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

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Message-ID: <1362073254.15793.43.camel@edumazet-glaptop>
Date:	Thu, 28 Feb 2013 09:40:54 -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 09:35 -0800, Eric Dumazet wrote:
> 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)
> 

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.




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