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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 */
* Memory poisoning.
#ifndef util_Poison_h
#define util_Poison_h
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/MemoryChecking.h"
#include <algorithm> // std::min
#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include "jstypes.h"
#include "js/Value.h"
#include "util/DiagnosticAssertions.h"
* Allow extra GC poisoning to be enabled in crash-diagnostics and zeal
* builds. Except in debug builds, this must be enabled by setting the
* JSGC_EXTRA_POISONING environment variable.
#if defined(JS_CRASH_DIAGNOSTICS) || defined(JS_GC_ZEAL)
namespace mozilla {
* Set the first |aNElem| T elements in |aDst| to |aSrc|.
template <typename T>
static MOZ_ALWAYS_INLINE void PodSet(T* aDst, const T& aSrc, size_t aNElem) {
for (const T* dstend = aDst + aNElem; aDst < dstend; ++aDst) {
*aDst = aSrc;
} /* namespace mozilla */
* Patterns used by SpiderMonkey to overwrite unused memory. If you are
* accessing an object with one of these pattern, you probably have a dangling
* pointer. These values should be odd, see the comment in IsThingPoisoned.
* Note: new patterns should also be added to the array in IsThingPoisoned!
* We try to keep our IRC bot, mrgiggles, up to date with these and other
* patterns:
const uint8_t JS_FRESH_NURSERY_PATTERN = 0x2F;
const uint8_t JS_SWEPT_NURSERY_PATTERN = 0x2B;
const uint8_t JS_FRESH_TENURED_PATTERN = 0x4F;
const uint8_t JS_MOVED_TENURED_PATTERN = 0x49;
const uint8_t JS_SWEPT_TENURED_PATTERN = 0x4B;
const uint8_t JS_FREED_HEAP_PTR_PATTERN = 0x6B;
const uint8_t JS_FREED_CHUNK_PATTERN = 0x8B;
const uint8_t JS_FREED_ARENA_PATTERN = 0x9B;
const uint8_t JS_SWEPT_TI_PATTERN = 0x6F;
const uint8_t JS_FRESH_MARK_STACK_PATTERN = 0x9F;
const uint8_t JS_RESET_VALUE_PATTERN = 0xBB;
const uint8_t JS_OOB_PARSE_NODE_PATTERN = 0xFF;
const uint8_t JS_LIFO_UNDEFINED_PATTERN = 0xcd;
// Even ones
* Ensure JS_SWEPT_CODE_PATTERN is a byte pattern that will crash immediately
* when executed, so either an undefined instruction or an instruction that's
* illegal in user mode.
#if defined(JS_CODEGEN_X86) || defined(JS_CODEGEN_X64) || \
defined(JS_CODEGEN_NONE) || defined(JS_CODEGEN_WASM32)
# define JS_SWEPT_CODE_PATTERN 0xED // IN instruction, crashes in user mode.
#elif defined(JS_CODEGEN_ARM) || defined(JS_CODEGEN_ARM64)
# define JS_SWEPT_CODE_PATTERN 0xA3 // undefined instruction
#elif defined(JS_CODEGEN_MIPS32) || defined(JS_CODEGEN_MIPS64)
# define JS_SWEPT_CODE_PATTERN 0x01 // undefined instruction
#elif defined(JS_CODEGEN_LOONG64)
# define JS_SWEPT_CODE_PATTERN 0x01 // undefined instruction
# error "JS_SWEPT_CODE_PATTERN not defined for this platform"
enum class MemCheckKind : uint8_t {
// Marks a region as poisoned. Memory sanitizers like ASan will crash when
// accessing it (both reads and writes).
// Marks a region as having undefined contents. In ASan builds this just
// unpoisons the memory. MSan and Valgrind can also use this to find
// reads of uninitialized memory.
static MOZ_ALWAYS_INLINE void SetMemCheckKind(void* ptr, size_t bytes,
MemCheckKind kind) {
switch (kind) {
case MemCheckKind::MakeUndefined:
case MemCheckKind::MakeNoAccess:
MOZ_CRASH("Invalid kind");
namespace js {
static inline void PoisonImpl(void* ptr, uint8_t value, size_t num) {
// Without a valid Value tag, a poisoned Value may look like a valid
// floating point number. To ensure that we crash more readily when
// observing a poisoned Value, we make the poison an invalid ObjectValue.
// Unfortunately, this adds about 2% more overhead, so we can only enable
// it in debug.
#if defined(DEBUG)
if (!num) {
uintptr_t poison;
memset(&poison, value, sizeof(poison));
# if defined(JS_PUNBOX64)
poison = poison & ((uintptr_t(1) << JSVAL_TAG_SHIFT) - 1);
# endif
JS::Value v = js::PoisonedObjectValue(poison);
# if defined(JS_NUNBOX32)
// On 32-bit arch, `ptr` is 4 bytes aligned, and it's less than
// `sizeof(JS::Value)` == 8 bytes.
// `mozilla::PodSet` with `v` requires the pointer to be 8 bytes aligned if
// `value_count > 0`.
// If the pointer isn't 8 bytes aligned, fill the leading 1-4 bytes
// separately here, so that either the pointer is 8 bytes aligned, or
// we have no more bytes to fill.
uintptr_t begin_count = std::min(num, uintptr_t(ptr) % sizeof(JS::Value));
if (begin_count) {
uint8_t* begin = static_cast<uint8_t*>(ptr);
mozilla::PodSet(begin, value, begin_count);
ptr = begin + begin_count;
num -= begin_count;
if (!num) {
# endif
MOZ_ASSERT(uintptr_t(ptr) % sizeof(JS::Value) == 0);
size_t value_count = num / sizeof(v);
size_t byte_count = num % sizeof(v);
mozilla::PodSet(reinterpret_cast<JS::Value*>(ptr), v, value_count);
if (byte_count) {
uint8_t* bytes = static_cast<uint8_t*>(ptr);
uint8_t* end = bytes + num;
mozilla::PodSet(end - byte_count, value, byte_count);
#else // !DEBUG
memset(ptr, value, num);
#endif // !DEBUG
// Unconditionally poison a region on memory.
static inline void AlwaysPoison(void* ptr, uint8_t value, size_t num,
MemCheckKind kind) {
PoisonImpl(ptr, value, num);
SetMemCheckKind(ptr, num, kind);
extern bool gExtraPoisoningEnabled;
// Conditionally poison a region of memory in debug builds and nightly builds
// when enabled by setting the JSGC_EXTRA_POISONING environment variable. Used
// by the GC in places where poisoning has a performance impact.
static inline void Poison(void* ptr, uint8_t value, size_t num,
MemCheckKind kind) {
if (js::gExtraPoisoningEnabled) {
PoisonImpl(ptr, value, num);
SetMemCheckKind(ptr, num, kind);
// Poison a region of memory in debug builds. Can be disabled by setting the
// JSGC_EXTRA_POISONING environment variable.
static inline void DebugOnlyPoison(void* ptr, uint8_t value, size_t num,
MemCheckKind kind) {
#if defined(DEBUG)
Poison(ptr, value, num, kind);
SetMemCheckKind(ptr, num, kind);
} // namespace js
#endif /* util_Poison_h */