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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:
* Copyright 2021 Mozilla Foundation
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#ifndef wasm_memory_h
#define wasm_memory_h
#include "mozilla/CheckedInt.h"
#include "mozilla/Maybe.h"
#include <stdint.h>
#include "js/Value.h"
#include "vm/NativeObject.h"
#include "wasm/WasmConstants.h"
#include "wasm/WasmValType.h"
namespace js {
namespace wasm {
// Limits are parameterized by an IndexType which is used to index the
// underlying resource (either a Memory or a Table). Tables are restricted to
// I32, while memories may use I64 when memory64 is enabled.
enum class IndexType : uint8_t { I32, I64 };
inline ValType ToValType(IndexType it) {
return it == IndexType::I64 ? ValType::I64 : ValType::I32;
extern bool ToIndexType(JSContext* cx, HandleValue value, IndexType* indexType);
extern const char* ToString(IndexType indexType);
// Pages is a typed unit representing a multiple of wasm::PageSize. We
// generally use pages as the unit of length when representing linear memory
// lengths so as to avoid overflow when the specified initial or maximum pages
// would overflow the native word size.
// Modules may specify pages up to 2^48 inclusive and so Pages is 64-bit on all
// platforms.
// We represent byte lengths using the native word size, as it is assumed that
// consumers of this API will only need byte lengths once it is time to
// allocate memory, at which point the pages will be checked against the
// implementation limits `MaxMemoryPages()` and will then be guaranteed to
// fit in a native word.
struct Pages {
// Pages are specified by limit fields, which in general may be up to 2^48,
// so we must use uint64_t here.
uint64_t value_;
constexpr Pages() : value_(0) {}
constexpr explicit Pages(uint64_t value) : value_(value) {}
// Get the wrapped page value. Only use this if you must, prefer to use or
// add new APIs to Page.
uint64_t value() const { return value_; }
// Converts from a byte length to pages, assuming that the length is an
// exact multiple of the page size.
static Pages fromByteLengthExact(size_t byteLength) {
MOZ_ASSERT(byteLength % PageSize == 0);
return Pages(byteLength / PageSize);
// Return whether the page length may overflow when converted to a byte
// length in the native word size.
bool hasByteLength() const {
mozilla::CheckedInt<size_t> length(value_);
length *= PageSize;
return length.isValid();
// Converts from pages to byte length in the native word size. Users must
// check for overflow, or be assured else-how that overflow cannot happen.
size_t byteLength() const {
mozilla::CheckedInt<size_t> length(value_);
length *= PageSize;
return length.value();
// Increment this pages by delta and return whether the resulting value
// did not overflow. If there is no overflow, then this is set to the
// resulting value.
bool checkedIncrement(Pages delta) {
mozilla::CheckedInt<uint64_t> newValue = value_;
newValue += delta.value_;
if (!newValue.isValid()) {
return false;
value_ = newValue.value();
return true;
// Implement pass-through comparison operators so that Pages can be compared.
bool operator==(Pages other) const { return value_ == other.value_; }
bool operator!=(Pages other) const { return value_ != other.value_; }
bool operator<=(Pages other) const { return value_ <= other.value_; }
bool operator<(Pages other) const { return value_ < other.value_; }
bool operator>=(Pages other) const { return value_ >= other.value_; }
bool operator>(Pages other) const { return value_ > other.value_; }
// The largest number of pages the application can request.
extern Pages MaxMemoryPages(IndexType t);
// The byte value of MaxMemoryPages(t).
static inline size_t MaxMemoryBytes(IndexType t) {
return MaxMemoryPages(t).byteLength();
// A value at least as large as MaxMemoryBytes(t) representing the largest valid
// bounds check limit on the system. (It can be larger than MaxMemoryBytes()
// because bounds check limits are rounded up to fit formal requirements on some
// platforms. Also see ComputeMappedSize().)
