linux-kernel - Re: [PATCH v3] [net]: Fix skb->csum update in inet_proto_csum

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Date:   Wed, 22 Jan 2020 12:43:33 +0100
From:   Florian Westphal <fw@...len.de>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Praveen Chaudhary <praveen5582@...il.com>, fw@...len.de,
        pablo@...filter.org, davem@...emloft.net, kadlec@...filter.org,
        netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Zhenggen Xu <zxu@...kedin.com>,
        Andy Stracner <astracner@...kedin.com>
Subject: Re: [PATCH v3] [net]: Fix skb->csum update in
 inet_proto_csum_replace16().

Daniel Borkmann <daniel@...earbox.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.