netdev - Re: [PATCH v3] net: Do not include padding in TCP GRO checksum

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Message-ID: <20131116004738.GA1491@gondor.apana.org.au>
Date:	Sat, 16 Nov 2013 08:47:38 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com,
	herbert@...dor.apana.org
Subject: Re: [PATCH v3] net: Do not include padding in TCP GRO checksum

On Fri, Nov 15, 2013 at 03:00:34PM -0800, Alexander Duyck wrote:
> In some recent tests I found the TCP checksum was being treated as valid
> for certain frames with padding on them.  On closer inspection I found the
> issue was that GRO was using the skb->len instead of the length recorded in
> the IP/IPv6 header to determine the number of bytes to checksum.  As such
> padded frames that actually had invalid checksums generated by adding the
> padding to the checksum were being incorrectly tagged as valid.
> 
> This change corrects that by using the tot_len from IPv4 headers and the
> payload_len from IPv6 headers to compute the total number of bytes to be
> included in the checksum.
> 
> To address the fact that skb->csum is invalid when a padded frame is
> received I have updated the code to fall though to the CHECKSUM_NONE path
> for CHECKSUM_COMPLETE frames that contain padding.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>

Nack.  This is needlessly complex.  As I said before, please
just do a pskb_trim_rcsum in inet_gro_receive and its IPv6
counterpart and this should all just work.

Also, if the padding occurs on every single packet or a fairly
large amount of packets then you should consider improving
pskb_trim_rcsum to keep the CHECKSUM_COMPLETE value.

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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