Re: [PATCH bpf-next 1/3] bpf: support nocsr patterns for calls to kfuncs

[Yonghong Song <yonghong.song@linux.dev>]

On 8/15/24 3:07 PM, Eduard Zingerman wrote:
> On Thu, 2024-08-15 at 14:24 -0700, Andrii Nakryiko wrote:
>
> [...]
>
>>> @@ -16140,6 +16140,28 @@ static bool verifier_inlines_helper_call(struct bpf_verifier_env *env, s32 imm)
>>>          }
>>>   }
>>>
>>> +/* Same as helper_nocsr_clobber_mask() but for kfuncs, see comment above */
>>> +static u32 kfunc_nocsr_clobber_mask(struct bpf_kfunc_call_arg_meta *meta)
>>> +{
>>> +       const struct btf_param *params;
>>> +       u32 vlen, i, mask;
>>> +
>>> +       params = btf_params(meta->func_proto);
>>> +       vlen = btf_type_vlen(meta->func_proto);
>>> +       mask = 0;
>>> +       if (!btf_type_is_void(btf_type_by_id(meta->btf, meta->func_proto->type)))
>>> +               mask |= BIT(BPF_REG_0);
>>> +       for (i = 0; i < vlen; ++i)
>>> +               mask |= BIT(BPF_REG_1 + i);
>> Somewhere deep in btf_dump implementation of libbpf, there is a
>> special handling of `<whatever> func(void)` (no args) function as
>> having vlen == 1 and type being VOID (i.e., zero). I don't know if
>> that still can happen, but I believe at some point we could get this
>> vlen==1 and type=VOID for no-args functions. So I wonder if we should
>> handle that here as well, or is it some compiler atavism we can forget
>> about?
>>
> I just checked BTF generated for 'int filelock_init(void)',
> for gcc compiled kernel using latest pahole and func proto looks as follows:
>
>    FUNC_PROTO '(anon)' ret_type_id=12 vlen=0
>
> So I assume this is an atavism.

Agree, for kernel vmlinux BTF, we should be fine.

>
> [...]
>
>