Re: [PATCH] reiserfs: Expand i_mutex to enclose lookup_one_len

[Al Viro <viro@ZenIV.linux.org.uk>]

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...