selftests/bpf: Add test for libbpf_bpf_link_type_str

This change adds a test for libbpf_bpf_link_type_str. The test retrieves
all variants of the bpf_link_type enumeration using BTF and makes sure
that the function under test works as expected for them.
Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220523230428.3077108-12-deso@posteo.net

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

@@ -59,6 +59,51 @@ static void test_libbpf_bpf_attach_type_str(void)
 	btf__free(btf);
 }
 
+/*
+ * Test case to check that all bpf_link_type variants are covered by
+ * libbpf_bpf_link_type_str.
+ */
+static void test_libbpf_bpf_link_type_str(void)
+{
+	struct btf *btf;
+	const struct btf_type *t;
+	const struct btf_enum *e;
+	int i, n, id;
+
+	btf = btf__parse("/sys/kernel/btf/vmlinux", NULL);
+	if (!ASSERT_OK_PTR(btf, "btf_parse"))
+		return;
+
+	/* find enum bpf_link_type and enumerate each value */
+	id = btf__find_by_name_kind(btf, "bpf_link_type", BTF_KIND_ENUM);
+	if (!ASSERT_GT(id, 0, "bpf_link_type_id"))
+		goto cleanup;
+	t = btf__type_by_id(btf, id);
+	e = btf_enum(t);
+	n = btf_vlen(t);
+	for (i = 0; i < n; e++, i++) {
+		enum bpf_link_type link_type = (enum bpf_link_type)e->val;
+		const char *link_type_name;
+		const char *link_type_str;
+		char buf[256];
+
+		if (link_type == MAX_BPF_LINK_TYPE)
+			continue;
+
+		link_type_name = btf__str_by_offset(btf, e->name_off);
+		link_type_str = libbpf_bpf_link_type_str(link_type);
+		ASSERT_OK_PTR(link_type_str, link_type_name);
+
+		snprintf(buf, sizeof(buf), "BPF_LINK_TYPE_%s", link_type_str);
+		uppercase(buf);
+
+		ASSERT_STREQ(buf, link_type_name, "exp_str_value");
+	}
+
+cleanup:
+	btf__free(btf);
+}
+
 /*
  * Test case to check that all bpf_map_type variants are covered by
  * libbpf_bpf_map_type_str.
@@ -151,6 +196,9 @@ void test_libbpf_str(void)
 	if (test__start_subtest("bpf_attach_type_str"))
 		test_libbpf_bpf_attach_type_str();
 
+	if (test__start_subtest("bpf_link_type_str"))
+		test_libbpf_bpf_link_type_str();
+
 	if (test__start_subtest("bpf_map_type_str"))
 		test_libbpf_bpf_map_type_str();