Re: [PATCH bpf-next 2/2] selftests/bpf: Test fentry link after tracing_multi link

[Leon Hwang <leon.hwang@linux.dev>]

On 2026/7/10 23:04, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] Test Flakiness due to Missing PID Filtering in BPF `fentry` Program
> --
> 
> --- Patch [2]: [PATCH bpf-next 2/2] selftests/bpf: Test fentry link after tracing_multi link ---
> commit 7dc95a7a3d3a0ca96e45e5ad2ebebe5157ae17ed
> Author: Leon Hwang <leon.hwang@linux.dev>
> 
> selftests/bpf: Test fentry link after tracing_multi link
> 
> This commit adds a selftest to verify that an fentry link can be attached
> safely after a tracing_multi link. It introduces a new fentry BPF program
> and a corresponding user-space test case.
> 
>> diff --git a/tools/testing/selftests/bpf/progs/tracing_multi_intersect_attach.c b/tools/testing/selftests/bpf/progs/tracing_multi_intersect_attach.c
>> index cd5be0bb6ffdd..5ebbe3b602a4f 100644
>> --- a/tools/testing/selftests/bpf/progs/tracing_multi_intersect_attach.c
>> +++ b/tools/testing/selftests/bpf/progs/tracing_multi_intersect_attach.c
> 
> [ ... ]
> 
>> @@ -11,6 +11,14 @@ __u64 test_result_fentry_1 = 0;
>>  __u64 test_result_fentry_2 = 0;
>>  __u64 test_result_fexit_1 = 0;
>>  __u64 test_result_fexit_2 = 0;
>> +__u64 test_result_fentry = 0;
>> +
>> +SEC("fentry/bpf_fentry_test1")
>> +int BPF_PROG(fentry, int a)
>> +{
>> +	test_result_fentry += a == 1;
> 
> [Severity: Medium]
> Can this cause test flakiness during parallel selftest runs?
> 
> Because bpf_fentry_test1 is a global kernel function widely used across
> various BPF selftests, parallel test executions might trigger this program.
> Since the counter is incremented blindly without filtering by the executing
> process PID, could parallel executions increase test_result_fentry above 1
> and cause the userland assertion ASSERT_EQ(skel->bss->test_result_fentry, 1)
> to sporadically fail?
> 


Correct.

I should use tracing_multi_arg_check() here, which would filter the PID.

Thanks,
Leon

>> +	return 0;
>> +}
>>  
>>  SEC("fentry.multi")
>>  int BPF_PROG(fentry_1)
>