selftests/bpf: Make sure libbpf doesn't enforce the signature of a func pointer.

The verifier in the kernel ensures that the struct_ops operators behave
correctly by checking that they access parameters and context
appropriately. The verifier will approve a program as long as it correctly
accesses the context/parameters, regardless of its function signature. In
contrast, libbpf should not verify the signature of function pointers and
functions to enable flexibility in loading various implementations of an
operator even if the signature of the function pointer does not match those
in the implementations or the kernel.

With this flexibility, user space applications can adapt to different
kernel versions by loading a specific implementation of an operator based
on feature detection.

This is a follow-up of the commit c911fc61 ("libbpf: Skip zeroed or
null fields if not found in the kernel type.")
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240404232342.991414-1-thinker.li@gmail.com

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

@@ -138,11 +138,35 @@ static void test_struct_ops_not_zeroed(void)
 	struct_ops_module__destroy(skel);
 }
 
+/* The signature of an implementation might not match the signature of the
+ * function pointer prototype defined in the BPF program. This mismatch
+ * should be allowed as long as the behavior of the operator program
+ * adheres to the signature in the kernel. Libbpf should not enforce the
+ * signature; rather, let the kernel verifier handle the enforcement.
+ */
+static void test_struct_ops_incompatible(void)
+{
+	struct struct_ops_module *skel;
+	struct bpf_link *link;
+
+	skel = struct_ops_module__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	link = bpf_map__attach_struct_ops(skel->maps.testmod_incompatible);
+	if (ASSERT_OK_PTR(link, "attach_struct_ops"))
+		bpf_link__destroy(link);
+
+	struct_ops_module__destroy(skel);
+}
+
 void serial_test_struct_ops_module(void)
 {
 	if (test__start_subtest("test_struct_ops_load"))
 		test_struct_ops_load();
 	if (test__start_subtest("test_struct_ops_not_zeroed"))
 		test_struct_ops_not_zeroed();
+	if (test__start_subtest("test_struct_ops_incompatible"))
+		test_struct_ops_incompatible();
 }
 

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

@@ -68,3 +68,16 @@ struct bpf_testmod_ops___zeroed testmod_zeroed = {
 	.test_1 = (void *)test_1,
 	.test_2 = (void *)test_2_v2,
 };
+
+struct bpf_testmod_ops___incompatible {
+	int (*test_1)(void);
+	void (*test_2)(int *a);
+	int data;
+};
+
+SEC(".struct_ops.link")
+struct bpf_testmod_ops___incompatible testmod_incompatible = {
+	.test_1 = (void *)test_1,
+	.test_2 = (void *)test_2,
+	.data = 3,
+};