Re: [PATCH bpf 2/2] selftests/bpf: Add test for >8 byte return value on fexit attach

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

On 7/10/26 8:29 AM, Leon Hwang wrote:
> On 2026/7/10 22:44, Yonghong Song wrote:
>> The BPF trampoline preserves only 8 bytes of the target's return value
>> (R0), so attaching an fexit/fmod_ret/fsession program to a function that
>> returns a >8 byte value is now rejected by the verifier.
>>
>> Add a bpf_testmod function returning __int128 and an fexit program that
>> targets it. The program is expected to fail to load with the
>> "with a >8 byte return value is not supported for this attach type"
>> message.
>>
>> __int128 is only available on 64-bit targets (where the compiler defines
>> __SIZEOF_INT128__). Guard the testmod function and the test with
>> __SIZEOF_INT128__ so a 32-bit build still compiles; on such builds
>> the subtest is simply not run.
>>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>>   .../selftests/bpf/prog_tests/tracing_failure.c      | 12 ++++++++++++
>>   tools/testing/selftests/bpf/progs/tracing_failure.c |  6 ++++++
>>   .../testing/selftests/bpf/test_kmods/bpf_testmod.c  | 13 +++++++++++++
>>   3 files changed, 31 insertions(+)
>>
>> diff --git a/tools/testing/selftests/bpf/prog_tests/tracing_failure.c b/tools/testing/selftests/bpf/prog_tests/tracing_failure.c
>> index f9f9e1cb87bf..6c199f5ff4db 100644
>> --- a/tools/testing/selftests/bpf/prog_tests/tracing_failure.c
>> +++ b/tools/testing/selftests/bpf/prog_tests/tracing_failure.c
>> @@ -76,6 +76,14 @@ static void test_fexit_noreturns(void)
>>   			       "Attaching fexit/fsession/fmod_ret to __noreturn function 'do_exit' is rejected.");
>>   }
>>   
>> +#ifdef __SIZEOF_INT128__
>> +static void test_fexit_int128_ret(void)
>> +{
>> +	test_tracing_fail_prog("fexit_int128_ret",
>> +			       "with a >8 byte return value is not supported for this attach type");
>> +}
>
> Seems that this subtest can be simplified to
>
> static void test_fexit_int128_ret(void)
> {
> #ifdef __SIZEOF_INT128__
> 	test_tracing_fail_prog("fexit_int128_ret",
> 			       "with a >8 byte return value is not supported for this attach
> type");
> #else
> 	test__skip();
> #endif
> }
>
> Then, no '#ifdef' below.

Thanks. Let me test this.

>
> Thanks,
> Leon
>
>> +#endif
>> +
>>   void test_tracing_failure(void)
>>   {
>>   	if (test__start_subtest("bpf_spin_lock"))
>> @@ -86,4 +94,8 @@ void test_tracing_failure(void)
>>   		test_tracing_deny();
>>   	if (test__start_subtest("fexit_noreturns"))
>>   		test_fexit_noreturns();
>> +#ifdef __SIZEOF_INT128__
>> +	if (test__start_subtest("fexit_int128_ret"))
>> +		test_fexit_int128_ret();
>> +#endif
>>   }
>> diff --git a/tools/testing/selftests/bpf/progs/tracing_failure.c b/tools/testing/selftests/bpf/progs/tracing_failure.c
>> index 65e485c4468c..f7a095767679 100644
>> --- a/tools/testing/selftests/bpf/progs/tracing_failure.c
>> +++ b/tools/testing/selftests/bpf/progs/tracing_failure.c
>> @@ -30,3 +30,9 @@ int BPF_PROG(fexit_noreturns)
>>   {
>>   	return 0;
>>   }
>> +
>> +SEC("?fexit/bpf_testmod_test_int128_ret")
>> +int BPF_PROG(fexit_int128_ret)
>> +{
>> +	return 0;
>> +}
>> diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> index 30f1cd23093c..902dbf658250 100644
>> --- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> +++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
>> @@ -161,6 +161,15 @@ bpf_testmod_test_arg_ptr_to_struct(struct bpf_testmod_struct_arg_1 *a) {
>>   	return bpf_testmod_test_struct_arg_result;
>>   }
>>   
>> +#ifdef __SIZEOF_INT128__
>> +noinline __int128
>> +bpf_testmod_test_int128_ret(int a)
>> +{
>> +	bpf_testmod_test_struct_arg_result = a;
>> +	return (__int128)a;
>> +}
>> +#endif
>> +
>>   __weak noinline void bpf_testmod_looooooooooooooooooooooooooooooong_name(void)
>>   {
>>   }
>> @@ -514,6 +523,10 @@ bpf_testmod_test_read(struct file *file, struct kobject *kobj,
>>   
>>   	(void)bpf_testmod_test_arg_ptr_to_struct(&struct_arg1_2);
>>   
>> +#ifdef __SIZEOF_INT128__
>> +	(void)bpf_testmod_test_int128_ret(i);
>> +#endif
>> +
>>   	(void)trace_bpf_testmod_test_raw_tp_null_tp(NULL);
>>   
>>   	bpf_testmod_test_struct_ops3();