Re: [PATCH bpf-next 1/5] selftests/bpf: Test by-value __int128 kfunc arguments at an odd slot

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

On 10/9/26 9:48 AM, bot+bpf-ci@kernel.org wrote:
>> selftests/bpf: Test by-value __int128 kfunc arguments at an odd slot
> TL;DR: On arm64 with an older pahole, the two new kfuncs lack BTF, so
> bpf_testmod.ko may fail to build or load, losing all bpf_testmod selftests,
> not just the new ones.
>
>> On arm64 the first two kfuncs get BTF only with the pahole patch
>> "dwarf_loader: Skip the argument register the arm64 ABI leaves as a hole"
>> [2], in pahole master after v1.32. An older pahole finds the argument
>> after the __int128 in an unexpected register, x4 rather than x3, and
>> leaves the function out of BTF, so the tests fail to load there.
>> diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> --- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> +++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
> [ ... ]
>
>> @@ -1763,6 +1786,9 @@ BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_deep)
>>   BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_ii)
>>   BTF_ID_FLAGS(func, bpf_kfunc_call_test_pair_arg)
>>   BTF_ID_FLAGS(func, bpf_kfunc_call_test_i128_arg)
>> +BTF_ID_FLAGS(func, bpf_kfunc_call_test_i128_arg_odd)
>> +BTF_ID_FLAGS(func, bpf_kfunc_call_test_i128_arg_odd_many)
>> +BTF_ID_FLAGS(func, bpf_kfunc_call_test_i128_arg_stack_hole)
>>   BTF_ID_FLAGS(func, bpf_kfunc_call_test_i128_arg_pad)
> Is the failure on arm64 with an older pahole really limited to the two new
> tests?

Yes, we use the latest pahole in github. Note that latest pahole is
also available to CI.

>
> Both bpf_kfunc_call_test_i128_arg_odd and
> bpf_kfunc_call_test_i128_arg_odd_many are added to
> bpf_testmod_check_kfunc_ids, the module's main kfunc set.
>
> With no BTF FUNC for them, resolve_btfids only warns ("no BTF func for
> kfunc ..." in collect_kfuncs() and "unresolved symbol ..." in id_patch())
> and leaves their set8 ids at 0.  With CONFIG_WERROR=y, scripts/Makefile.btf
> passes --fatal_warnings, so bpf_testmod.ko would fail to build.
>
> Otherwise, at module load bpf_testmod_init() calls
> register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &bpf_testmod_kfunc_set),
> and __register_btf_kfunc_id_set() checks every entry:
>
>      for (i = 0; i < kset->set->cnt; i++) {
>          ret = btf_check_kfunc_protos(btf, btf_relocate_id(btf, kset->set->pairs[i].id), ...);
>          if (ret)
>              goto err_out;
>      }
>
> btf_check_kfunc_protos() does:
>
>      func = btf_type_by_id(btf, func_id);
>      if (!func || !btf_type_is_func(func))
>          return -EINVAL;
>
> Id 0 resolves to btf_void in the base BTF (btf_relocate_id() maps 0 to 0),
> so this returns -EINVAL.
>
> That would reject the whole set, bpf_testmod_init() would return the error,
> and bpf_testmod.ko would not load.  test_progs then prints "Selftests
> relying on bpf_testmod.ko will be skipped", so every bpf_testmod-dependent
> selftest on arm64 would be lost, not only the two new ones.  The same
> configuration worked before this patch.
>
> Could the two arm64-hole kfuncs go into a separate kfunc set whose
> registration failure is tolerated?  Alternatively, should the commit message
> say that bpf_testmod.ko as a whole needs the newer pahole on arm64, rather
> than "the tests fail to load there"?

There is no need to test with old pahole. Rather we should use latest pahole.

>
>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/37897160282