> On 3/11/26 11:03 AM, Hari Bathini wrote:
>>
>> On 11/03/26 9:32 pm, Alexei Starovoitov wrote:
>>> On Wed, Mar 11, 2026 at 8:10 AM Hari Bathini
>>> <[email protected]> wrote:
>>>>
>>>>>
>>>>>> +
>>>>>> + /* Check zero-extension */
>>>>>> + if (val != (unsigned long)a)
>>>>>> + return 1;
>>>>>> + /* Check no sign-extension */
>>>>>> + if (val < 0)
>>>>>> + return 2;
>>>>>> +
>>>>>> + val = b;
>>>>>> + if (val != (unsigned long)b)
>>>>>> + return 3;
>>>>>> + if (val < 0)
>>>>>> + return 4;
>>>>>> +
>>>>>> + val = c;
>>>>>> + if (val != (unsigned long)c)
>>>>>> + return 5;
>>>>>> + if (val < 0)
>>>>>> + return 6;
>>>>>> +
>>>>>> + return 0;
>>>>>> +}
>>>>>
>>>>> Overall this looks very useful.
>>>>> I would expand with another test where a,b,c are s8,s16,s32.
>>>>
>>>> Slightly different approach but kfunc_call_test4/bpf_kfunc_call_test4
>>>> cover signed arguments already?
>>>
>>> Ahh. Then may be tweak it to adopt similar fine grained
>>> error reporting as your bpf_kfunc_call_test5()
>>
>> I Prefer this.
>>
>> Does the below change to bpf_kfunc_call_test4 look fine:
>>
>> diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> index 48dcaf93bb9f..6237c2222633 100644
>> --- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> +++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> @@ -760,8 +760,30 @@ __bpf_kfunc struct sock
>> *bpf_kfunc_call_test3(struct sock *sk)
>>
>> __bpf_kfunc long noinline bpf_kfunc_call_test4(signed char a, short
>> b, int c, long d)
>
> I might regret for bringing this up as it could be yet another ABI
> fiasco between gcc and llvm.
char is signed in BPF in both compilers, matching x86.
> As per C standard, sign of unadorned char (i.e. w/o explicit signed or
> unsigned prefix) is ABI defined.
> For gcc-bpf char is specified to be signed.
> So test4 has s8, while new test5 has u8. Would it make sense to have
> an additional test without signed/unsigned annotation for char ?
> This will flag any discrepancy between the two compilers.
>
> Thx,
> -Vineet
>
>> {
>> + /*
>> + * Make val as volatile to avoid compiler optimizations.
>> + * Verify that negative signed values remain negative after
>> + * sign-extension (JIT must sign-extend, not zero-extend).
>> + */
>> + volatile long val;
>> +
>> + /* val will be positive, if JIT does zero-extension instead
>> of sign-extension */
>> + val = a;
>> + if (val >= 0)
>> + return 1;
>> +
>> + val = b;
>> + if (val >= 0)
>> + return 2;
>> +
>> + val = c;
>> + if (val >= 0)
>> + return 3;
>> +
>> /* Provoke the compiler to assume that the caller has
>> sign-extended a,
>> * b and c on platforms where this is required (e.g. s390x).
>> + *
>> + * Original behavior: return sum for backward compatibility
>> */
>> return (long)a + (long)b + (long)c + d;
>> }
>>
>>
>> - Hari
