netdev - Re: [PATCH] tcp: Reallocate headroom if it would overflow csum

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Date:	Thu, 11 Apr 2013 18:52:45 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Thomas Graf <tgraf@...g.ch>, <davem@...emloft.net>,
	<netdev@...r.kernel.org>
Subject: Re: [PATCH] tcp: Reallocate headroom if it would overflow csum_start

On Thu, 2013-04-11 at 08:49 -0700, Eric Dumazet wrote:
> 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@...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>
> > ---
> > 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)

You dropped the unlikely() and added redundant parentheses, which may be
clearer but is still equivalent.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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