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

From: Florian Westphal <[email-masked]>
Date: Thu, 23 Jan 2020 09:21:06 +0100

Daniel Borkmann <[email-masked]> wrote:
> On 1/22/20 12:43 PM, Florian Westphal wrote:
> > Daniel Borkmann <[email-masked]> 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?