Disallow calling Map on currently-mapped SharedMemory

base/memory/shared_memory_posix.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 261 matching lines @@
 
 #endif  // !defined(OS_ANDROID)
 
 bool SharedMemory::MapAt(off_t offset, size_t bytes) {
   if (mapped_file_ == -1)
     return false;
 
   if (bytes > static_cast<size_t>(std::numeric_limits<int>::max()))
     return false;
 
+  if (memory_)
+    return false;
+
 #if defined(OS_ANDROID)
   // On Android, Map can be called with a size and offset of zero to use the
   // ashmem-determined size.
   if (bytes == 0) {
     DCHECK_EQ(0, offset);
     int ashmem_bytes = ashmem_get_size_region(mapped_file_);
     if (ashmem_bytes < 0)
       return false;
     bytes = ashmem_bytes;
   }
@@ skipping 163 matching lines @@
   new_handle->fd = new_fd;
   new_handle->auto_close = true;
 
   if (close_self)
     Close();
 
   return true;
 }
 
 }  // namespace base