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

[Trond Myklebust <trond.myklebust@fys.uio.no>]

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@gmail.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@redhat.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?

Trond