Implement off-the-wire validation scheme for enum types.

ipc/ipc_message_macros.h

Index: ipc/ipc_message_macros.h
diff --git a/ipc/ipc_message_macros.h b/ipc/ipc_message_macros.h
index 2993014f0468539f89afaf4c0089d6c8d2689e7f..142cccfb710a4c35ed8bd7b2f1ca70cca87822f8 100644
--- a/ipc/ipc_message_macros.h
+++ b/ipc/ipc_message_macros.h
@@ -142,7 +142,19 @@
 // IPC_STRUCT_TRAITS_END().
 //
 // Enum types are registered with a single IPC_ENUM_TRAITS() macro.  There
-// is no need to enumerate each value to the IPC mechanism.
+// is no need to enumerate each value to the IPC mechanism. By default, enums
+// are not range-checked when they are read from the wire, and callers must
+// not trust values received from less privileged processes. In order for
+// IPC to perform validaion, enum authors must provide a specialization for

[aedla, 2013/05/31 08:35:15]

nit: 'validaion' typo

+// the function template declared in base/enum_validator.h. If such a
+// specialization exists for the enum type, then IPC will reject messages
+// that fail this validation. For example:
+//
+//     #include "base/enum_validator.h"
+//     enum Color { RED, GREEN, BLUE };
+//     template <> inline bool IsIntValidForEnum<Color>(int x) {
+//       return x >= RED && x <= BLUE;
+//     }
 //
 // Do not place semicolons following these IPC_ macro invocations.  There
 // is no reason to expect that their expansion corresponds one-to-one with