netdev - Re: [PATCH 1/4] pch_gbe: Fix the checksum fill to the error location

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Date:	Tue, 17 Jul 2012 10:48:33 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Zhong Hongbo <hongbo.zhong@...driver.com>
Cc:	Andy Cress <andy.cress@...kontron.com>, netdev@...r.kernel.org
Subject: Re: [PATCH 1/4] pch_gbe: Fix the checksum fill to the error
 location

On Tue, 2012-07-17 at 16:04 +0800, Zhong Hongbo wrote:

> Hi Eric,
> 
> When forwarding network packages at the network layer, the variable
> value of skb->transport_header is unknown. In my test, the variable
> value of skb->transport_header is equal to skb->network_header. So
> When you count the checksum as following:
> 
> offset = skb_transport_offset(skb);
> 
> skb->csum = skb_checksum(skb, offset, skb->len - offset, 0);
> We should only count the TCP checksum, But it maybe include IP part.
> 
> tcp_hdr(skb)->check = csum_tcpudp_magic(iph->saddr, iph->daddr,
> skb->len - offset, IPPROTO_TCP, skb->csum);
> We should fill the checksum in TCP package, But maybe fill it in other
> location and cover the useful information, such as source ip.
> 
> So We should count the TCP checksum and fill it in the correct
> location. Or else the forwarding network package will be drop for the
> error checksum.
> 



So maybe you should instead test 

if (skb->ip_summed == CHECKSUM_PARTIAL) {
	...
	skb_checksum_start_offset(skb); /* is valid */
}


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