Re: [PATCH] tcp: Reallocate headroom if it would overflow csum_start

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2013-04-11 at 13:19 +0200, 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.
> 
> Verify if the acking and collapsing resulted in a headroom exceeding
> what csum_start can cover and reallocate the headroom if so.
> 
> LLNL has been running the patch for a while and has not seen the
> problem occur since.
> 
> A big thank you to Jim Foraker <foraker1@llnl.gov> and the team at
> LLNL for helping out with the investigation and testing.
> 
> Reported-by: Jim Foraker <foraker1@llnl.gov>
> Signed-off-by: Thomas Graf <tgraf@suug.ch>
> ---
> v2: reallocate headroom instead of preventing further collapsing
> 
>  net/ipv4/tcp_output.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index b44cf81..bf6ceb7 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2388,8 +2388,11 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
>  	 */
>  	TCP_SKB_CB(skb)->when = tcp_time_stamp;
>  
> -	/* make sure skb->data is aligned on arches that require it */
> -	if (unlikely(NET_IP_ALIGN && ((unsigned long)skb->data & 3))) {
> +	/* make sure skb->data is aligned on arches that require it
> +	 * and check if ack-trimming & collapsing extended the headroom
> +	 * beyond what csum_start can cover. */
> +	if (unlikely(NET_IP_ALIGN && ((unsigned long)skb->data & 3) ||
> +		     skb_headroom(skb) >= 0xFFFF)) {
>  		struct sk_buff *nskb = __pskb_copy(skb, MAX_TCP_HEADER,
>  						   GFP_ATOMIC);
>  		return nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :

Strange... It was tested on an arch with NET_IP_ALIGN == 2 I presume ?

This fix should also be done for other arches (x86 for example)

I would code the condition like that instead

if ((NET_IP_ALIGN && ((unsigned long)skb->data & 3)) ||
    skb_headroom(skb) >= 0xFFFF)