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

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Date:	Tue, 10 Jun 2008 21:27:07 +0100
From:	"Daniel J Blueman" <daniel.blueman@...il.com>
To:	"Jeff Layton" <jlayton@...hat.com>
Cc:	"Trond Myklebust" <trond.myklebust@....uio.no>,
	nfsv4@...ux-nfs.org, linux-nfs@...r.kernel.org,
	"Linux Kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [2.6.26-rc4] mount.nfsv4/memory poisoning issues...

On Tue, Jun 10, 2008 at 8:18 PM, Jeff Layton <jlayton@...hat.com> wrote:
> On Tue, 10 Jun 2008 15:13:57 -0400
> Jeff Layton <jlayton@...hat.com> wrote:
>
>> On Tue, 10 Jun 2008 14:54:48 -0400
>> Trond Myklebust <trond.myklebust@....uio.no> wrote:
>>
>> > On Wed, 2008-06-04 at 20:35 -0400, Jeff Layton wrote:
>> > > On Thu, 5 Jun 2008 00:33:54 +0100
>> > > "Daniel J Blueman" <daniel.blueman@...il.com> wrote:
>> > >
>> > > > Having experienced 'mount.nfs4: internal error' when mounting nfsv4 in
>> > > > the past, I have a minimal test-case I sometimes run:
>> > > >
>> > > > $ while :; do mount -t nfs4 filer:/store /store; umount /store; done
>> > > >
>> > > > After ~100 iterations, I saw the 'mount.nfs4: internal error',
>> > > > followed by symptoms of memory corruption [1], a locking issue with
>> > > > the reporting [2] and another (related?) memory-corruption issue
>> > > > (off-by-1?) [3]. A little analysis shows memory being overwritten by
>> > > > (likely) a poison value, which gets complicated if it's not
>> > > > use-after-free...
>> > > >
>> > > > Anyone dare confirm this issue? NFSv4 server is x86-64 Ubuntu 8.04
>> > > > 2.6.24-18, client U8.04 2.6.26-rc4; batteries included [4].
>> > > >
>> > > > I'm happy to decode addresses, test patches etc.
>> > > >
>> > > > Daniel
>> > > >
>> > >
>> > > Looks like it fell down while trying to take down the kthread during a
>> > > failed mount attempt. I have to wonder if I might have introduced a
>> > > race when I changed nfs4 callback thread to kthread API. I think we may
>> > > need the BKL around the last 2 statements in the main callback thread
>> > > function. If you can easily reproduce this, could you test the
>> > > following patch and let me know if it helps?
>> > >
>> > > Note that this patch is entirely untested, so test it someplace
>> > > non-critical ;-).
>> > >
>> > > Signed-off-by: Jeff Layton <jlayton@...hat.com>
>> > >
>> > >
>> > > diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
>> > > index c1e7c83..a3e83f9 100644
>> > > --- a/fs/nfs/callback.c
>> > > +++ b/fs/nfs/callback.c
>> > > @@ -90,9 +90,9 @@ nfs_callback_svc(void *vrqstp)
>> > >           preverr = err;
>> > >           svc_process(rqstp);
>> > >   }
>> > > - unlock_kernel();
>> > >   nfs_callback_info.task = NULL;
>> > >   svc_exit_thread(rqstp);
>> > > + unlock_kernel();
>> > >   return 0;
>> > >  }
>> >
>> > We certainly need to protect nfs_callback_info.task (and I believe I
>> > explained this earlier), but why do we need to protect svc_exit_thread?
>> >
>> > Also, looking at the general use of the BKL in that code, I thought we
>> > agreed that there was no need to hold the BKL while taking the
>> > nfs_callback_mutex?
>> >
>>
>> Hmm, I don't remember that discussion, but I'll take your word for it...
>>
>> 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?
>>
>
> To clarify for Dan...
>
> I don't think that this patch will help the problem you're having. This
> is essentially a cleanup patch to remove some locking that doesn't
> appear to be needed.
>
> The original patch that Trond commented on above is also probably
> unnecessary (assuming I'm right about the locking here).

Thanks for the head-up, Jeff. I took it at face value, so didn't
harbour the notion it would fix the memory corruption.

Let's see If I can get time for this git bisect sooner rather than later...
-- 
Daniel J Blueman
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