Mojo cpp bindings: validation logic for incoming messages

mojo/public/cpp/bindings/message.h

Index: mojo/public/cpp/bindings/message.h
diff --git a/mojo/public/cpp/bindings/message.h b/mojo/public/cpp/bindings/message.h
index b8298360b0d1c3971c0df620bff824c1ec5bb9b5..beb1cd9036ffb45a9e53f46b2ce7ee059974617a 100644
--- a/mojo/public/cpp/bindings/message.h
+++ b/mojo/public/cpp/bindings/message.h
@@ -63,6 +63,10 @@ class Message {
   uint8_t* mutable_payload() {
     return reinterpret_cast<uint8_t*>(data_) + data_->header.num_bytes;
   }
+  uint32_t payload_num_bytes() const {
+    assert(data_num_bytes_ >= data_->header.num_bytes);
+    return data_num_bytes_ - data_->header.num_bytes;
+  }
 
   // Access the handles.
   const std::vector<Handle>* handles() const { return &handles_; }