	Mon, 04 May 2009 00:51:20 -0400
From:	Jeff Mahoney <jeffm@...e.com>
To:	Al Viro <viro@...IV.linux.org.uk>
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

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Al Viro wrote:
> On Sun, May 03, 2009 at 09:52:36AM +0100, Al Viro wrote:
>> 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?
> 
> BTW, what in the name of everything unholy is ->xattr_root?  Never
> assigned a non-NULL value...

Huh. I didn't see that still in there. That's an artifact of an earlier
version of the code where I kept a reference to /.reiserfs_priv/xattrs.
Then I decided that .reiserfs_priv was all I needed to cache (to avoid
recursive i_mutex locking on the fs root) and dropped the caching of
xattrs. Looks like it didn't get totally cleared out.

- -Jeff
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