ipc: Create an unforgeable mechanism to get the process id of the sender.

ipc/ipc_channel_nacl.cc

 // Copyright (c) 2012 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 "ipc/ipc_channel_nacl.h"
 
 #include <errno.h>
 #include <stddef.h>
 #include <sys/types.h>
 
@@ skipping 361 matching lines @@
   // When the input data buffer is empty, the fds should be too.
   return input_fds_.empty();
 }
 
 void ChannelNacl::HandleInternalMessage(const Message& msg) {
   // The trusted side IPC::Channel should handle the "hello" handshake; we
   // should not receive the "Hello" message.
   NOTREACHED();
 }
 
+base::ProcessId ChannelNacl::GetSenderPID() {
+  // The untrusted side of the IPC::Channel should never have to worry about
+  // sender's process id.
+  return base::kNullProcessId;
+}
+
 // Channel's methods
 
 // static
 scoped_ptr<Channel> Channel::Create(const IPC::ChannelHandle& channel_handle,
                                     Mode mode,
                                     Listener* listener,
                                     AttachmentBroker* broker) {
   return scoped_ptr<Channel>(
       new ChannelNacl(channel_handle, mode, listener, broker));
 }
 
 }  // namespace IPC