netdev - Re: [PATCH net] tcp: Don't collapse if resulting skb could overflow skb->csum

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Date:	Thu, 28 Feb 2013 19:23:51 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, foraker1@...l.gov
Subject: Re: [PATCH net] tcp: Don't collapse if resulting skb could overflow
 skb->csum_start

On 02/28/13 at 09:40am, Eric Dumazet wrote:
> On Thu, 2013-02-28 at 09:35 -0800, Eric Dumazet wrote:
> > This had a followup with 22b4a4f22da4
> > (tcp: fix retransmit of partially acked frames)
> > 
> > I also wonder if there is not another similar potential problem in
> > __tcp_retransmit_skb) after call to 
> > 
> > tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq)
> > 
> > csum_start can overflow again because of MAX_TCP_HEADER headroom
> > reserve.
> > 
> > So maybe we should limit TCP MTU to (64K - MAX_TCP_HEADER)
> > 
> 
> Or adapt the test at line 2390 in net/ipv4/tcp_output.c
> 
> to force in the case there could be an overflow a :
> 
> struct sk_buff *nskb = __pskb_copy()
> 
> So that we have a new skb with minimal headroom.

I would favour the __pskb_copy() variant as that would be more
generic. I'll redo the patch.
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