mfbt/Poison.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /*
   8  * A poison value that can be used to fill a memory space with
   9  * an address that leads to a safe crash when dereferenced.
  10  */
  11
  12 #ifndef mozilla_Poison_h
  13 #define mozilla_Poison_h
  14
  15 #include "mozilla/Assertions.h"
  16 #include "mozilla/Types.h"
  17
  18 #include <stdint.h>
  19 #include <string.h>
  20
  21 MOZ_BEGIN_EXTERN_C
  22
  23 extern MFBT_DATA uintptr_t gMozillaPoisonValue;
  24
  25 /**
  26  * @return the poison value.
  27  */
  28 inline uintptr_t mozPoisonValue() { return gMozillaPoisonValue; }
  29
  30 /**
  31  * Overwrite the memory block of aSize bytes at aPtr with the poison value.
  32  * Only a multiple of sizeof(uintptr_t) bytes are overwritten, the last
  33  * few bytes (if any) are not overwritten.
  34  */
  35 inline void mozWritePoison(void* aPtr, size_t aSize) {
  36   const uintptr_t POISON = mozPoisonValue();
  37   char* p = (char*)aPtr;
  38   char* limit = p + (aSize & ~(sizeof(uintptr_t) - 1));
  39   MOZ_ASSERT(aSize >= sizeof(uintptr_t), "poisoning this object has no effect");
  40   for (; p < limit; p += sizeof(uintptr_t)) {
  41     memcpy(p, &POISON, sizeof(POISON));
  42   }
  43 }
  44
  45 /* Values annotated by CrashReporter */
  46 extern MFBT_DATA uintptr_t gMozillaPoisonBase;
  47 extern MFBT_DATA uintptr_t gMozillaPoisonSize;
  48
  49 MOZ_END_EXTERN_C
  50
  51 #if defined(__cplusplus)
  52
  53 namespace mozilla {
  54
  55 /**
  56  * A version of CorruptionCanary that is suitable as a member of objects that
  57  * are statically allocated.
  58  */
  59 class CorruptionCanaryForStatics {
  60  public:
  61   constexpr CorruptionCanaryForStatics() : mValue(kCanarySet) {}
  62
  63   // This is required to avoid static constructor bloat.
  64   ~CorruptionCanaryForStatics() = default;
  65
  66   void Check() const {
  67     if (mValue != kCanarySet) {
  68       MOZ_CRASH("Canary check failed, check lifetime");
  69     }
  70   }
  71
  72  protected:
  73   uintptr_t mValue;
  74
  75  private:
  76   static const uintptr_t kCanarySet = 0x0f0b0f0b;
  77 };
  78
  79 /**
  80  * This class is designed to cause crashes when various kinds of memory
  81  * corruption are observed. For instance, let's say we have a class C where we
  82  * suspect out-of-bounds writes to some members.  We can insert a member of type
  83  * Poison near the members we suspect are being corrupted by out-of-bounds
  84  * writes.  Or perhaps we have a class K we suspect is subject to use-after-free
  85  * violations, in which case it doesn't particularly matter where in the class
  86  * we add the member of type Poison.
  87  *
  88  * In either case, we then insert calls to Check() throughout the code.  Doing
  89  * so enables us to narrow down the location where the corruption is occurring.
  90  * A pleasant side-effect of these additional Check() calls is that crash
  91  * signatures may become more regular, as crashes will ideally occur
  92  * consolidated at the point of a Check(), rather than scattered about at
  93  * various uses of the corrupted memory.
  94  */
  95 class CorruptionCanary : public CorruptionCanaryForStatics {
  96  public:
  97   constexpr CorruptionCanary() = default;
  98
  99   ~CorruptionCanary() {
 100     Check();
 101     mValue = mozPoisonValue();
 102   }
 103 };
 104
 105 }  // namespace mozilla
 106
 107 #endif
 108
 109 #endif /* mozilla_Poison_h */