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

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Date:	Thu, 28 Feb 2013 08:18:46 -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 10:26 +0000, Thomas Graf wrote:
> If a TCP retransmission gets partially ACKed and collapsed multiple
> times it is possible for the headroom to grow beyond 64K which will
> overflow the 16bit skb->csum_start which is based on the start of
> the headroom. It has been observed rarely in the wild with IPoIB due
> to the 64K MTU.
> 
> With this patch, the overflow has not been observed for over a week
> while previously it would occur within ~ 1 day.
> 
> A big thank you to Jim Foraker <foraker1@...l.gov> and the team at
> LLNL for helping out with the investigation and testing.
> 
> Reported-by: Jim Foraker <foraker1@...l.gov>
> Signed-off-by: Thomas Graf <tgraf@...g.ch>
> ---
>  net/ipv4/tcp_output.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index e2b4461..1902fee 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2305,6 +2305,12 @@ static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
>  		if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
>  			break;
>  
> +		/* Never collapse if the resulting headroom + data exceeds
> +		 * 64K as that is the maximum csum_start can cover.
> +		 */
> +		if (skb_headroom(to) + to->len + skb->len > 0xFFFF)
> +			break;
> +
>  		tcp_collapse_retrans(sk, to);
>  	}
>  }

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;




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