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/*
* Copyright (C) 2024 Apple Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#pragma once
#include "EnsureStillAliveHere.h"
namespace JSC {
class JSCell;
// This class is used return data owned by a JSCell. Consider:
// int foo(JSString* jsString)
// {
// String& string = jsString->value(globalObject);
// // Do something that triggers a GC (e.g. any object allocation)
// return string.length();
// }
// The C++ compiler is technically free to "drop" the last reference to jsString
// after we call value. Since our GC relies on live C++ values still existing on
// the stack when we trigger a GC we could collect jsString freeing string and
// causing a UAF. This class helps avoid that problem by forcing the C++ compiler
// to keep a reference to the owner GCed object of our data while being as
// unobtrusive as possible. That said there are a few caveats for callers:
// 1) **NEVER** do:
// Data& data = foo->getData();
// or:
// Data* data = foo->getData();
// this will **NOT** prevent the GC from collecting foo before data goes
// out of scope.
//
// 2) The preferred way to call a function returning a GCOwnedDataScope is:
// auto data = foo->getData();
// this will ensure foo exists long enough prevent a GC before data goes
// out of scope.
//
//
// 3) It's ok to bind to / return a retained value:
// Data data = foo->getData();
// or:
// return foo->getData();
// These aren't ideal as they trigger a retain and maybe release on data
// but this can't be avoided in some cases.
template<typename T>
struct GCOwnedDataScope {
GCOwnedDataScope(const JSCell* cell, T value)
: owner(cell)
, data(value)
{ }
~GCOwnedDataScope() { ensureStillAliveHere(owner); }
operator const T() const { return data; }
operator T() { return data; }
std::remove_reference_t<T>* operator->() requires (!std::is_pointer_v<T>) { return &data; }
const std::remove_reference_t<T>* operator->() const requires (!std::is_pointer_v<T>) { return &data; }
T operator->() requires (std::is_pointer_v<T>) { return data; }
const T operator->() const requires (std::is_pointer_v<T>) { return data; }
auto operator[](unsigned index) const { return data[index]; }
// Convenience conversion for String -> StringView
operator StringView() const requires (std::is_same_v<std::decay_t<T>, String>) { return data; }
const JSCell* owner;
T data;
};
}
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