Re: [PATCH bpf-next v3 1/2] bpf: Support struct btf_struct_meta via KF_IMPLICIT_ARGS

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

On 3/20/26 8:49 AM, Mykyta Yatsenko wrote:
> Ihor Solodrai <ihor.solodrai@linux.dev> writes:
> 
>> [...]
>> +/*
>> + * A kfunc with KF_IMPLICIT_ARGS has two prototypes in BTF:
>> + *   - the _impl prototype with full arg list (this is meta->func_proto)
>> + *   - the BPF API prototype w/o implicit args (func->type in BTF)
>> + * To determine whether an argument is implicit, we compare its position
>> + * against the number of arguments of both prototypes.
>> + */
>> +static bool is_kfunc_arg_implicit(const struct bpf_kfunc_call_arg_meta *meta, u32 arg_idx)
>> +{
>> +	const struct btf_type *func, *func_proto;
>> +	u32 argn, full_argn;
>> +
>> +	if (!(meta->kfunc_flags & KF_IMPLICIT_ARGS))
>> +		return false;
>> +
>> +	full_argn = btf_type_vlen(meta->func_proto);
>> +
>> +	func = btf_type_by_id(meta->btf, meta->func_id);
>> +	func_proto = btf_type_by_id(meta->btf, func->type);
>> +	argn = btf_type_vlen(func_proto);
>> +
>> +	return argn <= arg_idx && arg_idx < full_argn;
> The `arg_idx < full_argn` condition is not necessary, is it?
> arg_idx is always less than full_argn because full_argn is the number of
> arguments in the _impl variant of the function, which is supposed to be
> greater than non-_impl variant that arg_idx tracks.
> arg_idx >= full_argn is an invariant violation, not the implicit
> argument condition, if I understand this right.

Hi Mykyta, thanks for the review.

Yes, I think you're right.

I don't think this helper should error out on arg_idx >= full_argn,
since that would be an error caught in check_kfunc_call(). And returning
an error will make the helper a little more complicated.

At the same time, it seems logical to me that the answer to
"is arg 6 implicit for kfunc with 5 args in _impl proto?" 
should be false.

Do you have a specific suggestion?

> 
> The rest of the refactoring looks good to me.
>> +}
>> +
>>  [...]
>> -- 
>> 2.53.0