Implement SharedMemory::NewAnonymousReadOnly(contents).

base/memory/shared_memory_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 "base/memory/shared_memory.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
@@ skipping 122 matching lines @@
 void SharedMemory::Lock() {
   NOTIMPLEMENTED();
 }
 
 void SharedMemory::Unlock() {
   NOTIMPLEMENTED();
 }
 
 bool SharedMemory::ShareToProcessCommon(ProcessHandle process,
                                         SharedMemoryHandle *new_handle,
-                                        bool close_self) {
+                                        bool close_self,
+                                        ShareMode share_mode) {
+  if (share_mode == SHARE_READONLY) {
+    // Untrusted code can't create descriptors or handles, which is needed to
+    // drop permissions.
+    return false;
+  }
   const int new_fd = dup(mapped_file_);
   if (new_fd < 0) {
     DPLOG(ERROR) << "dup() failed.";
     return false;
   }
 
   new_handle->fd = new_fd;
   new_handle->auto_close = true;
 
   if (close_self)
     Close();
   return true;
 }
 
 }  // namespace base