Re: [PATCH bpf-next v3 5/9] selftests/bpf: Add kfunc __arena and __arena_nullable argument tests

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2026-08-04 at 13:24 -0700, Eduard Zingerman wrote:
> On Tue, 2026-08-04 at 22:14 +0200, Kumar Kartikeya Dwivedi wrote:
> 
> ...
> 
> > > Which leads to several conclusions:
> > > - We should reach a consensus regarding KF_ARENA* usage, as these
> > >   kfuncs would clearly benefit from having __arena annotations on
> > >   parameters (on the BPF side, on the host side these should be
> > >   declared as plain pointers).
> > 
> > Yes, but even if we do, for the time begin, we won't have support for
> > automatically emitting it until resolve_btfids changes are done. That said, I
> > can manually add the annotation for now.
> 
> We can fast-track resolve_btfids.
> 
> > > - In patch #1, it appears that there is no need to allow PTR_TO_ARENA
> > >   for parameters passed to such a function. At-least I don't see a C
> > >   code not involving manual casts that would require such support.
> > 
> > This one I don't understand. Why wouldn't we get PTR_TO_ARENA typed paramters as
> > arguments?
> 
> `(u64 *)val` is translated as an address space cast instruction.
> This instruction yields PTR_TO_ARENA. Except for explicit casts,
> how else can you pass PTR_TO_ARENA to such a function?

For example, `foo->ptr` yields a scalar from the verifier point of view.

> 
> ...