[PATCH bpf-next v2 6/6] selftests/bpf: make sure bpf_testmod handling racing link destroying well.

[Kui-Feng Lee <thinker.li@gmail.com>]

Subsystems that manage struct_ops objects may attempt to detach a link when
the link has been released or is about to be released. The test in
this patch demonstrate to developers the correct way to handle this
situation using a locking mechanism and atomic64_inc_not_zero().

A subsystem must ensure that a link is valid when detaching the link. In
order to achieve that, the subsystem may need to obtain a lock to safeguard
a table that holds the pointer to the link being detached. However, the
subsystem cannot invoke link->ops->detach() while holding the lock because
other tasks may be in the process of unregistering, which could lead to a
deadlock. This is why atomic64_inc_not_zero() is used to maintain the
link's validity. (Refer to bpf_dummy_do_link_detach() in the previous patch
for more details.)

This test make sure the pattern mentioned above work correctly.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
---
 .../bpf/prog_tests/test_struct_ops_module.c   | 44 +++++++++++++++++++
 1 file changed, 44 insertions(+)

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 9f6657b53a93..1e37037cfd8a 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
@@ -292,6 +292,48 @@ static void test_subsystem_detach(void)
 	struct_ops_detach__destroy(skel);
 }
 
+/* A subsystem detachs a link while the link is going to be free. */
+static void test_subsystem_detach_free(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, topts,
+		    .data_in = &pkt_v4,
+		    .data_size_in = sizeof(pkt_v4));
+	struct struct_ops_detach *skel;
+	struct bpf_link *link = NULL;
+	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;
+
+	bpf_link__destroy(link);
+
+	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;
+
+	/* The link may have zero refcount value and may have been
+	 * unregistered, so the detachment from the subsystem should fail.
+	 */
+	ASSERT_EQ(topts.retval, (u32)-ENOENT, "start_detach_run retval");
+
+	/* Sync RCU to make sure the link is freed without any crash */
+	ASSERT_OK(kern_sync_rcu(), "sync rcu");
+
+cleanup:
+	struct_ops_detach__destroy(skel);
+}
+
 void serial_test_struct_ops_module(void)
 {
 	if (test__start_subtest("test_struct_ops_load"))
@@ -304,5 +346,7 @@ void serial_test_struct_ops_module(void)
 		test_detach_link();
 	if (test__start_subtest("test_subsystem_detach"))
 		test_subsystem_detach();
+	if (test__start_subtest("test_subsystem_detach_free"))
+		test_subsystem_detach_free();
 }
 
-- 
2.34.1