netdev - Re: ipv4 gro receive IHL

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Date:	Tue, 23 Oct 2012 08:01:34 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	edumazet@...gle.com, netdev@...r.kernel.org
Subject: Re: ipv4 gro receive IHL

On Mon, 2012-10-22 at 23:46 -0400, David Miller wrote:
> Eric why do we force the length field to '5' in the ip_fast_csum()
> call of inet_gro_receive()?  This happened as part of commit
> a9e050f4e7f9d36afe0dcc0bddba864ee442715e ("net: tcp: GRO should be ECN
> friendly")
> 
> This will cause the checksum check to fail for any IPV4 header with
> options.
> 
> If that's intended, the way to do this is to reject by checking the
> iph->ihl value, rather than doing so obscurely via the checksum call.
> --

Hi David

GRO on IPv4 only supports ihl = 5, this is tested in :

if (*(u8 *)iph != 0x45)
    goto out_unlock;


I also was planning to add some code to ip_fast_csum(addr, X) to have a
fast path (no conditional jump) for the case where X is the constant 5


Thanks !


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