Re: [PATCH bpf-next v2 5/6] selftests/bpf: detach a struct_ops link from the subsystem managing it.

[Martin KaFai Lau <martin.lau@linux.dev>]

On 5/6/24 10:55 PM, Kui-Feng Lee wrote:
> Not only a user space program can detach a struct_ops link, the subsystem
> managing a link can also detach the link. This patch add a kfunc to
> simulate detaching a link by the subsystem managing it.
> 
> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
> ---
>   .../selftests/bpf/bpf_testmod/bpf_testmod.c   | 21 ++++++
>   .../bpf/prog_tests/test_struct_ops_module.c   | 65 +++++++++++++++++++
>   .../selftests/bpf/progs/struct_ops_detach.c   |  6 ++
>   3 files changed, 92 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> index c89a6414c69f..0bf1acc1767a 100644
> --- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> +++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> @@ -502,6 +502,26 @@ __bpf_kfunc void bpf_kfunc_call_test_sleepable(void)
>   static DEFINE_MUTEX(detach_mutex);
>   static struct bpf_link *link_to_detach;
>   
> +__bpf_kfunc int bpf_dummy_do_link_detach(void)
> +{
> +	struct bpf_link *link;
> +	int ret = -ENOENT;
> +
> +	mutex_lock(&detach_mutex);
> +	link = link_to_detach;
> +	/* Make sure the link is still valid by increasing its refcnt */
> +	if (link && !atomic64_inc_not_zero(&link->refcnt))

It is better to reuse the bpf_link_inc_not_zero().

> +		link = NULL;
> +	mutex_unlock(&detach_mutex);
> +
> +	if (link) {
> +		ret = link->ops->detach(link);
> +		bpf_link_put(link);
> +	}
> +
> +	return ret;
> +}
> +
>   BTF_KFUNCS_START(bpf_testmod_check_kfunc_ids)
>   BTF_ID_FLAGS(func, bpf_testmod_test_mod_kfunc)
>   BTF_ID_FLAGS(func, bpf_kfunc_call_test1)
> @@ -529,6 +549,7 @@ BTF_ID_FLAGS(func, bpf_kfunc_call_test_destructive, KF_DESTRUCTIVE)
>   BTF_ID_FLAGS(func, bpf_kfunc_call_test_static_unused_arg)
>   BTF_ID_FLAGS(func, bpf_kfunc_call_test_offset)
>   BTF_ID_FLAGS(func, bpf_kfunc_call_test_sleepable, KF_SLEEPABLE)
> +BTF_ID_FLAGS(func, bpf_dummy_do_link_detach)

It should need KF_SLEEPABLE. mutex is used.

>   BTF_KFUNCS_END(bpf_testmod_check_kfunc_ids)
>   
>   static int bpf_testmod_ops_init(struct btf *btf)
> diff --git a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
> index f39455b81664..9f6657b53a93 100644
> --- a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
> +++ b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
> @@ -229,6 +229,69 @@ static void test_detach_link(void)
>   	struct_ops_detach__destroy(skel);
>   }
>   
> +/* Detach a link from the subsystem that the link was registered to */
> +static void test_subsystem_detach(void)
> +{
> +	LIBBPF_OPTS(bpf_test_run_opts, topts,
> +		    .data_in = &pkt_v4,
> +		    .data_size_in = sizeof(pkt_v4));
> +	struct epoll_event ev, events[2];
> +	struct struct_ops_detach *skel;
> +	struct bpf_link *link = NULL;
> +	int fd, epollfd = -1, nfds;
> +	int prog_fd;
> +	int err;
> +
> +	skel = struct_ops_detach__open_and_load();
> +	if (!ASSERT_OK_PTR(skel, "struct_ops_detach_open_and_load"))
> +		return;
> +
> +	link = bpf_map__attach_struct_ops(skel->maps.testmod_do_detach);
> +	if (!ASSERT_OK_PTR(link, "attach_struct_ops"))
> +		goto cleanup;
> +
> +	fd = bpf_link__fd(link);
> +	if (!ASSERT_GE(fd, 0, "link_fd"))
> +		goto cleanup;
> +
> +	prog_fd = bpf_program__fd(skel->progs.start_detach);
> +	if (!ASSERT_GE(prog_fd, 0, "start_detach_fd"))
> +		goto cleanup;
> +
> +	/* Do detachment from the registered subsystem */
> +	err = bpf_prog_test_run_opts(prog_fd, &topts);
> +	if (!ASSERT_OK(err, "start_detach_run"))
> +		goto cleanup;
> +
> +	if (!ASSERT_EQ(topts.retval, 0, "start_detach_run retval"))
> +		goto cleanup;
> +
> +	epollfd = epoll_create1(0);
> +	if (!ASSERT_GE(epollfd, 0, "epoll_create1"))
> +		goto cleanup;
> +
> +	ev.events = EPOLLHUP;
> +	ev.data.fd = fd;
> +	err = epoll_ctl(epollfd, EPOLL_CTL_ADD, fd, &ev);
> +	if (!ASSERT_OK(err, "epoll_ctl"))
> +		goto cleanup;
> +
> +	/* Wait for EPOLLHUP */
> +	nfds = epoll_wait(epollfd, events, 2, 5000);
> +	if (!ASSERT_EQ(nfds, 1, "epoll_wait"))
> +		goto cleanup;
> +
> +	if (!ASSERT_EQ(events[0].data.fd, fd, "epoll_wait_fd"))
> +		goto cleanup;
> +	if (!ASSERT_TRUE(events[0].events & EPOLLHUP, "events[0].events"))
> +		goto cleanup;
> +
> +cleanup:
> +	close(epollfd);
> +	bpf_link__destroy(link);
> +	struct_ops_detach__destroy(skel);
> +}
> +
>   void serial_test_struct_ops_module(void)
>   {
>   	if (test__start_subtest("test_struct_ops_load"))
> @@ -239,5 +302,7 @@ void serial_test_struct_ops_module(void)
>   		test_struct_ops_incompatible();
>   	if (test__start_subtest("test_detach_link"))
>   		test_detach_link();
> +	if (test__start_subtest("test_subsystem_detach"))
> +		test_subsystem_detach();
>   }
>   
> diff --git a/tools/testing/selftests/bpf/progs/struct_ops_detach.c b/tools/testing/selftests/bpf/progs/struct_ops_detach.c
> index aeb355b3bea3..139f9a5c5601 100644
> --- a/tools/testing/selftests/bpf/progs/struct_ops_detach.c
> +++ b/tools/testing/selftests/bpf/progs/struct_ops_detach.c
> @@ -29,3 +29,9 @@ struct bpf_testmod_ops testmod_do_detach = {
>   	.test_1 = (void *)test_1,
>   	.test_2 = (void *)test_2,
>   };
> +
> +SEC("tc")

The bpf_dummy_do_link_detach() uses a mutex. There is no lockdep splat in the test?

> +int start_detach(void *skb)
> +{
> +	return bpf_dummy_do_link_detach();
> +}