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

[Alexei Starovoitov]

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

While at it reformat the comment to proper kernel style.

the rest lgtm.