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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* 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 http://mozilla.org/MPL/2.0/. */
/*
* 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>
MOZ_BEGIN_EXTERN_C
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.
* aPtr MUST be aligned at a sizeof(uintptr_t) boundary.
* Only an even number of sizeof(uintptr_t) bytes are overwritten, the last
* few bytes (if any) is not overwritten.
*/
inline void mozWritePoison(void* aPtr, size_t aSize)
{
const uintptr_t POISON = mozPoisonValue();
char* p = (char*)aPtr;
char* limit = p + aSize;
MOZ_ASSERT((uintptr_t)aPtr % sizeof(uintptr_t) == 0, "bad alignment");
MOZ_ASSERT(aSize >= sizeof(uintptr_t), "poisoning this object has no effect");
for (; p < limit; p += sizeof(uintptr_t)) {
*((uintptr_t*)p) = POISON;
}
}
/**
* Initialize the poison value.
* This should only be called once.
*/
extern MFBT_API void mozPoisonValueInit();
/* Values annotated by CrashReporter */
extern MFBT_DATA uintptr_t gMozillaPoisonBase;
extern MFBT_DATA uintptr_t gMozillaPoisonSize;
MOZ_END_EXTERN_C
#if defined(__cplusplus)
namespace mozilla {
/**
* 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:
CorruptionCanary() {
mValue = kCanarySet;
}
~CorruptionCanary() {
Check();
mValue = mozPoisonValue();
}
void Check() const {
if (mValue != kCanarySet) {
MOZ_CRASH("Canary check failed, check lifetime");
}
}
private:
static const uintptr_t kCanarySet = 0x0f0b0f0b;
uintptr_t mValue;
};
} // mozilla
#endif
#endif /* mozilla_Poison_h */
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