C types and utilities to validate and access struct and message headers

mojo/public/c/bindings/lib/message.c

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "mojo/public/c/bindings/message.h"
+
+#include <stdint.h>
+
+_Static_assert(sizeof(mojo_message_header_t) == 16u,
+               "mojo_message_header_t should be 16 bytes");
+
+_Static_assert(sizeof(mojo_message_header_with_request_id_t) == 24u,
+               "mojo_message_header_t should be 24 bytes");
+
+bool mojo_validate_message_header(const mojo_struct_header_t* header,
+                                  size_t size) {
+  if (header->num_bytes < sizeof(mojo_message_header_t) ||
+      size < sizeof(mojo_message_header_t) || size > UINT32_MAX) {
+    return false;
+  }
+
+  const mojo_message_header_t* message_header =
+      (const mojo_message_header_t*)header;
+
+  // Message expects response and message is response flags are mutually
+  // exclusive.
+  if ((message_header->flags & MOJO_MESSAGE_HEADER_FLAGS_EXPECTS_RESPONSE) &&
+      (message_header->flags & MOJO_MESSAGE_HEADER_FLAGS_IS_RESPONSE)) {
+    return false;
+  }
+
+  if (header->version == 0u) {
+    if (header->num_bytes != sizeof(mojo_message_header_t)) {
+      return false;
+    }
+
+    // Version 0 has no request id and should not have either of these flags.
+    if ((message_header->flags & MOJO_MESSAGE_HEADER_FLAGS_EXPECTS_RESPONSE) ||
+        (message_header->flags & MOJO_MESSAGE_HEADER_FLAGS_IS_RESPONSE)) {
+      return false;
+    }
+  } else if (header->version == 1u) {
+    if (header->num_bytes != sizeof(mojo_message_header_with_request_id_t)) {
+      return false;
+    }
+  }
+  // Accept unknown versions of the message header to be future-proof.
+
+  return true;
+}