Re: [PATCH bpf-next v5 09/11] selftests/bpf: Add C tests for 16-byte returns in R0:R2

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

On 8/14/26 5:47 PM, Eduard Zingerman wrote:
> On Thu, 2026-08-13 at 13:02 -0700, Yonghong Song wrote:
>> Add selftests that exercise a 16-byte return value passed in the R0:R2
>> register pair, written in C so that they depend on the compiler lowering
>> the register-pair return. Covered are an __int128 return, a 16-byte struct
>> return (from a static and from a global subprogram) and a 16-byte union
>> return, plus __int128 and 16-byte struct returns from a kfunc. The union
>> program shares an object with the struct ones.
>>
>> The R0:R2 convention is only emitted by LLVM 23 and newer, and a by-value
>> aggregate return does not compile at all before that, so the programs sit
>> behind a __clang_major__ guard. An older compiler builds the dummy test in
>> the #else branch instead, which keeps each object non-empty and says in
>> its description why nothing was exercised.
>>
>> The kfunc tests are tagged __arch_x86_64/__arch_arm64 and skip elsewhere.
>> Those are the architectures whose JIT advertises
>> bpf_jit_supports_kfunc_ret_reg_pair(), which bpf_add_kfunc_call() requires
>> before it accepts a kfunc returning more than 8 bytes, and they are also
>> the only ones building the kfuncs.
>>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
> Acked-by: Eduard Zingerman <eddyz87@gmail.com>
>
>>   .../selftests/bpf/prog_tests/aggregate_ret.c  |  13 ++
>>   .../bpf/progs/aggregate_ret_int128_c.c        |  49 ++++++++
>>   .../bpf/progs/aggregate_ret_kfunc_c.c         |  65 ++++++++++
>>   .../bpf/progs/aggregate_ret_struct_c.c        | 114 ++++++++++++++++++
> Nit: I'd merge all three files into one and move loader to prog_tests/verifier.c.
>       Also not sure about union vs struct tests.

Okay, will do.

>
>> diff --git a/tools/testing/selftests/bpf/progs/aggregate_ret_int128_c.c b/tools/testing/selftests/bpf/progs/aggregate_ret_int128_c.c
>> new file mode 100644
>> index 000000000000..913cc374215d
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/progs/aggregate_ret_int128_c.c
> ...
>
>> +#if defined(__clang_major__) && __clang_major__ >= 23
> ...
>
>> +#else
>> +
>> +SEC("socket")
>> +__description("aggregate_ret_int128_c: needs LLVM 23, dummy test")
>> +__success
>> +int dummy_test(void)
>> +{
>> +	return 0;
>> +}
> Q: what would happen w/o #else branch? Is loading an empty object an error?

With #else branch, we will have:
#1/1     aggregate_ret/aggregate_ret_int128_c: needs LLVM 23, dummy test:OK
#1/2     aggregate_ret/aggregate_ret_struct_c: needs LLVM 23, dummy test:OK
#1/3     aggregate_ret/aggregate_ret_kfunc_c: needs LLVM 23, dummy test:OK

Without #else branch, all the above three subtests will be gone.
I probably will keep it so user knows the current limitation. Once CI compiler
becomes llvm23, the above three will go through real tests.
   

>
>> +#endif
> ...