selftests/bpf: validate generic bpf_object and subskel APIs work together

Add a new subtest validating that bpf_object loaded and initialized
through generic APIs is still interoperable with BPF subskeleton,
including initialization and reading of global variables.

Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 	return result;
 }
 
-void test_subskeleton(void)
+/* initialize and load through skeleton, then instantiate subskeleton out of it */
+static void subtest_skel_subskeleton(void)
 {
 	int err, result;
 	struct test_subskeleton *skel;
···
 
 cleanup:
 	test_subskeleton__destroy(skel);
+}
+
+/* initialize and load through generic bpf_object API, then instantiate subskeleton out of it */
+static void subtest_obj_subskeleton(void)
+{
+	int err, result;
+	const void *elf_bytes;
+	size_t elf_bytes_sz = 0, rodata_sz = 0, bss_sz = 0;
+	struct bpf_object *obj;
+	const struct bpf_map *map;
+	const struct bpf_program *prog;
+	struct bpf_link *link = NULL;
+	struct test_subskeleton__rodata *rodata;
+	struct test_subskeleton__bss *bss;
+
+	elf_bytes = test_subskeleton__elf_bytes(&elf_bytes_sz);
+	if (!ASSERT_OK_PTR(elf_bytes, "elf_bytes"))
+		return;
+
+	obj = bpf_object__open_mem(elf_bytes, elf_bytes_sz, NULL);
+	if (!ASSERT_OK_PTR(obj, "obj_open_mem"))
+		return;
+
+	map = bpf_object__find_map_by_name(obj, ".rodata");
+	if (!ASSERT_OK_PTR(map, "rodata_map_by_name"))
+		goto cleanup;
+
+	rodata = bpf_map__initial_value(map, &rodata_sz);
+	if (!ASSERT_OK_PTR(rodata, "rodata_get"))
+		goto cleanup;
+
+	rodata->rovar1 = 10;
+	rodata->var1 = 1;
+	subskeleton_lib_setup(obj);
+
+	err = bpf_object__load(obj);
+	if (!ASSERT_OK(err, "obj_load"))
+		goto cleanup;
+
+	prog = bpf_object__find_program_by_name(obj, "handler1");
+	if (!ASSERT_OK_PTR(prog, "prog_by_name"))
+		goto cleanup;
+
+	link = bpf_program__attach(prog);
+	if (!ASSERT_OK_PTR(link, "prog_attach"))
+		goto cleanup;
+
+	/* trigger tracepoint */
+	usleep(1);
+
+	map = bpf_object__find_map_by_name(obj, ".bss");
+	if (!ASSERT_OK_PTR(map, "bss_map_by_name"))
+		goto cleanup;
+
+	bss = bpf_map__initial_value(map, &bss_sz);
+	if (!ASSERT_OK_PTR(rodata, "rodata_get"))
+		goto cleanup;
+
+	result = subskeleton_lib_subresult(obj) * 10;
+	ASSERT_EQ(bss->out1, result, "out1");
+
+cleanup:
+	bpf_link__destroy(link);
+	bpf_object__close(obj);
+}
+
+
+void test_subskeleton(void)
+{
+	if (test__start_subtest("skel_subskel"))
+		subtest_skel_subskeleton();
+	if (test__start_subtest("obj_subskel"))
+		subtest_obj_subskeleton();
 }