Re: [PATCH bpf-next v6 02/10] bpf: Add helpers to describe the R0:R2 return register pair

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2026-08-18 at 12:51 -0700, Eduard Zingerman wrote:
> On Sun, 2026-08-16 at 21:21 -0700, Yonghong Song wrote:
> 
> ...
> 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index b3c474ba7140..f1f1268d29c6 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -385,27 +385,70 @@ bool bpf_subprog_is_global(const struct bpf_verifier_env *env, int subprog)
> 
> ...
> 
> > +static int bpf_compute_subprog_ret_regs(struct bpf_verifier_env *env)
> > +{
> > +	const struct btf *btf = env->prog->aux->btf;
> > +	const struct btf_type *type;
> > +	int subprog;
> > +	u32 size;
> > +
> > +	if (!env->prog->jit_requested || bpf_prog_is_offloaded(env->prog->aux))
> > +		return 0;
> > +
> > +	/*
> > +	 * Skip the main program: its return value is the program's exit code,
> > +	 * read out of R0, so it never uses the register pair. An extension does
> > +	 * have a real prototype for subprog 0, but bpf_check_attach_target()
> > +	 * refuses to replace a function returning more than 8 bytes.
> > +	 */
> > +	for (subprog = 1; subprog < env->subprog_cnt; subprog++) {
> > +		type = subprog_ret_type(env, subprog);
> > +		/*
> > +		 * This runs before btf_validate_return_type(), so apply the same
> > +		 * type filter here.
> > +		 */
> > +		if (!type || !(btf_type_is_struct(type) || btf_type_is_scalar(type)))
>                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> I still think that only !type check is necessary here:

Actually: if (!type || !btf_type_is_void(type)) ...
since btf_resolve_size() does not like void.

> - btf_resolve_size can handle any type
> - BTF validation should guarantee that return type is not e.g. BTF_KIND_VAR
>   that would trigger -EINVAL
> - for local subprograms verifier checks if r0/r2 bear compatible types anyway
> - for global subprograms BTF compliance is enforced.
> 
> Could you please explain why would you want to keep it here?
> 
> > +			continue;
> > +		if (verifier_bug_if(IS_ERR(btf_resolve_size(btf, type, &size)), env,
> > +				    "cannot size return type of subprog %d", subprog))
> > +			return -EFAULT;
> > +		if (ret_regs_cnt(size) > 1) {
> > +			subprog_info(env, subprog)->ret_reg_pair = true;
> > +			env->prog->jit_required = 1;
> > +		}
> > +	}
> > +
> > +	return 0;
> >  }
> >  
> >  const char *bpf_subprog_name(const struct bpf_verifier_env *env, int subprog)