linux-kernel - Re: [2.6.26-rc4] mount.nfsv4/memory poisoning issues...

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Date:	Tue, 10 Jun 2008 16:13:52 -0400
From:	Jeff Layton <jlayton@...hat.com>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	Daniel J Blueman <daniel.blueman@...il.com>,
	linux-nfs@...r.kernel.org, nfsv4@...ux-nfs.org,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [2.6.26-rc4] mount.nfsv4/memory poisoning issues...

On Tue, 10 Jun 2008 15:58:29 -0400
Trond Myklebust <trond.myklebust@....uio.no> wrote:

> On Tue, 2008-06-10 at 15:13 -0400, Jeff Layton wrote:
> 
> > I think you're basically correct, but it looks to me like the
> > nfs_callback_mutex actually protects nfs_callback_info.task as well.
> > 
> > If we're starting the thread, then we can't call kthread_stop on it
> > until we release the mutex. So the thread can't exit until we release
> > the mutex, and we can be guaranteed that this:
> > 
> >      nfs_callback_info.task = NULL;
> > 
> > ...can't happen until after kthread_run returns and nfs_callback_up
> > sets it.
> > 
> > If that's right, then maybe this (untested, RFC only) patch would make sense?
> 
> Hmm... I suppose that is correct, but what if nfs_alloc_client() does
> 
> 	nfs_callback_up();
> 	<kstrdup() fails>
> 	nfs_callback_down();
> 
> AFAICS, if nfs_callback_down() gets called before the kthread() function
> gets scheduled back in, then you can get left with a value of
> nfs_callback_info.task != NULL, since nfs_callback_svc() will never be
> called.
> 

I don't see this race.

We can't call nfs_callback_down() until after nfs_callback_up()
returns, so we're guaranteed to have "task" set to a valid task
(presuming that nfs_callback_up() doesn't return error). We also can't
return from nfs_callback_down() until after the nfs_callback_svc() has
exited. kthread_stop() will block until it does.

> Wouldn't it therefore make more sense to clear nfs_callback_info.task in
> nfs_callback_down()?
> 

I suppose that makes just as much sense. It also seems more symmetrical
given that we also set the var in nfs_callback_up(). I'll roll that into
the BKL removal patch, and give it some testing. Look for it in a day
or two...

Thanks,
-- 
Jeff Layton <jlayton@...hat.com>
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