netdev - Re: Bug in skb_segment: fskb->len != len

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Date:	Mon, 28 Oct 2013 14:28:00 +0100
From:	Christoph Paasch <christoph.paasch@...ouvain.be>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	netdev <netdev@...r.kernel.org>
Subject: Re: Bug in skb_segment: fskb->len != len

On 28/10/13 - 06:21:11, Eric Dumazet wrote:
> On Mon, 2013-10-28 at 12:55 +0100, Christoph Paasch wrote:
> > I have been seeing the below BUG in skb_segment with the latest net-next
> > head on my router.
> > 
> > I am forwarding Multipath TCP-traffic on this router. The MPTCP-sender is simply
> > doing an iperf-session. Strangely, I cannot reproduce the bug when sending
> > regular TCP-traffic across the router.
> > Note: The crash happens on a vanilla net-next kernel. It does not has any
> > MPTCP-code in it.
> > 
> > I bisected it down to 8a29111c7c (net: gro: allow to build full sized skb),
> > but I guess 8a29111c7c is just revealing a more fundamental bug in skb_segment.
> > 
> > Some info I found:
> > In skb_segment, when the bug happens, fskb->len is 4284 but the mss and len is 1428.
> 
> fskb seems to contain 3 segments -> 3*1428 = 4284, so it looks fine
> 
> But what do you mean by 'len is 1428' ?

I meant that the variable "len" equals 1428. And thus BUG_ON(fskb->len != len) triggers.

> > Shortly before the bug happens, skb_gro_receive is building a packet where
> > lp->len is equal to 4284 inside the frag_list.
> > 
> > 
> > Seems like skb_segment cannot handle those bigger skb's in the frag_list.
> > 
> 
> Thanks for the report, I'll take a look.
> 
> As mentioned earlier, building very large skbs (with a frag_list) for a
> router makes little sense, because we need to segment them before NIC
> ndo_start_xmit()
> 
> But we also need to fix the skb_segment() bug anyway.
> 
> Thanks !

Let me know if I should provide more info or test a patch.


Cheers,
Christoph

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