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

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Message-ID: <1362072952.15793.41.camel@edumazet-glaptop>
Date:	Thu, 28 Feb 2013 09:35:52 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Thomas Graf <tgraf@...g.ch>
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 Thu, 2013-02-28 at 16:45 +0000, Thomas Graf wrote:
> On 02/28/13 at 08:18am, Eric Dumazet wrote:
> > but.... what is the value of skb_availroom(to) ?
> > 
> > The earlier test at line 2302 should already guard this case ?
> > 
> >                /* Punt if not enough space exists in the first SKB for
> >                  * the data in the second
> >                  */
> >                 if (skb->len > skb_availroom(to))
> >                         break;
> 
> Only if it is guaranteed that we never see an MSS > 64K.
> 
> Assuming that is true forever, then a21d45726 (tcp: avoid order-1
> allocations on wifi and tx path) does in fact resolve this
> issue at the cost of not being able to use the extra room
> kmalloc() might have given us in __alloc_skb() for collapsing.
> 
> Was that an intentional side effect of a21d45726?

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)


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