netdev - Re: [PATCH -next] net: preserve geometry of fragment sizes when forwarding

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20150518.153922.1873846748456446294.davem@davemloft.net>
Date:	Mon, 18 May 2015 15:39:22 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	fw@...len.de
Cc:	netdev@...r.kernel.org, hannes@...essinduktion.org,
	edumazet@...gle.com
Subject: Re: [PATCH -next] net: preserve geometry of fragment sizes when
 forwarding

From: Florian Westphal <fw@...len.de>
Date: Thu,  7 May 2015 23:04:24 +0200

> There was interest in keeping geometry of original fragments on forward.
> 
> This (re)enables this feature.
> 
> on router with mtu 1500 on all interfaces and netfilter conntrack enabled:
 ...
> Caveat:
> This disables the optimization made in commit
> 3cc4949269e01f39443d0 ("ipv4: use skb coalescing in defragmentation") for
> everyone as soon as nf_defrag_ipv4 modules are loaded (conntrack defrag
> hooks earlier than ipv4 stacks own defragmentation for local delivery),
> and there is no way to easily determine if we will forward the skb at that
> stage.
> 
> ip_fragment checks the size of the frag skbs vs. the outgoing device mtu
> before using them so if device mtu is smaller than the frag skb length
> the device mtu will be used instead for refragmentation.
> 
> Cc: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Florian Westphal <fw@...len.de>

Indeed, I agree that we should only modify the packet's geomtry if we
know it's to be locally delivered.

But paying the cost just because a netfilter module is loaded, that's
really heavy handed and shows really bad engineering on our part.

When I hear "happens when netfilter modules are loaded", it translates
into my head as "all the time".  And for you it should too, because
effectively that's how the world operates.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html