selftests/bpf: Test if shadow types work correctly.

Change the values of fields, including scalar types and function pointers,
and check if the struct_ops map works as expected.

The test changes the field "test_2" of "testmod_1" from the pointer to
test_2() to pointer to test_3() and the field "data" to 13. The function
test_2() and test_3() both compute a new value for "test_2_result", but in
different way. By checking the value of "test_2_result", it ensures the
struct_ops map works as expected with changes through shadow types.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240229064523.2091270-6-thinker.li@gmail.com

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -539,6 +539,15 @@ static int bpf_testmod_ops_init_member(const struct btf_type *t,
 				       const struct btf_member *member,
 				       void *kdata, const void *udata)
 {
+	if (member->offset == offsetof(struct bpf_testmod_ops, data) * 8) {
+		/* For data fields, this function has to copy it and return
+		 * 1 to indicate that the data has been handled by the
+		 * struct_ops type, or the verifier will reject the map if
+		 * the value of the data field is not zero.
+		 */
+		((struct bpf_testmod_ops *)kdata)->data = ((struct bpf_testmod_ops *)udata)->data;
+		return 1;
+	}
 	return 0;
 }
 
@@ -559,7 +568,7 @@ static int bpf_dummy_reg(void *kdata)
 	 * initialized, so we need to check for NULL.
 	 */
 	if (ops->test_2)
-		ops->test_2(4, 3);
+		ops->test_2(4, ops->data);
 
 	return 0;
 }

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.h

@@ -35,6 +35,14 @@ struct bpf_testmod_ops {
 	void (*test_2)(int a, int b);
 	/* Used to test nullable arguments. */
 	int (*test_maybe_null)(int dummy, struct task_struct *task);
+
+	/* The following fields are used to test shadow copies. */
+	char onebyte;
+	struct {
+		int a;
+		int b;
+	} unsupported;
+	int data;
 };
 
 #endif /* _BPF_TESTMOD_H */

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

@@ -32,17 +32,23 @@ cleanup:
 
 static void test_struct_ops_load(void)
 {
-	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts);
 	struct struct_ops_module *skel;
 	struct bpf_map_info info = {};
 	struct bpf_link *link;
 	int err;
 	u32 len;
 
-	skel = struct_ops_module__open_opts(&opts);
+	skel = struct_ops_module__open();
 	if (!ASSERT_OK_PTR(skel, "struct_ops_module_open"))
 		return;
 
+	skel->struct_ops.testmod_1->data = 13;
+	skel->struct_ops.testmod_1->test_2 = skel->progs.test_3;
+	/* Since test_2() is not being used, it should be disabled from
+	 * auto-loading, or it will fail to load.
+	 */
+	bpf_program__set_autoload(skel->progs.test_2, false);
+
 	err = struct_ops_module__load(skel);
 	if (!ASSERT_OK(err, "struct_ops_module_load"))
 		goto cleanup;
@@ -56,8 +62,13 @@ static void test_struct_ops_load(void)
 	link = bpf_map__attach_struct_ops(skel->maps.testmod_1);
 	ASSERT_OK_PTR(link, "attach_test_mod_1");
 
-	/* test_2() will be called from bpf_dummy_reg() in bpf_testmod.c */
-	ASSERT_EQ(skel->bss->test_2_result, 7, "test_2_result");
+	/* test_3() will be called from bpf_dummy_reg() in bpf_testmod.c
+	 *
+	 * In bpf_testmod.c it will pass 4 and 13 (the value of data) to
+	 * .test_2.  So, the value of test_2_result should be 20 (4 + 13 +
+	 * 3).
+	 */
+	ASSERT_EQ(skel->bss->test_2_result, 20, "check_shadow_variables");
 
 	bpf_link__destroy(link);
 

tools/testing/selftests/bpf/progs/struct_ops_module.c

@@ -21,9 +21,17 @@ void BPF_PROG(test_2, int a, int b)
 	test_2_result = a + b;
 }
 
+SEC("struct_ops/test_3")
+int BPF_PROG(test_3, int a, int b)
+{
+	test_2_result = a + b + 3;
+	return a + b + 3;
+}
+
 SEC(".struct_ops.link")
 struct bpf_testmod_ops testmod_1 = {
 	.test_1 = (void *)test_1,
 	.test_2 = (void *)test_2,
+	.data = 0x1,
 };