Re: [PATCH bpf-next 1/2] libbpf: implement bpf_usdt_arg_size BPF function

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

On 2/24/25 3:25 PM, Andrii Nakryiko wrote:
> On Mon, Feb 24, 2025 at 3:15 PM Ihor Solodrai <ihor.solodrai@linux.dev> wrote:
>>
>> On 2/24/25 2:05 PM, Andrii Nakryiko wrote:
>>
>>> [...]
>>>
>>> arg_bitshift is stored as char (which could be signed), so that's why
>>> you were getting signed division, just cast to unsigned and keep
>>> division:
>>>
>>> return (64 - (unsigned)arg_spec->arg_bitshift) / 8;
>>
>> As it turns out, this doesn't work either. Presumably because
>> (64 - (u8)x) is still a signed int.
>
> hm... ok, surprising, but fine
>
>>
>> This works, however:
>>
>>     return (unsigned char)(64 - arg_spec->arg_bitshift) / 8;
>>
>
>
> nit: just unsigned (int), BPF doesn't have single-byte division
> anyways, so it will be upconverted to 32-bit (or 64-bit for noalu32).
> So let's be bold and use 32-bits here ;)

oh

I specified a type because:

    $ ./scripts/checkpatch.pl /tmpfs/0001-libbpf-implement-bpf_usdt_arg_size-BPF-function.patch 
    WARNING: Prefer 'unsigned int' to bare use of 'unsigned'
    #56: FILE: tools/lib/bpf/usdt.bpf.h:140:
    +	return (64 - (unsigned)arg_spec->arg_bitshift) / 8;

(unsigned int) it is then

>
>>>
>>>> [...]