linux-kernel - Re: [PATCH nf-next] netfilter: xt_CHECKSUM: avoid bad offload warnings on GSO packets

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Message-ID: <20170825092819.hjgazfcvvrlibkd4@unicorn.suse.cz>
Date:   Fri, 25 Aug 2017 11:28:19 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Florian Westphal <fw@...len.de>
Cc:     Davide Caratti <dcaratti@...hat.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Markos Chandras <markos.chandras@...e.com>
Subject: Re: [PATCH nf-next] netfilter: xt_CHECKSUM: avoid bad offload
 warnings on GSO packets

On Thu, Aug 24, 2017 at 03:17:22PM +0200, Florian Westphal wrote:
> Davide Caratti <dcaratti@...hat.com> wrote:
> > Small nit: may I suggest you to call skb_csum_hwoffload_help() instead of
> > skb_checksum_help(), so that we avoid corrupting SCTP packets in case they
> > hit xt_CHECKSUM target?
> 
> Alternatively we could restrict the target to udp only.
> 
> AFAIU the only reason this thing exists is to fix up udp checksum
> for old dhcp clients that use AF_PACKET without evaluating the extra
> metadata that indicates when a 'bad' checksum is in fact ok because it
> is supposed to be filled in by hardware later.
> 
> This can happen in virtual environemnt when such skb is directly passed
> to vm.

Based on what the documentation and the commit message of the commit
introducing xt_CHECKSUM module say, it seems so. But I must admit I'm
not sure where is the target is used and how (and why). In particular,
our issue was most likely result of

  https://github.com/openstack/openstack-ansible-tests/blob/master/test-prepare-host.yml#L196-L197

where they explicitely confine it to TCP packets. Unfortunately these
lines come from "Initial testing commit" so it's hard to say what the
intention behind that was.

                                                       Michal Kubecek