selftests/bpf: Test that module can't be unloaded with attached trampoline

Adding test to verify that once we attach module's trampoline,
the module can't be unloaded.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210414195147.1624932-7-jolsa@kernel.org

tools/testing/selftests/bpf/prog_tests/module_attach.c

@@ -45,12 +45,18 @@ static int trigger_module_test_write(int write_sz)
 	return 0;
 }
 
+static int delete_module(const char *name, int flags)
+{
+	return syscall(__NR_delete_module, name, flags);
+}
+
 void test_module_attach(void)
 {
 	const int READ_SZ = 456;
 	const int WRITE_SZ = 457;
 	struct test_module_attach* skel;
 	struct test_module_attach__bss *bss;
+	struct bpf_link *link;
 	int err;
 
 	skel = test_module_attach__open();
@@ -84,6 +90,23 @@ void test_module_attach(void)
 	ASSERT_EQ(bss->fexit_ret, -EIO, "fexit_tet");
 	ASSERT_EQ(bss->fmod_ret_read_sz, READ_SZ, "fmod_ret");
 
+	test_module_attach__detach(skel);
+
+	/* attach fentry/fexit and make sure it get's module reference */
+	link = bpf_program__attach(skel->progs.handle_fentry);
+	if (!ASSERT_OK_PTR(link, "attach_fentry"))
+		goto cleanup;
+
+	ASSERT_ERR(delete_module("bpf_testmod", 0), "delete_module");
+	bpf_link__destroy(link);
+
+	link = bpf_program__attach(skel->progs.handle_fexit);
+	if (!ASSERT_OK_PTR(link, "attach_fexit"))
+		goto cleanup;
+
+	ASSERT_ERR(delete_module("bpf_testmod", 0), "delete_module");
+	bpf_link__destroy(link);
+
 cleanup:
 	test_module_attach__destroy(skel);
 }