Re: [PATCH bpf-next v5 8/8] selftests: net: add test for XDP_PASS skb checksum invalidation

[Stanislav Fomichev <sdf.kernel@gmail.com>]

On 07/22, Jakub Kicinski wrote:
> On Wed, 22 Jul 2026 11:00:07 -0700 Stanislav Fomichev wrote:
> > > > This is about current drivers that only report UNNECESSARY with xdp: the xdp
> > > > prog has to maintain in-packet checksum if it changes the payload. I think
> > > > it's a fair assumption?  
> > > 
> > > What about decap? If we decap the header that device validated 
> > > as UNNECESSARY there's no possibility of maintaining the checksum
> > > (by which IIUC you mean correcting it in along the payload changes).
> > > 
> > > I think we'd need some API to "decrement the csum level" ?
> > > Or some heuristic in the drivers to decrement if the head moved
> > > by at least len(IP+UDP) into the packet ?  
> > 
> > True :-/ I guess one more case to document?
> 
> What would we be documenting? That it's a known bug and we have no idea
> how to address it? Or am I missing something?

Yeah, that UNECESSARY + decap in xdp has this corner case of inner
csum being not validated (and currently no way to flip it to NONE from
bpf).

(idk whether we really have nics that do csum_level>=2, so I'd just add
bpf_invalidate_csum or something, but it's a separate conversation)

> Again: 
>  .. the ask was to sketch out the plan of explicitly
>  updating/invalidating the checksum even if we don't 
>  implement it today

SGTM, I'll leave it up to Lorenzo. I had this TODAY vs TOMORROW summary,
we can expand on that with UNNECESSARY+csum_level plus maybe more
details on the shape of kfuncs for TOMORROW section..