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

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

On 3/19/26 5:25 AM, Jiri Olsa wrote:
> On Wed, Mar 18, 2026 at 04:42:09PM -0700, Ihor Solodrai wrote:
> 
> SNIP
> 
>>  
>> +/*
>> + * 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;
> 
> hi,

Hi Jiri, thank you for taking a look.

> I understand above is faster, but should we rather check for exact
> types with something like:
> 
> __is_kfunc_ptr_arg_type(btf, arg, KF_ARG_PROG_AUX_ID ||
> __is_kfunc_ptr_arg_type(btf, arg, KF_ARG_BTF_STRUCT_META

This is an open question, see my discussion with Andrii [1].

IMO we don't have to enforce the argument type in
is_kfunc_arg_implicit(). The only way I can see this check causing
problems is by an invalid kfunc declaration, or a relevant bug in
resolve_btfids causing bad/wrong BTF. My understanding is that
verifier can trust kernel's BTF as long as it's valid BTF.

But let me know if I'm missing something here.

[1] https://lore.kernel.org/bpf/3d069965-2992-421f-bb94-827bcb177f17@linux.dev/

> 
> jirka