Add support for loading pak files from arbitrary file regions.

base/files/memory_mapped_file_win.cc

Index: base/files/memory_mapped_file_win.cc
diff --git a/base/files/memory_mapped_file_win.cc b/base/files/memory_mapped_file_win.cc
index f3822873bfdddb877bd73afd9fa2548ca5cd83f2..74d8619e9a6a99592bbf30529c386066b80e738c 100644
--- a/base/files/memory_mapped_file_win.cc
+++ b/base/files/memory_mapped_file_win.cc
@@ -18,7 +18,7 @@ bool MemoryMappedFile::InitializeAsImageSection(const FilePath& file_name) {
   return Initialize(file_name);
 }
 
-bool MemoryMappedFile::MapFileToMemory() {
+bool MemoryMappedFile::MapFileToMemory(const base::File::Region& region) {
   ThreadRestrictions::AssertIOAllowed();
 
   if (!file_.IsValid())
@@ -27,6 +27,10 @@ bool MemoryMappedFile::MapFileToMemory() {
   int64 len = file_.GetLength();
   if (len <= 0 || len > kint32max)

[mkosiba (inactive), 2014/07/17 00:38:01]

do we want to apply the same check to offset? then the check below would be
guaranteed to not overflow.

[Primiano Tucci (use gerrit), 2014/07/17 01:15:17]

Actually,  now that I think more about this, we shouldn'd do any check at all
here. mmap / MapViewOfFile is just supposed to handle this case and fail. There
is no reason IMHO to replicate this behavior on our side.

     return false;
+  if (region.offset + region.size > len) {
+    DLOG(ERROR) << "Region bounds invalid";
+    return false;
+  }
   length_ = static_cast<size_t>(len);
 
   int flags = image_ ? SEC_IMAGE | PAGE_READONLY : PAGE_READONLY;
@@ -36,8 +40,12 @@ bool MemoryMappedFile::MapFileToMemory() {
   if (!file_mapping_.IsValid())
     return false;
 
+  LARGE_INTEGER offset = {.QuadPart = region.offset};
   data_ = static_cast<uint8*>(::MapViewOfFile(file_mapping_.Get(),
-                                              FILE_MAP_READ, 0, 0, 0));
+                                              FILE_MAP_READ,
+                                              offset.HighPart,
+                                              offset.LowPart,
+                                              region.size));
   return data_ != NULL;
 }