netdev - Re: [PATCH] xt_TCPMSS: SYN packets are allowed to contain data

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Date:	Tue, 19 Jan 2010 13:53:13 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Simon Arlott <simon@...e.lp0.eu>
CC:	William Allen Simpson <william.allen.simpson@...il.com>,
	netdev <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Netfilter Development Mailinglist 
	<netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH] xt_TCPMSS: SYN packets are allowed to contain data

Simon Arlott wrote:
> On Tue, January 19, 2010 09:30, Patrick McHardy wrote:
>> William Allen Simpson wrote:
>>> Simon Arlott wrote:
>>>> This moves the check for (header size != packet size) to after
>>>> attempting to modify an existing MSS option. Another check is
>>>> needed before looking through the header to ensure it doesn't
>>>> claim to be larger than the packet size.
>>>>
>>> What's the path from tcp_v[4,6]_rcv() to these tests?
>>>
>>> 1) Header larger than the packet is already tested in about 5 places,
>>> and my patch "tcp: harmonize tcp_vx_rcv header length assumptions"
>>> tries to get them all down to just *one* test.
>> We're talking about a netfilter module here, which has to deal
>> with forwarded traffic and can only rely on the IP header checks
>> done in ip_rcv().
> 
> My gateway (where these error messages occur) is running 2.6.29,
> and skb->len (from the prink) is 40 bytes.
> 
> If this is 20 (IPv4 Header) + 20 (TCP Header) = 40 bytes, then there
> is no data and the header offset is wrong so it hasn't been checked.

That's odd. If the packet is really only 40 bytes large, then there
are no TCP options, so your patch shouldn't have any effect.
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