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::MapFileRegionToMemory(const File::Region& region) {
   ThreadRestrictions::AssertIOAllowed();
 
-  struct stat file_stat;
-  if (fstat(file_.GetPlatformFile(), &file_stat) == -1 ) {
-    DPLOG(ERROR) << "fstat " << file_.GetPlatformFile();
+  off_t map_start = 0;
+  size_t map_size = 0;
+  int32 data_offset = 0;
+
+  if (region == File::Region::kWholeFile) {
+    int64 file_len = file_.GetLength();
+    if (file_len == -1) {
+      DPLOG(ERROR) << "fstat " << file_.GetPlatformFile();
+      return false;
+    }
+    map_size = static_cast<size_t>(file_len);
+    length_ = map_size;
+  } else {
+    // The region can be arbitrarily aligned. mmap, instead, requires both the
+    // start and size to be page-aligned. Hence, we map here the page-aligned
+    // outer region [|aligned_start|, |aligned_start| + |size|] which contains
+    // |region| and then add up the |data_offset| displacement.
+    int64 aligned_start = 0;
+    int64 aligned_size = 0;
+    CalculateVMAlignedBoundaries(region.offset,
+                                 region.size,
+                                 &aligned_start,
+                                 &aligned_size,
+                                 &data_offset);
+
+    // Ensure that the casts in the mmap call below are sane.
+    if (aligned_start < 0 || aligned_size < 0 ||
+        aligned_start > std::numeric_limits<off_t>::max() ||
+        static_cast<uint64>(aligned_size) >
+            std::numeric_limits<size_t>::max() ||
+        static_cast<uint64>(region.size) > std::numeric_limits<size_t>::max()) {
+      DLOG(ERROR) << "Region bounds are not valid for mmap";

[willchan no longer on Chromium, 2014/07/25 21:21:24]

How about NOTREACHED()?

[Primiano Tucci (use gerrit), 2014/07/28 12:52:11]

Hmm technically that is reachable if the client calling this API is being
stupid:
If you do memory_mapped_file.Initialize(some_file,
File::Region(0xffffffffffffffffLL, 10248576)), you'll hit this.

I might be wrong, but I think that NOTREACHED should emphasize the concept of
"if you hit this, there might be a bug / invalid assumption in *this* file
(mmap..posix.cc)".
What I really want to express here, instead, is: "if you hit this, you (caller)
are doing something wrong".
And I think a DLOG (and returning false) is more suitable, isn't it?

+      return false;
+    }
+
+    map_start = static_cast<off_t>(aligned_start);
+    map_size = static_cast<size_t>(aligned_size);
+    length_ = static_cast<size_t>(region.size);
+  }
+
+  data_ = static_cast<uint8*>(mmap(NULL,
+                                   map_size,
+                                   PROT_READ,
+                                   MAP_SHARED,
+                                   file_.GetPlatformFile(),
+                                   map_start));
+  if (data_ == MAP_FAILED) {
+    DPLOG(ERROR) << "mmap " << file_.GetPlatformFile();
     return false;
   }
-  length_ = file_stat.st_size;
 
-  data_ = static_cast<uint8*>(
-      mmap(NULL, length_, PROT_READ, MAP_SHARED, file_.GetPlatformFile(), 0));
-  if (data_ == MAP_FAILED)
-    DPLOG(ERROR) << "mmap " << file_.GetPlatformFile();
-
-  return data_ != MAP_FAILED;
+  data_ += data_offset;
+  return true;
 }
 
 void MemoryMappedFile::CloseHandles() {
   ThreadRestrictions::AssertIOAllowed();
 
   if (data_ != NULL)
     munmap(data_, length_);
   file_.Close();
 
   data_ = NULL;
   length_ = 0;
 }
 
 }  // namespace base