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

[Daniel J Blueman]

On Wed, Jun 18, 2008 at 1:07 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> On Tue, 10 Jun 2008 21:27:07 +0100
> "Daniel J Blueman" <daniel.blueman@xxxxxxxxx> wrote:
>
>> On Tue, Jun 10, 2008 at 8:18 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>> > On Tue, 10 Jun 2008 15:13:57 -0400
>> > Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>> >
>> >> On Tue, 10 Jun 2008 14:54:48 -0400
>> >> Trond Myklebust <trond.myklebust@xxxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxxx>
>> >> > >
>> >> > >
>> >> > > 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...
>
> I've tried reproducing this, but haven't had much success (probably some
> differences in my kernel config).
>
> I suspect that Trond is correct here and the race has something to
> do with the kthread being spawned but nfs_callback_svc() never getting
> a chance to run. I posted a patchset to the list late last week with the
> intro email:
>
> [PATCH 0/3] fix potential races in lockd and nfs4-callback startup/shutdown
>
> Dan, could you apply that patchset to your kernel and see if it helps
> this problem?

Yes, I confirm the problem reproduces without this patchset and I'm
unable to reproduce it with the patchset, so looks good!

Thanks again for the all the help and fixes!
  Dan
-- 
Daniel J Blueman
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