selftests/bpf: Add BTF_KIND_DECL_TAG typedef unit tests

Test good and bad variants of typedef BTF_KIND_DECL_TAG encoding.
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211021195633.4019472-1-yhs@fb.com

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

@@ -3903,6 +3903,42 @@ static struct btf_raw_test raw_tests[] = {
 	.btf_load_err = true,
 	.err_str = "Invalid component_idx",
 },
+{
+	.descr = "decl_tag test #13, typedef, well-formed",
+	.raw_types = {
+		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),	/* [1] */
+		BTF_TYPEDEF_ENC(NAME_TBD, 1),			/* [2] */
+		BTF_DECL_TAG_ENC(NAME_TBD, 2, -1),
+		BTF_END_RAW,
+	},
+	BTF_STR_SEC("\0t\0tag"),
+	.map_type = BPF_MAP_TYPE_ARRAY,
+	.map_name = "tag_type_check_btf",
+	.key_size = sizeof(int),
+	.value_size = 4,
+	.key_type_id = 1,
+	.value_type_id = 1,
+	.max_entries = 1,
+},
+{
+	.descr = "decl_tag test #14, typedef, invalid component_idx",
+	.raw_types = {
+		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),	/* [1] */
+		BTF_TYPEDEF_ENC(NAME_TBD, 1),			/* [2] */
+		BTF_DECL_TAG_ENC(NAME_TBD, 2, 0),
+		BTF_END_RAW,
+	},
+	BTF_STR_SEC("\0local\0tag"),
+	.map_type = BPF_MAP_TYPE_ARRAY,
+	.map_name = "tag_type_check_btf",
+	.key_size = sizeof(int),
+	.value_size = 4,
+	.key_type_id = 1,
+	.value_type_id = 1,
+	.max_entries = 1,
+	.btf_load_err = true,
+	.err_str = "Invalid component_idx",
+},
 
 }; /* struct btf_raw_test raw_tests[] */