Re: [PATCH v3] [net]: Fix skb->csum update in inet_proto_csum_replace16().

From: Daniel Borkmann <daniel@iogearbox.net>
To: Florian Westphal <fw@strlen.de>
Cc: Praveen Chaudhary <praveen5582@gmail.com>,
	pablo@netfilter.org, davem@davemloft.net, kadlec@netfilter.org,
	netfilter-devel@vger.kernel.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org, Zhenggen Xu <zxu@linkedin.com>,
	Andy Stracner <astracner@linkedin.com>
Subject: Re: [PATCH v3] [net]: Fix skb->csum update in inet_proto_csum_replace16().
Date: Thu, 23 Jan 2020 15:20:39 +0100	[thread overview]
Message-ID: <1c1fc75d-e69c-f2f6-78ce-de9dc8aa89ca@iogearbox.net> (raw)
In-Reply-To: <20200123082106.GT795@breakpoint.cc>

On 1/23/20 9:21 AM, Florian Westphal wrote:
> Daniel Borkmann <daniel@iogearbox.net> wrote:
>> On 1/22/20 12:43 PM, Florian Westphal wrote:
>>> Daniel Borkmann <daniel@iogearbox.net> wrote:
>>>>> @@ -449,9 +464,6 @@ void inet_proto_csum_replace16(__sum16 *sum, struct sk_buff *skb,
>>>>>     	if (skb->ip_summed != CHECKSUM_PARTIAL) {
>>>>>     		*sum = csum_fold(csum_partial(diff, sizeof(diff),
>>>>>     				 ~csum_unfold(*sum)));
>>>>> -		if (skb->ip_summed == CHECKSUM_COMPLETE && pseudohdr)
>>>>> -			skb->csum = ~csum_partial(diff, sizeof(diff),
>>>>> -						  ~skb->csum);
>>>>
>>>> What is the technical rationale in removing this here but not in any of the
>>>> other inet_proto_csum_replace*() functions? You changelog has zero analysis
>>>> on why here but not elsewhere this change would be needed?
>>>
>>> Right, I think it could be dropped everywhere BUT there is a major caveat:
>>>
>>> At least for the nf_nat case ipv4 header manipulation (which uses the other
>>> helpers froum utils.c) will eventually also update iph->checksum field
>>> to account for the changed ip addresses.
>>>
>>> And that update doesn't touch skb->csum.
>>>
>>> So in a way the update of skb->csum in the other helpers indirectly account
>>> for later ip header checksum update.
>>>
>>> At least that was my conclusion when reviewing the earlier incarnation
>>> of the patch.
>>
>> Mainly asking because not inet_proto_csum_replace16() but the other ones are
>> exposed via BPF and they are all in no way fundamentally different to each
>> other, but my concern is that depending on how the BPF prog updates the csums
>> things could start to break. :/
> 
> I'm reasonably sure removing the skb->csum update from the other
> helpers will also break ipv4 nat :)
> 
> So, AFAIU from what you're saying above the patch seems fine as-is and
> just needs a more verbose commit message explaining why replace16()
> doesn't update skb->csum while all the other ones do.
> 
> Is that correct?

Probably better a comment in the code to avoid confusion on why it's not done in
inet_proto_csum_replace16() but all the other cases; mainly to avoid some folks
in future sending random cleanup patches w/ removal attempts.

Thanks,
Daniel