Re: [PATCH bpf-next v5 4/5] bpf: verifier: Support eliding map lookup nullness

[Eduard Zingerman <eddyz87@gmail.com>]

On Sat, 2024-12-14 at 00:10 +0100, Kumar Kartikeya Dwivedi wrote:

[...]

> > @@ -11199,10 +11266,17 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> >                                 "kernel subsystem misconfigured verifier\n");
> >                         return -EINVAL;
> >                 }
> > +
> > +               if (func_id == BPF_FUNC_map_lookup_elem &&
> > +                   can_elide_value_nullness(meta.map_ptr->map_type) &&
> > +                   meta.const_map_key >= 0 &&
> > +                   meta.const_map_key < meta.map_ptr->max_entries)
> > +                       ret_flag &= ~PTR_MAYBE_NULL;
> 
> I think we probably need mark_chain_precision applied on the constant
> key since its concrete value is made use of here to prevent pruning on
> it. If it's already happening and I missed it, I think we should
> atleast add a comment.
> 
> For context of a similar case with tail calls, see commit
> cc52d9140aa9 ("bpf: Fix record_func_key to perform backtracking on r3")
> for what happens when it is missed.

Great point, I'm sure this does not happen.

[...]