Re: [PATCH bpf-next v1] selftests/bpf: Fix flakiness of task_local_storage/sys_enter_exit

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

Ihor Solodrai <ihor.solodrai@linux.dev> writes:

> The test_sys_enter_exit test was setting target_pid before attaching
> the BPF programs, which causes syscalls made during the attach phase
> to be counted. This is flaky because, apparently, there is no
> guarantee that both on_enter and on_exit will trigger during the
> attachment.
>
> Move the target_pid assignment to after task_local_storage__attach()
> so that only explicit sys_gettid() calls are counted.
>
> Reported-by: BPF CI Bot (Claude Opus 4.6) <bot+bpf-ci@kernel.org>
> Closes: https://github.com/kernel-patches/vmtest/issues/448
> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
>
> ---
>
> I've been experimenting with running AI on BPF CI to investigate test
> failures. This is an example of a thing it may come up with.
>
> I don't want to spam the list with these, so for starters I'll be
> relaying only patches that I evaluated and/or tested.
>
> The AI generated reports will be posted with "[bpf-ci-bot]" prefix
> here: https://github.com/kernel-patches/vmtest/issues
>
> The goal of this particular application of AI is to make BPF CI more
> stable, less noisy/flaky, and potentially find and fix more kernel
> bugs. We'll see how it goes.
>
> ---
>  .../selftests/bpf/prog_tests/task_local_storage.c  | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/task_local_storage.c b/tools/testing/selftests/bpf/prog_tests/task_local_storage.c
> index 7bee33797c71..2820a604aaa6 100644
> --- a/tools/testing/selftests/bpf/prog_tests/task_local_storage.c
> +++ b/tools/testing/selftests/bpf/prog_tests/task_local_storage.c
> @@ -25,24 +25,28 @@
>  static void test_sys_enter_exit(void)
>  {
>  	struct task_local_storage *skel;
> +	pid_t pid = sys_gettid();
>  	int err;
>  
>  	skel = task_local_storage__open_and_load();
>  	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
>  		return;
>  
> -	skel->bss->target_pid = sys_gettid();
> -
>  	err = task_local_storage__attach(skel);
>  	if (!ASSERT_OK(err, "skel_attach"))
>  		goto out;
>  
> +	/* Set target_pid after attach so that syscalls made during
> +	 * attach are not counted.
> +	 */
> +	skel->bss->target_pid = pid;
> +
>  	sys_gettid();
>  	sys_gettid();
Maybe a simpler and less fragile fix would be to add syscall number
filter in the BPF program, so that we don't count those unexpected
syscalls.
>  
> -	/* 3x syscalls: 1x attach and 2x gettid */
> -	ASSERT_EQ(skel->bss->enter_cnt, 3, "enter_cnt");
> -	ASSERT_EQ(skel->bss->exit_cnt, 3, "exit_cnt");
> +	/* 2x gettid syscalls */
> +	ASSERT_EQ(skel->bss->enter_cnt, 2, "enter_cnt");
> +	ASSERT_EQ(skel->bss->exit_cnt, 2, "exit_cnt");
>  	ASSERT_EQ(skel->bss->mismatch_cnt, 0, "mismatch_cnt");
>  out:
>  	task_local_storage__destroy(skel);
> -- 
> 2.53.0