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..4e7e934e1c858823f13a2c7842a1daa0e390591b 100644
--- a/base/files/memory_mapped_file_win.cc
+++ b/base/files/memory_mapped_file_win.cc
@@ -18,17 +18,13 @@ bool MemoryMappedFile::InitializeAsImageSection(const FilePath& file_name) {
   return Initialize(file_name);
 }
 
-bool MemoryMappedFile::MapFileToMemory() {
+bool MemoryMappedFile::MapFileRegionToMemory(
+    const MemoryMappedFile::Region& region) {
   ThreadRestrictions::AssertIOAllowed();
 
   if (!file_.IsValid())
     return false;
 
-  int64 len = file_.GetLength();
-  if (len <= 0 || len > kint32max)
-    return false;
-  length_ = static_cast<size_t>(len);
-
   int flags = image_ ? SEC_IMAGE | PAGE_READONLY : PAGE_READONLY;
 
   file_mapping_.Set(::CreateFileMapping(file_.GetPlatformFile(), NULL,
@@ -36,9 +32,49 @@ bool MemoryMappedFile::MapFileToMemory() {
   if (!file_mapping_.IsValid())
     return false;
 
+  LARGE_INTEGER map_start = {0};
+  SIZE_T map_size = 0;
+  int32 data_offset = 0;
+
+  if (region == MemoryMappedFile::Region::kWholeFile) {
+    int64 file_len = file_.GetLength();
+    if (file_len <= 0 || file_len > kint32max)
+      return false;
+    length_ = static_cast<size_t>(file_len);
+  } else {
+    // The region can be arbitrarily aligned. MapViewOfFile, instead, requires
+    // that the start address is aligned to the VM granularity (which is
+    // typically larger than a page size, for instance 32k).
+    // Also, conversely to POSIX's mmap, the |map_size| doesn't have to be
+    // aligned and must be less than or equal the mapped file size.
+    // We map here the outer region [|aligned_start|, |aligned_start+size|]
+    // which contains |region| and then add up the |data_offset| displacement.
+    int64 aligned_start = 0;
+    int64 ignored = 0;
+    CalculateVMAlignedBoundaries(
+        region.offset, region.size, &aligned_start, &ignored, &data_offset);
+    int64 size = region.size + data_offset;
+
+    // Ensure that the casts below in the MapViewOfFile call are sane.
+    if (aligned_start < 0 || size < 0 ||
+        static_cast<uint64>(size) > std::numeric_limits<SIZE_T>::max()) {
+      DLOG(ERROR) << "Region bounds are not valid for MapViewOfFile";
+      return false;
+    }
+    map_start.QuadPart = aligned_start;
+    map_size = static_cast<SIZE_T>(size);
+    length_ = static_cast<size_t>(region.size);
+  }
+
   data_ = static_cast<uint8*>(::MapViewOfFile(file_mapping_.Get(),
-                                              FILE_MAP_READ, 0, 0, 0));
-  return data_ != NULL;
+                                              FILE_MAP_READ,
+                                              map_start.HighPart,
+                                              map_start.LowPart,
+                                              map_size));
+  if (data_ == NULL)
+    return false;
+  data_ += data_offset;
+  return true;
 }
 
 void MemoryMappedFile::CloseHandles() {