Add namespace sandbox class.

sandbox/linux/services/namespace_utils.cc

Index: sandbox/linux/services/namespace_utils.cc
diff --git a/sandbox/linux/services/namespace_utils.cc b/sandbox/linux/services/namespace_utils.cc
index f03fe4080aa32ea49e383e784463dd30d24c8de2..81e096c1262d706ab3e1b3d922fc8b85f0a49c10 100644
--- a/sandbox/linux/services/namespace_utils.cc
+++ b/sandbox/linux/services/namespace_utils.cc
@@ -18,7 +18,7 @@
 #include "base/logging.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/process/launch.h"
-#include "base/strings/stringprintf.h"
+#include "base/strings/safe_sprintf.h"
 #include "base/third_party/valgrind/valgrind.h"
 
 namespace sandbox {
@@ -38,11 +38,14 @@ bool NamespaceUtils::WriteToIdMapFile(const char* map_file, generic_id_t id) {
 
   const generic_id_t inside_id = id;
   const generic_id_t outside_id = id;
-  const std::string mapping =
-      base::StringPrintf("%d %d 1\n", inside_id, outside_id);
-  const size_t len = mapping.size();
-  const ssize_t rc = HANDLE_EINTR(write(fd.get(), mapping.c_str(), len));
-  return rc == static_cast<ssize_t>(len);
+
+  // This function needs to be async-signal-safe, as it is called in between
+  // fork and exec.
+  char mapping[64];
+  ssize_t len =
+      base::strings::SafeSPrintf(mapping, "%d %d 1\n", inside_id, outside_id);
+  const ssize_t rc = HANDLE_EINTR(write(fd.get(), mapping, len));
+  return rc == len;
 }
 
 // static