Add offset support to BitstreamBuffer.

content/common/gpu/media/shared_memory_region.cc

Index: content/common/gpu/media/shared_memory_region.cc
diff --git a/content/common/gpu/media/shared_memory_region.cc b/content/common/gpu/media/shared_memory_region.cc
new file mode 100644
index 0000000000000000000000000000000000000000..972c4eee2d15dcf0d599185a50b715b8e36e1f21
--- /dev/null
+++ b/content/common/gpu/media/shared_memory_region.cc
@@ -0,0 +1,38 @@
+// Copyright (c) 2015 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/sys_info.h"
+#include "content/common/gpu/media/shared_memory_region.h"
+
+namespace content {
+
+SharedMemoryRegion::SharedMemoryRegion(const base::SharedMemoryHandle& handle,
+                                       off_t offset,
+                                       size_t size,
+                                       bool read_only)
+    : shm_(handle, read_only),
+      offset_(offset),
+      size_(size),
+      extra_size_(offset % base::SysInfo::VMAllocationGranularity()) {
+  DCHECK_LT(offset_, 0) << "Invalid offset: " << offset_;

[Pawel Osciak, 2016/01/05 06:22:51]

DCHECK_GE ?

[Owen Lin, 2016/01/06 03:45:46]

Done.

+}
+
+SharedMemoryRegion::SharedMemoryRegion(
+    const media::BitstreamBuffer& bitstream_buffer,
+    bool read_only)
+    : SharedMemoryRegion(bitstream_buffer.handle(),
+                         bitstream_buffer.offset(),
+                         bitstream_buffer.size(),
+                         read_only) {}
+
+bool SharedMemoryRegion::Map() {
+  return shm_.MapAt(offset_ - extra_size_, size_ + extra_size_);

[Pawel Osciak, 2016/01/05 06:22:51]

I'm wondering, if offset is < 0, then extra_size_ will be negative, but this
could possibly succeed, since we subtract a negative, so we will actually be
adding (it'd also require size < shared memory size, but still possible). Then
in memory() we'd be adding a negative, so subtracting and we may provide an
invalid pointer, to before mapped memory... That would be very unexpected for
Map() to succeed, but memory() to provide an invalid pointer.

Perhaps we should explicitly disallow offset < 0, maybe failing here if
extra_size_ < 0?

[Owen Lin, 2016/01/06 03:45:46]

Done.

+}
+
+void* SharedMemoryRegion::memory() {
+  int8_t* addr = reinterpret_cast<int8_t*>(shm_.memory());
+  return addr ? addr + extra_size_ : nullptr;
+}
+
+}  // namespace content