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

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Message-ID: <4B557BA2.7080407@trash.net>
Date:	Tue, 19 Jan 2010 10:30:10 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	William Allen Simpson <william.allen.simpson@...il.com>
CC:	Simon Arlott <simon@...e.lp0.eu>, 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

William Allen Simpson wrote:
> Simon Arlott wrote:
>> The check for data only needs to apply where the packet length
>> could be increased by adding the MSS option. (The MSS option
>> itself applies to the sender's maximum receive size which is
>> not relevant to any data in its own packet.)
>>
>> 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().
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