NaCl: preallocate shared-memory object on Linux as on MacOSX

chrome/browser/nacl_host/nacl_process_host.cc

 // Copyright (c) 2011 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 "build/build_config.h"
 
 #include "chrome/browser/nacl_host/nacl_process_host.h"
 
 #if defined(OS_POSIX)
 #include <fcntl.h>
@@ skipping 480 matching lines @@
       return;
     }
   }
 
   // Send over the IRT file handle.  We don't close our own copy!
   if (!SendHandleToSelLdr(handle(), irt_file, false, &handles_for_sel_ldr)) {
     delete this;
     return;
   }
 
-#if defined(OS_MACOSX)
-  // For dynamic loading support, NaCl requires a file descriptor that
-  // was created in /tmp, since those created with shm_open() are not
-  // mappable with PROT_EXEC.  Rather than requiring an extra IPC
-  // round trip out of the sandbox, we create an FD here.
+#if defined(OS_POSIX)
+  // For dynamic loading support, NaCl requires a file descriptor on an
+  // anonymous file that can have PROT_EXEC applied to its mappings.
+  // Rather than requiring an extra IPC round trip out of the sandbox,
+  // we create an FD here.
   base::SharedMemory memory_buffer;
   if (!memory_buffer.CreateAnonymous(/* size= */ 1)) {
     LOG(ERROR) << "Failed to allocate memory buffer";
     delete this;
     return;
   }
   nacl::FileDescriptor memory_fd;
   memory_fd.fd = dup(memory_buffer.handle().fd);
   if (memory_fd.fd < 0) {
     LOG(ERROR) << "Failed to dup() a file descriptor";
     delete this;
     return;
   }
   memory_fd.auto_close = true;
   handles_for_sel_ldr.push_back(memory_fd);
 #endif
 
   Send(new NaClProcessMsg_Start(handles_for_sel_ldr));
   internal_->sockets_for_sel_ldr.clear();
 }
 
 bool NaClProcessHost::OnMessageReceived(const IPC::Message& msg) {
   NOTREACHED() << "Invalid message with type = " << msg.type();
   return false;
 }
 
 bool NaClProcessHost::CanShutdown() {
   return true;
 }