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

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Message-ID: <20170824105118.GA15739@breakpoint.cc>
Date:   Thu, 24 Aug 2017 12:51:18 +0200
From:   Florian Westphal <fw@...len.de>
To:     Michal Kubecek <mkubecek@...e.cz>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        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

Michal Kubecek <mkubecek@...e.cz> wrote:
> When --checksum_fill action is applied to a GSO packet, checksum_tg() calls
> skb_checksum_help() which is only meant to be applied to non-GSO packets so
> that it issues a warning.
> 
> This can be easily triggered by using e.g.
> 
>   iptables -t mangle -A OUTPUT -j CHECKSUM --checksum-fill
> 
> and sending TCP stream via a device with GSO enabled.
> 
> While this can be considered a misconfiguration, I believe the bad offload
> warning is supposed to catch bugs in drivers and networking stack, not
> misconfigured firewalls. So let's ignore such packets and only issue a one
> time warning with pr_warn_once() rather than a WARN with stack trace and
> tainted kernel.

Why issue a warning at all?
What kind of action should be taken upon seeing such warning?