netdev - Re: [Bug #16626] Machine hangs with EIP at skb_copy_and_csum

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Date:	Wed, 1 Sep 2010 17:05:04 +0200
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Plamen Petrov <pvp-lsts@...uni-ruse.bg>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org
Subject: Re: [Bug #16626] Machine hangs with EIP at skb_copy_and_csum_dev

On Wed, Sep 01, 2010 at 03:57:41PM +0200, Eric Dumazet wrote:
> Le mercredi 01 septembre 2010 ?? 11:20 +0000, Jarek Poplawski a écrit :
> > On Wed, Sep 01, 2010 at 12:50:51PM +0200, Eric Dumazet wrote:
> > > Plamen, could you test following patch ?
> > > 
> > > I reproduced problem on a dev machine and following patch cured it.
> > > 
> > > Thanks
> > > 
> > > [PATCH] gro: fix different skb headrooms
> > > 
> > > packets entering GRO might have different headrooms, even for a given
> > > flow (because of implementation details in drivers, like copybreak).
> > > We cant force drivers to deliver packets with a fixed headroom.
> > > 
> > > 1) fix skb_segment()
> > > 
> > > skb_segment() makes the false assumption headrooms of fragments are same
> > > than the head. When CHECKSUM_PARTIAL is used, this can give csum_start
> > > errors, and crash later in skb_copy_and_csum_dev()
> > 
> > Eric, probably I missed something, but since the same test as in
> > skb_copy_and_csum_dev() gave different result a bit earlier on exactly
> > the same skb, I've suspected some sharing (or use after free)
> > problems, so I'm not sure your current diagnose can explain this.
> > (Unless this old test was dismissed later.)
> 
> Oh, this is because your patch had an error for the gso part that read :
> 
> -               rc = ops->ndo_start_xmit(nskb, dev);
> +               if (skb_csum_start_bug(skb, 50)) {
> +                       kfree_skb(skb);
> +                       rc = NETDEV_TX_OK;
> +               } else
> +                       rc = ops->ndo_start_xmit(nskb, dev);
> +
>                 if (unlikely(rc != NETDEV_TX_OK)) {
>                         if (rc & ~NETDEV_TX_MASK)
>                                 goto out_kfree_gso_skb;
> 
> You called skb_csum_start_bug(skb, 50) instead of
> skb_csum_start_bug(nskb, 50)
> 
> Hope this clarify a bit ;)

All clear! Sorry for the false alarm!

Thanks,
Jarek P.
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