netdev - Re: [PATCH -next 0/3] net: cap size to original frag size when refragmenting

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Message-Id: <20150416.114323.2122666447487730641.davem@davemloft.net>
Date:	Thu, 16 Apr 2015 11:43:23 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	hannes@...essinduktion.org
Cc:	herbert@...dor.apana.org.au, kaber@...sh.net, fw@...len.de,
	netdev@...r.kernel.org
Subject: Re: [PATCH -next 0/3] net: cap size to original frag size when
 refragmenting

From: Hannes Frederic Sowa <hannes@...essinduktion.org>
Date: Thu, 16 Apr 2015 14:11:42 +0200

> On Thu, Apr 16, 2015, at 07:29, Herbert Xu wrote:
>> On Thu, Apr 16, 2015 at 06:24:00AM +0100, Patrick McHardy wrote:
>> >
>> > Netfilter may change the contents of the packet, even change its size.
>> > It is *really* hard to do this while keeping the original fragments
>> > intact.
>> 
>> Perhaps we should provide better helpers to facilitate this?
>> 
>> So instead of directly manipulating the content of the skb you
>> would so so through helpers and the helpers can then try to do
>> sensible things with the fragments.
> 
> When Florian and me started discussing how to solve this we also wanted
> to be as transparent as possible. But looking at all possible
> fragmentation scenarios, this seems to be too complicated.
> 
> Even imagine a fragment with overlapping offsets and some of the
> fragments got duplicated. If we had to keep this in frag_list and now
> netfilter has to change any of this contents, this will become a total
> mess, like changing one port in multiple skbs at different offsets.
> 
> I doubt it is worth the effort.

You guys keep talking about exceptional cases rather than what is
unquestionable the common case, and the one worth handling in the
fast paths.
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