base::ReadFile() should return the number of read bytes on Windows.

base/file_util_win.cc

 // Copyright (c) 2012 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 "base/file_util.h"
 
 #include <windows.h>
 #include <psapi.h>
 #include <shellapi.h>
 #include <shlobj.h>
@@ skipping 553 matching lines @@
 
   return true;
 }
 
 FILE* OpenFile(const FilePath& filename, const char* mode) {
   ThreadRestrictions::AssertIOAllowed();
   std::wstring w_mode = ASCIIToWide(std::string(mode));
   return _wfsopen(filename.value().c_str(), w_mode.c_str(), _SH_DENYNO);
 }
 
-int ReadFile(const FilePath& filename, char* data, int size) {
+int ReadFile(const FilePath& filename, char* data, int max_size) {
   ThreadRestrictions::AssertIOAllowed();
   base::win::ScopedHandle file(CreateFile(filename.value().c_str(),
                                           GENERIC_READ,
                                           FILE_SHARE_READ | FILE_SHARE_WRITE,
                                           NULL,
                                           OPEN_EXISTING,
                                           FILE_FLAG_SEQUENTIAL_SCAN,
                                           NULL));
   if (!file)
     return -1;
 
   DWORD read;
-  if (::ReadFile(file, data, size, &read, NULL) &&
-      static_cast<int>(read) == size)
+  if (::ReadFile(file, data, max_size, &read, NULL))
     return read;
+
   return -1;
 }
 
 int WriteFile(const FilePath& filename, const char* data, int size) {
   ThreadRestrictions::AssertIOAllowed();
   base::win::ScopedHandle file(CreateFile(filename.value().c_str(),
                                           GENERIC_WRITE,
                                           0,
                                           NULL,
                                           CREATE_ALWAYS,
@@ skipping 180 matching lines @@
     // Like Move, this function is not transactional, so we just
     // leave the copied bits behind if deleting from_path fails.
     // If to_path exists previously then we have already overwritten
     // it by now, we don't get better off by deleting the new bits.
   }
   return false;
 }
 
 }  // namespace internal
 }  // namespace base