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

[Daniel Borkmann]

On 1/23/20 9:21 AM, Florian Westphal wrote:
> Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> > On 1/22/20 12:43 PM, Florian Westphal wrote:
> > > Daniel Borkmann <daniel@xxxxxxxxxxxxx> 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