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

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Message-ID: <1383059293.5464.31.camel@edumazet-glaptop.roam.corp.google.com>
Date:	Tue, 29 Oct 2013 08:08:13 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Christoph Paasch <christoph.paasch@...ouvain.be>,
	netdev <netdev@...r.kernel.org>
Subject: Re: Bug in skb_segment: fskb->len != len

On Tue, 2013-10-29 at 22:41 +0800, Herbert Xu wrote:
> On Mon, Oct 28, 2013 at 06:15:08PM -0700, Eric Dumazet wrote:
> > On Mon, 2013-10-28 at 06:21 -0700, Eric Dumazet wrote:
> > 
> > > But we also need to fix the skb_segment() bug anyway.
> > 
> > Hi Christoph
> > 
> > I cooked a minimal patch, could you please try it ?
> > 
> > I'll refactor skb_segment() to be smarter for the next release
> > (linux-3.14).
> 
> I think this patch is just papering over a deeper issue.
> 
> We should either be building skbs in pages, or using frag_list.
> In the latter case each frag_list must be exactly mss bytes,
> except for the last one.
> 
> So if we're crashing here it means that we got mixed up on the
> receive side, either because the driver was sending us bogus skbs
> or we're simply buggy.
> 
> So we need to figure out why the receive-side (i.e., GRO) is building
> these bogus packets, and not papering over them on the transmit-side.

It looks like you missed a lot of recent changes.

GRO layer was updated to be able to stack two or three sk_buff,
fully populated with page frags.

Thats quite mandatory to support line rate for 40Gb links.

We now have to make skb_segment() aware of this, I missed this part.



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