bpf: btf: Ensure t->type == 0 for BTF_KIND_FWD

The t->type in BTF_KIND_FWD is not used.  It must be 0.
This patch ensures that and also adds a test case in test_btf.c

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/btf.c

@@ -1286,8 +1286,27 @@ static struct btf_kind_operations ptr_ops = {
 	.seq_show = btf_ptr_seq_show,
 };
 
+static s32 btf_fwd_check_meta(struct btf_verifier_env *env,
+			      const struct btf_type *t,
+			      u32 meta_left)
+{
+	if (btf_type_vlen(t)) {
+		btf_verifier_log_type(env, t, "vlen != 0");
+		return -EINVAL;
+	}
+
+	if (t->type) {
+		btf_verifier_log_type(env, t, "type != 0");
+		return -EINVAL;
+	}
+
+	btf_verifier_log_type(env, t, NULL);
+
+	return 0;
+}
+
 static struct btf_kind_operations fwd_ops = {
-	.check_meta = btf_ref_type_check_meta,
+	.check_meta = btf_fwd_check_meta,
 	.resolve = btf_df_resolve,
 	.check_member = btf_df_check_member,
 	.log_details = btf_ref_type_log,

tools/testing/selftests/bpf/test_btf.c

@@ -1242,6 +1242,28 @@ static struct btf_raw_test raw_tests[] = {
 	.err_str = "Invalid btf_info",
 },
 
+{
+	.descr = "fwd test. t->type != 0\"",
+	.raw_types = {
+		/* int */				/* [1] */
+		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),
+		/* fwd type */				/* [2] */
+		BTF_TYPE_ENC(0, BTF_INFO_ENC(BTF_KIND_FWD, 0, 0), 1),
+		BTF_END_RAW,
+	},
+	.str_sec = "",
+	.str_sec_size = sizeof(""),
+	.map_type = BPF_MAP_TYPE_ARRAY,
+	.map_name = "fwd_test_map",
+	.key_size = sizeof(int),
+	.value_size = sizeof(int),
+	.key_type_id = 1,
+	.value_type_id = 1,
+	.max_entries = 4,
+	.btf_load_err = true,
+	.err_str = "type != 0",
+},
+
 }; /* struct btf_raw_test raw_tests[] */
 
 static const char *get_next_str(const char *start, const char *end)