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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this file,
* You can obtain one at */
#include "mozilla/ipc/SerializedStructuredCloneBuffer.h"
#include "js/StructuredClone.h"
namespace IPC {
void ParamTraits<JSStructuredCloneData>::Write(MessageWriter* aWriter,
const paramType& aParam) {
MOZ_ASSERT(!(aParam.Size() % sizeof(uint64_t)));
// We can only construct shared memory regions up to 4Gb in size, making that
// the maximum possible JSStructuredCloneData size.
mozilla::CheckedUint32 size = aParam.Size();
if (!size.isValid()) {
aWriter->FatalError("JSStructuredCloneData over 4Gb in size");
WriteParam(aWriter, size.value());
MessageBufferWriter bufWriter(aWriter, size.value());
aParam.ForEachDataChunk([&](const char* aData, size_t aSize) {
return bufWriter.WriteBytes(aData, aSize);
bool ParamTraits<JSStructuredCloneData>::Read(MessageReader* aReader,
paramType* aResult) {
uint32_t length = 0;
if (!ReadParam(aReader, &length)) {
aReader->FatalError("JSStructuredCloneData length read failed");
return false;
MOZ_ASSERT(!(length % sizeof(uint64_t)));
// Borrowing is not suitable to use for IPC to hand out data because we often
// want to store the data somewhere for processing after IPC has released the
// underlying buffers.
// This deserializer previously used a mechanism to transfer ownership over
// the underlying buffers from IPC into the JSStructuredCloneData. This was
// removed when support for serializing over shared memory was added, as the
// benefit for avoiding copies was limited due to it only functioning for
// buffers under 64k in size (as larger buffers would be serialized using
// shared memory), and it added substantial complexity to the BufferList type
// and the IPC serialization layer due to things like buffer alignment. This
// can be revisited in the future if it turns out to be a noticable
// performance regression. (bug 1783242)
mozilla::BufferList<js::SystemAllocPolicy> buffers(0, 0, 4096);
MessageBufferReader bufReader(aReader, length);
uint32_t read = 0;
while (read < length) {
size_t bufLen;
char* buf = buffers.AllocateBytes(length - read, &bufLen);
if (!buf) {
// Would be nice to allow actor to control behaviour here (bug 1784307)
NS_ABORT_OOM(length - read);
return false;
if (!bufReader.ReadBytesInto(buf, bufLen)) {
aReader->FatalError("JSStructuredCloneData ReadBytesInto failed");
return false;
read += bufLen;
MOZ_ASSERT(read == length);
*aResult = JSStructuredCloneData(
std::move(buffers), JS::StructuredCloneScope::DifferentProcess,
return true;
} // namespace IPC