Fix silent truncations when extracting values from CheckedNumeric

chrome/utility/safe_browsing/mac/hfs.cc

 // 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.
 
 #include "chrome/utility/safe_browsing/mac/hfs.h"
 
 #include <libkern/OSByteOrder.h>
 #include <stddef.h>
 #include <sys/stat.h>
 
@@ skipping 365 matching lines @@
       current_extent_data_.resize(extent_size.ValueOrDie());
       if (!hfs_->stream()->ReadExact(&current_extent_data_[0],
                                      extent_size.ValueOrDie())) {
         DLOG(ERROR) << "Failed to read extent " << current_extent_;
         return false;
       }
 
       read_current_extent_ = true;
     }
 
-    size_t extent_offset = fork_logical_offset_ % extent_size.ValueOrDie();
-    size_t bytes_to_copy =
-        std::min(std::min(static_cast<size_t>(fork_.logicalSize) -
-                              fork_logical_offset_,
-                          extent_size.ValueOrDie() - extent_offset),
-                 buffer_space_remaining);
+    size_t extent_offset = (fork_logical_offset_ % extent_size).ValueOrDie();
+    size_t bytes_to_copy = std::min(
+        std::min(
+            static_cast<size_t>(fork_.logicalSize) - fork_logical_offset_,
+            static_cast<size_t>((extent_size - extent_offset).ValueOrDie())),
+        buffer_space_remaining);
 
     memcpy(&buffer[buffer_size - buffer_space_remaining],
            &current_extent_data_[extent_offset],
            bytes_to_copy);
 
     buffer_space_remaining -= bytes_to_copy;
     *bytes_read += bytes_to_copy;
     fork_logical_offset_ += bytes_to_copy;
 
     // If the fork's data have been read, then end the loop.
@@ skipping 243 matching lines @@
   return object;
 }
 
 bool HFSBTreeIterator::IsKeyUnexported(const base::string16& key) {
   return key == kHFSDirMetadataFolder ||
          key == kHFSMetadataFolder;
 }
 
 }  // namespace dmg
 }  // namespace safe_browsing