Re: [PATCH bpf-next 2/2 v3] selftests/bpf: Test LINK_DETACH for perf link

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, May 01, 2026 at 06:09:01PM +0200, Florian Lehner wrote:
> Add test_perf_link_detach() to verify that the new LINK_DETACH support for
> BPF perf links works correctly. The test creates a link to a BPF program
> for a software perf event, confirms the program is executed, calls
> bpf_link_detach() to exercise the BPF_LINK_DETACH syscall path, and then
> verifies the program is no longer invoked after detach.
> 
> Signed-off-by: Florian Lehner <dev@der-flo.net>

hi,
you mentioned in here [1] the primary use case is to disable uprobes
temporarily.. I was expecting this as a selftest and curious to see
how that works.. could you add selftest for that?

thanks,
jirka


[1] https://lore.kernel.org/bpf/aJOhPoTLdYnZmHYA@der-flo.net/


> ---
>  .../selftests/bpf/prog_tests/perf_link.c      | 79 ++++++++++++++++---
>  1 file changed, 67 insertions(+), 12 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/perf_link.c b/tools/testing/selftests/bpf/prog_tests/perf_link.c
> index 9e3a0d217af8..b75112c1b67d 100644
> --- a/tools/testing/selftests/bpf/prog_tests/perf_link.c
> +++ b/tools/testing/selftests/bpf/prog_tests/perf_link.c
> @@ -18,29 +18,84 @@ static void burn_cpu(void)
>  		barrier();
>  }
>  
> -void test_perf_link(void)
> +static int perf_link_setup(struct test_perf_link **skel, int *pfd)
>  {
> -	struct test_perf_link *skel = NULL;
>  	struct perf_event_attr attr;
> -	int pfd = -1, link_fd = -1, err;
> -	int run_cnt_before, run_cnt_after;
> -	struct bpf_link_info info;
> -	__u32 info_len = sizeof(info);
> -	__u64 timeout_time_ns;
>  
> -	/* create perf event */
>  	memset(&attr, 0, sizeof(attr));
>  	attr.size = sizeof(attr);
>  	attr.type = PERF_TYPE_SOFTWARE;
>  	attr.config = PERF_COUNT_SW_CPU_CLOCK;
>  	attr.freq = 1;
>  	attr.sample_freq = 1000;
> -	pfd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, PERF_FLAG_FD_CLOEXEC);
> -	if (!ASSERT_GE(pfd, 0, "perf_fd"))
> +	*pfd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, PERF_FLAG_FD_CLOEXEC);
> +	if (!ASSERT_GE(*pfd, 0, "perf_fd"))
> +		return -1;
> +
> +	*skel = test_perf_link__open_and_load();
> +	if (!ASSERT_OK_PTR(*skel, "skel_load"))
> +		return -1;
> +
> +	return 0;
> +}
> +
> +void test_perf_link_detach(void)
> +{
> +	struct test_perf_link *skel = NULL;
> +	int pfd = -1, link_fd = -1, err;
> +	int run_cnt_before, run_cnt_after;
> +	__u64 timeout_time_ns;
> +
> +	if (perf_link_setup(&skel, &pfd))
> +		goto cleanup;
> +
> +	link_fd = bpf_link_create(bpf_program__fd(skel->progs.handler), pfd,
> +				  BPF_PERF_EVENT, NULL);
> +	if (!ASSERT_GE(link_fd, 0, "link_fd"))
>  		goto cleanup;
>  
> -	skel = test_perf_link__open_and_load();
> -	if (!ASSERT_OK_PTR(skel, "skel_load"))
> +	/* ensure we get at least one perf_event prog execution */
> +	timeout_time_ns = get_time_ns() + BURN_TIMEOUT_NS;
> +	while (true) {
> +		burn_cpu();
> +		if (skel->bss->run_cnt > 0)
> +			break;
> +		if (!ASSERT_LT(get_time_ns(), timeout_time_ns, "run_cnt_timeout"))
> +			goto cleanup;
> +	}
> +
> +	/* detach via BPF_LINK_DETACH - BPF program should no longer be executed */
> +	err = bpf_link_detach(link_fd);
> +	if (!ASSERT_OK(err, "link_detach"))
> +		goto cleanup;
> +
> +	/* make sure there are no stragglers */
> +	kern_sync_rcu();
> +
> +	run_cnt_before = skel->bss->run_cnt;
> +	burn_cpu();
> +	run_cnt_after = skel->bss->run_cnt;
> +
> +	ASSERT_EQ(run_cnt_before, run_cnt_after, "run_cnt_detached");
> +
> +cleanup:
> +	if (link_fd >= 0)
> +		close(link_fd);
> +	if (pfd >= 0)
> +		close(pfd);
> +	test_perf_link__destroy(skel);
> +}
> +
> +void test_perf_link(void)
> +{
> +	struct test_perf_link *skel = NULL;
> +	int pfd = -1, link_fd = -1, err;
> +	int run_cnt_before, run_cnt_after;
> +	struct bpf_link_info info;
> +	__u32 info_len = sizeof(info);
> +	__u64 timeout_time_ns;
> +
> +	if (perf_link_setup(&skel, &pfd))
>  		goto cleanup;
>  
>  	link_fd = bpf_link_create(bpf_program__fd(skel->progs.handler), pfd,
> -- 
> 2.53.0
>