Re: [PATCH dwarves v1 2/2] btf_encoder: implement KF_IMPLICIT_PROG_AUX_ARG kfunc flag handling

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 9/24/25 6:22 PM, Eduard Zingerman wrote:
> On Wed, 2025-09-24 at 14:15 -0700, Ihor Solodrai wrote:
>> When a kfunc is marked with KF_IMPLICIT_PROG_AUX_ARG, do not emit the
>> last parameter of this function to BTF.
> 
> [...]
> 
>> @@ -887,6 +923,12 @@ static int32_t btf_encoder__add_func_proto_for_state(struct btf_encoder *encoder
>>  	nr_params = state->nr_parms;
>>  	type_id = state->ret_type_id;
>>  
>> +	if (is_kfunc_state(state) && KF_IMPLICIT_PROG_AUX_ARG & state->elf->kfunc_flags) {
>> +		if (validate_kfunc_with_implicit_prog_aux_arg(state))
>> +			return -1;
>> +		nr_params--;
>> +	}
>> +
>>  	id = btf_encoder__emit_func_proto(encoder, type_id, nr_params);
>>  	if (id < 0)
>>  		return id;
> 
> This change hides the fact that function accepts one more parameter
> both from kernel BTF and from program BTF (via vmlinux.h).

Right, this is intentional.

> Do we anticipate other implicit parameter types?

It's very plausible, but I don't know of specific examples.

> Because if we do, it seems like having some generic KF_IMPLICIT_ARG
> and hiding it only from vmlinux.h seem more flexible.

I'm not sure how generic KF_IMPLICIT_ARG would even work.

Any *implicit* parameter requires a very concrete implementation in
the verifier: an actual pointer of a particular type is injected after
the verification. So we have to do a type check on pahole side to
catch invalid kfunc declarations. And the verifier of course must be
very strict about where it can pass pointers to kernel objects.

From a couple of discussions with Andrii, my impression is that it
would be beneficial to have some kind of generic "execution context"
available to BPF programs and/or kfuncs to cover all potential
implicit arguments. But that's a separate big discussion.

Supporting bpf_prog_aux specifically is a pragmatic improvement to the
current inconvenience that sched_ext has to deal with [1].

[1] https://lore.kernel.org/bpf/20250920005931.2753828-42-tj@kernel.org/