netdev - Re: [PATCH] tcp_output: Re: rare bad TCP checksum with 2.6.19?

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Date:	Fri, 19 Jan 2007 13:14:52 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Jarek Poplawski <jarkao2@...pl>
CC:	Michael Tokarev <mjt@....msk.ru>, netdev@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH] tcp_output: Re: rare bad TCP checksum with 2.6.19?

Jarek Poplawski wrote:
> Here is my patch proposal. If I'm not totally wrong,
> there is a possibility that, during collapsing, empty
> skb with FIN is added to "normal" packet and changes
> its ip_summed field to CHECKSUM_NONE.
> 
> diff -Nurp linux-2.6.19-/net/ipv4/tcp_output.c linux-2.6.19/net/ipv4/tcp_output.c
> --- linux-2.6.19-/net/ipv4/tcp_output.c	2006-11-29 22:57:37.000000000 +0100
> +++ linux-2.6.19/net/ipv4/tcp_output.c	2007-01-19 07:58:39.000000000 +0100
> @@ -1590,7 +1590,8 @@ static void tcp_retrans_try_collapse(str
>  
>  		memcpy(skb_put(skb, next_skb_size), next_skb->data, next_skb_size);
>  
> -		skb->ip_summed = next_skb->ip_summed;
> +		if (next_skb->ip_summed == CHECKSUM_PARTIAL)
> +			skb->ip_summed = CHECKSUM_PARTIAL;
>  
>  		if (skb->ip_summed != CHECKSUM_PARTIAL)
>  			skb->csum = csum_block_add(skb->csum, next_skb->csum, skb_size);
> 

I noticed this too, but I can't see how it could lead to
a partial checksum on the wire since the checksumming is
done after changing ip_summed to CHECKSUM_NONE. Is this
patch verified to fix Michael's problem?

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