Add namespace sandbox class.

sandbox/linux/services/namespace_utils.h

 // Copyright 2015 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.
 
 #ifndef SANDBOX_LINUX_SERVICES_NAMESPACE_UTILS_H_
 #define SANDBOX_LINUX_SERVICES_NAMESPACE_UTILS_H_
 
 #include <sys/types.h>
 
+#include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/template_util.h"
 #include "sandbox/sandbox_export.h"
 
 namespace sandbox {
 
 // Utility functions for using Linux namepaces.
 class SANDBOX_EXPORT NamespaceUtils {
  public:
   COMPILE_ASSERT((base::is_same<uid_t, gid_t>::value), UidAndGidAreSameType);
   // generic_id_t can be used for either uid_t or gid_t.
   typedef uid_t generic_id_t;
 
-  // Write a uid or gid mapping from |id| to |id| in |map_file|.
-  static bool WriteToIdMapFile(const char* map_file, generic_id_t id);
+  // Write a uid or gid mapping from |id| to |id| in |map_file|. This function
+  // is async-signal-safe.
+  static bool WriteToIdMapFile(const char* map_file,
+                               generic_id_t id) WARN_UNUSED_RESULT;
 
   // Returns true if unprivileged namespaces of type |type| is supported
   // (meaning that both CLONE_NEWUSER and type are are supported).  |type| must
   // be one of CLONE_NEWIPC, CLONE_NEWNET, CLONE_NEWNS, CLONE_NEWPID,
   // CLONE_NEWUSER, or CLONE_NEWUTS. This relies on access to /proc, so it will
   // not work from within a sandbox.
   static bool KernelSupportsUnprivilegedNamespace(int type);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(NamespaceUtils);
 };
 
 }  // namespace sandbox
 
 #endif  // SANDBOX_LINUX_SERVICES_NAMESPACE_UTILS_H_