netdev - Re: [PATCH net] ipv6: update skb->csum when CE mark is propagated

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Date:	Fri, 15 Jan 2016 07:44:26 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <tom@...bertland.com>
Subject: Re: [PATCH net] ipv6: update skb->csum when CE mark is propagated

On Fri, 2016-01-15 at 21:45 +0800, Herbert Xu wrote:
> On Fri, Jan 15, 2016 at 04:56:56AM -0800, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> > 
> > When a tunnel decapsulates the outer header, it has to comply
> > with RFC 6080 and eventually propagate CE mark into inner header.
> > 
> > It turns out IP6_ECN_set_ce() does not correctly update skb->csum
> > for CHECKSUM_COMPLETE packets, triggering infamous "hw csum failure"
> > messages and stack traces.
> > 
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> 
> Good catch!

Thanks Herbert

Note that I considered to use 

skb->csum = csum_add(skb->csum, from ^ to);

instead of the more generic

skb->csum = csum_add(csum_sub(skb->csum, from), to);

I can spin a v2 if you guys prefer the optimized version (but a little
more hacky...)