[Mac] Use raw disks (/dev/rdisk#) in image_writer_mac.

chrome/utility/image_writer/image_writer_mac.cc

 // Copyright 2014 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 <CoreFoundation/CoreFoundation.h>
 #include <IOKit/IOBSD.h>
 #include <IOKit/IOKitLib.h>
 #include <IOKit/storage/IOBlockStorageDevice.h>
 #include <IOKit/storage/IOMedia.h>
 #include <IOKit/storage/IOStorageProtocolCharacteristics.h>
@@ skipping 77 matching lines @@
   base::ScopedFD child_socket(sockets[1]);
 
   // Map the client socket to the client's STDOUT.
   base::FileHandleMappingVector fd_map;
   fd_map.push_back(std::pair<int, int>(child_socket.get(), STDOUT_FILENO));
   options.fds_to_remap = &fd_map;
 
   // Find the file path to open.
   base::FilePath real_device_path;
   if (device_path_.IsAbsolute()) {
+    // This only occurs for tests where the device path is mocked with a
+    // temporary file.
     real_device_path = device_path_;
   } else {
-    real_device_path = base::FilePath("/dev").Append(device_path_);
+    // Get the raw device file. Writes need to be in multiples of
+    // DAMediaBlockSize (usually 512). This is fine since WriteChunk() writes in
+    // multiples of kMemoryAlignment.
+    real_device_path =
+        base::FilePath("/dev").Append("r" + device_path_.BaseName().value());
   }
 
   // Build the command line.
   std::string rdwr = base::StringPrintf("%d", O_RDWR);
 
   base::CommandLine cmd_line = base::CommandLine(base::FilePath(kAuthOpenPath));
   cmd_line.AppendSwitch("-stdoutpipe");
   // Using AppendSwitchNative will use an equal-symbol which we don't want.
   cmd_line.AppendArg("-o");
   cmd_line.AppendArg(rdwr);
@@ skipping 47 matching lines @@
 
   if (child_exit_status) {
     LOG(ERROR) << "Child process returned failure.";
     return false;
   }
 
   return device_file_.IsValid();
 }
 
 }  // namespace image_writer