Re: [PATCH bpf-next v3] selftests/bpf: veristat: Minimize map size during verification

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2026-07-10 at 14:44 -0700, Andrii Nakryiko wrote:

[...]

> > @@ -1248,6 +1248,29 @@ static void fixup_obj_maps(struct bpf_object *obj)
> >
> >                 /* fix up map size, if necessary */
> >                 switch (bpf_map__type(map)) {
> > +               /*
> > +                * if the verifier doesn't use max_entries
> > +                * then set to 1 to avoid -ENOMEM
> > +                */
> > +               case BPF_MAP_TYPE_HASH:
>
>
> I'm wondering if we should add ARRAY here as that one is actually with
> high likelihood will be large, though there are some corner cases
> where size might matter (e.g., existing code omits error checking
> knowing that verifier will prove it can never fail; resizing to 1
> might pessimize such check, probably, if index is larger than 1). Can
> you actually try the latter scenario with some small program, I'm
> curious how the verifier will behave?

For array maps there are several complications: first is the constant
maps handling and second is null check elision. The latter is somewhat
nasty, see check_helper_call():

         case RET_PTR_TO_MAP_VALUE:
                 ...
                 if (func_id == BPF_FUNC_map_lookup_elem &&
                     can_elide_value_nullness(meta.map.ptr) && <-------- true for array maps
                     meta.const_map_key >= 0 &&
                     meta.const_map_key < meta.map.ptr->max_entries) <-- checks max_entries
                         ret_flag &= ~PTR_MAYBE_NULL;

[...]