netdev - Re: [PATCH net] netfilter: xt_TCPMSS: handle CHECKSUM_COMPLETE in tcpmss

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Message-Id: <20160115.140449.1324683966791337294.davem@davemloft.net>
Date:	Fri, 15 Jan 2016 14:04:49 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	tom@...bertland.com, pablo@...filter.org, kaber@...sh.net,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
	herbert@...dor.apana.org.au
Subject: Re: [PATCH net] netfilter: xt_TCPMSS: handle CHECKSUM_COMPLETE in
 tcpmss_tg6()

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 15 Jan 2016 09:18:44 -0800

> On Fri, 2016-01-15 at 08:43 -0800, Tom Herbert wrote:
> 
>> Probably should have a utility function do this. Maybe something like
>> update_hdr_field(skb, old, new)? Could return new value so above could
>> just be:
>> 
>> ipv6h->payload_len = update_hdr_field(skb, ipv6h->payload_len,
>> htons(ntohs(oldlen) + ret);
> 
> Well, oldlen would still need to be ipv6h->payload_len
> 
> Not sure a helper will be nice for a stable submission.
> 
> Probably better for net-next ?

Agreed.  We already have those csum_replace{2,4}() helpers in net/checksum.h
which we could augment with ones that can handle length changes too.