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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 */
* A poison value that can be used to fill a memory space with
* an address that leads to a safe crash when dereferenced.
#ifndef mozilla_Poison_h
#define mozilla_Poison_h
#include "mozilla/Assertions.h"
#include "mozilla/Types.h"
#include <stdint.h>
#include <string.h>
extern MFBT_DATA uintptr_t gMozillaPoisonValue;
* @return the poison value.
inline uintptr_t mozPoisonValue() { return gMozillaPoisonValue; }
* Overwrite the memory block of aSize bytes at aPtr with the poison value.
* Only a multiple of sizeof(uintptr_t) bytes are overwritten, the last
* few bytes (if any) are not overwritten.
inline void mozWritePoison(void* aPtr, size_t aSize) {
const uintptr_t POISON = mozPoisonValue();
char* p = (char*)aPtr;
char* limit = p + (aSize & ~(sizeof(uintptr_t) - 1));
MOZ_ASSERT(aSize >= sizeof(uintptr_t), "poisoning this object has no effect");
for (; p < limit; p += sizeof(uintptr_t)) {
memcpy(p, &POISON, sizeof(POISON));
/* Values annotated by CrashReporter */
extern MFBT_DATA uintptr_t gMozillaPoisonBase;
extern MFBT_DATA uintptr_t gMozillaPoisonSize;
#if defined(__cplusplus)
namespace mozilla {
* A version of CorruptionCanary that is suitable as a member of objects that
* are statically allocated.
class CorruptionCanaryForStatics {
constexpr CorruptionCanaryForStatics() : mValue(kCanarySet) {}
// This is required to avoid static constructor bloat.
~CorruptionCanaryForStatics() = default;
void Check() const {
if (mValue != kCanarySet) {
MOZ_CRASH("Canary check failed, check lifetime");
uintptr_t mValue;
static const uintptr_t kCanarySet = 0x0f0b0f0b;
* This class is designed to cause crashes when various kinds of memory
* corruption are observed. For instance, let's say we have a class C where we
* suspect out-of-bounds writes to some members. We can insert a member of type
* Poison near the members we suspect are being corrupted by out-of-bounds
* writes. Or perhaps we have a class K we suspect is subject to use-after-free
* violations, in which case it doesn't particularly matter where in the class
* we add the member of type Poison.
* In either case, we then insert calls to Check() throughout the code. Doing
* so enables us to narrow down the location where the corruption is occurring.
* A pleasant side-effect of these additional Check() calls is that crash
* signatures may become more regular, as crashes will ideally occur
* consolidated at the point of a Check(), rather than scattered about at
* various uses of the corrupted memory.
class CorruptionCanary : public CorruptionCanaryForStatics {
constexpr CorruptionCanary() = default;
~CorruptionCanary() {
mValue = mozPoisonValue();
} // namespace mozilla
#endif /* mozilla_Poison_h */