Re: [PATCH bpf 3/4] bpf: track changes_pkt_data property for global functions

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2024-12-06 at 12:43 -0800, Alexei Starovoitov wrote:
> On Thu, Dec 5, 2024 at 8:03 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
> > 
> > diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> > index f4290c179bee..48b7b2eeb7e2 100644
> > --- a/include/linux/bpf_verifier.h
> > +++ b/include/linux/bpf_verifier.h
> > @@ -659,6 +659,7 @@ struct bpf_subprog_info {
> >         bool args_cached: 1;
> >         /* true if bpf_fastcall stack region is used by functions that can't be inlined */
> >         bool keep_fastcall_stack: 1;
> > +       bool changes_pkt_data: 1;
> 
> since freplace was brought up in the other thread.
> Let's fix it all in one patch.
> I think propagating changes_pkt_data flag into prog_aux and
> into map->owner should do it.
> The handling will be similar to existing xdp_has_frags.
> 
> Otherwise tail_call from static subprog will have the same issue.
> xdp_has_frags compatibility requires equality. All progs either
> have it or don't.
> changes_pkt_data flag doesn't need to be that strict:
> A prog with changes_pkt_data can be freplaced by prog without
> and tailcall into prog without it.
> But not the other way around.

Ack, will do.

(Note: the change Andrii suggested with change to global subprogram
       visit order looks plausible and not hard to implement,
       after I though about it a bit more).