Add support for loading pak files from arbitrary file regions.

base/files/memory_mapped_file_posix.cc

 // Copyright 2013 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/files/memory_mapped_file.h"
 
 #include <sys/mman.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
 #include "base/logging.h"
 #include "base/threading/thread_restrictions.h"
 
 namespace base {
 
 MemoryMappedFile::MemoryMappedFile() : data_(NULL), length_(0) {
 }
 
-bool MemoryMappedFile::MapFileToMemory() {
+bool MemoryMappedFile::MapFileToMemory(const base::File::Region& region) {
   ThreadRestrictions::AssertIOAllowed();
 
-  struct stat file_stat;
-  if (fstat(file_.GetPlatformFile(), &file_stat) == -1 ) {
+  int64 file_len = file_.GetLength();
+
+  if (file_len == -1) {
     DPLOG(ERROR) << "fstat " << file_.GetPlatformFile();
     return false;
   }
-  length_ = file_stat.st_size;
 
-  data_ = static_cast<uint8*>(
-      mmap(NULL, length_, PROT_READ, MAP_SHARED, file_.GetPlatformFile(), 0));
+  if (region.offset + region.size > file_.GetLength()) {
+    DLOG(ERROR) << "Region bounds invalid";
+    return false;
+  }
+
+  length_ = static_cast<size_t>(region.size);
+  data_ = static_cast<uint8*>(mmap(NULL,
+                                   length_,
+                                   PROT_READ,
+                                   MAP_SHARED,
+                                   file_.GetPlatformFile(),
+                                   region.offset));
   if (data_ == MAP_FAILED)
     DPLOG(ERROR) << "mmap " << file_.GetPlatformFile();
 
   return data_ != MAP_FAILED;
 }
 
 void MemoryMappedFile::CloseHandles() {
   ThreadRestrictions::AssertIOAllowed();
 
   if (data_ != NULL)
     munmap(data_, length_);
   file_.Close();
 
   data_ = NULL;
   length_ = 0;
 }
 
 }  // namespace base