Re: [PATCH] selftests/bpf: improve test coverage for kfunc call

[Hari Bathini <hbathini@linux.ibm.com>]

On 11/03/26 9:32 pm, Alexei Starovoitov wrote:
> On Wed, Mar 11, 2026 at 8:10 AM Hari Bathini <hbathini@linux.ibm.com> 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)
  {
+       /*
+        * 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