extern size_t MaxMemoryBoundsCheckLimit(IndexType t);
static inline uint64_t MaxMemoryLimitField(IndexType indexType) {
return indexType == IndexType::I32 ? MaxMemory32LimitField
: MaxMemory64LimitField;
// Compute the 'clamped' maximum size of a memory. See
// 'WASM Linear Memory structure' in ArrayBufferObject.cpp for background.
extern Pages ClampedMaxPages(IndexType t, Pages initialPages,
const mozilla::Maybe<Pages>& sourceMaxPages,
bool useHugeMemory);
// For a given WebAssembly/asm.js 'clamped' max pages, return the number of
// bytes to map which will necessarily be a multiple of the system page size and
// greater than clampedMaxPages in bytes. See "Wasm Linear Memory Structure" in
// vm/ArrayBufferObject.cpp.
extern size_t ComputeMappedSize(Pages clampedMaxPages);
extern size_t GetMaxOffsetGuardLimit(bool hugeMemory);
// Return whether the given immediate satisfies the constraints of the platform.
extern bool IsValidBoundsCheckImmediate(uint32_t i);
// Return whether the given immediate is valid on arm.
extern bool IsValidARMImmediate(uint32_t i);
// Return the next higher valid immediate that satisfies the constraints of the
// platform.
extern uint64_t RoundUpToNextValidBoundsCheckImmediate(uint64_t i);
// Return the next higher valid immediate for arm.
extern uint64_t RoundUpToNextValidARMImmediate(uint64_t i);
// On WASM_SUPPORTS_HUGE_MEMORY platforms, every asm.js or WebAssembly 32-bit
// memory unconditionally allocates a huge region of virtual memory of size
// wasm::HugeMappedSize. This allows all memory resizing to work without
// reallocation and provides enough guard space for most offsets to be folded
// into memory accesses. See "Linear memory addresses and bounds checking" in
// wasm/WasmMemory.cpp for more information.
// Reserve 4GiB to support any i32 index.
static const uint64_t HugeIndexRange = uint64_t(UINT32_MAX) + 1;
// Reserve 32MiB to support most offset immediates. Any immediate that is over
// this will require a bounds check to be emitted. 32MiB was chosen to
// generously cover the max offset immediate, 20MiB, found in a corpus of wasm
// modules.
static const uint64_t HugeOffsetGuardLimit = 1 << 25;
// Reserve a wasm page (64KiB) to support slop on unaligned accesses.
static const uint64_t HugeUnalignedGuardPage = PageSize;
// Compute the total memory reservation.
static const uint64_t HugeMappedSize =
HugeIndexRange + HugeOffsetGuardLimit + HugeUnalignedGuardPage;
// Try to keep the memory reservation aligned to the wasm page size. This
// ensures that it's aligned to the system page size.
static_assert(HugeMappedSize % PageSize == 0);
// The size of the guard page for non huge-memories.
static const size_t GuardSize = PageSize;
// The size of the guard page that included NULL pointer. Reserve a smallest
// range for typical hardware, to catch near NULL pointer accesses, e.g.
// for a structure fields operations.
static const size_t NullPtrGuardSize = 4096;
// Check if a range of wasm memory is within bounds, specified as byte offset
// and length (using 32-bit indices). Omits one check by converting from
// uint32_t to uint64_t, at which point overflow cannot occur.
static inline bool MemoryBoundsCheck(uint32_t offset, uint32_t len,
size_t memLen) {
uint64_t offsetLimit = uint64_t(offset) + uint64_t(len);
return offsetLimit <= memLen;
// Check if a range of wasm memory is within bounds, specified as byte offset
// and length (using 64-bit indices).
static inline bool MemoryBoundsCheck(uint64_t offset, uint64_t len,
size_t memLen) {
uint64_t offsetLimit = offset + len;
bool didOverflow = offsetLimit < offset;
bool tooLong = memLen < offsetLimit;
return !didOverflow && !tooLong;
} // namespace wasm
} // namespace js
#endif // wasm_memory_h