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  * aPtr MUST be aligned at a sizeof(uintptr_t) boundary.
  33  * Only an even number of sizeof(uintptr_t) bytes are overwritten, the last
  34  * few bytes (if any) is not overwritten.
  35  */
  36 inline void mozWritePoison(void* aPtr, size_t aSize) {
  37   const uintptr_t POISON = mozPoisonValue();
  38   char* p = (char*)aPtr;
  39   char* limit = p + (aSize & ~(sizeof(uintptr_t) - 1));
  40   MOZ_ASSERT(aSize >= sizeof(uintptr_t), "poisoning this object has no effect");
  41   for (; p < limit; p += sizeof(uintptr_t)) {
  42     memcpy(p, &POISON, sizeof(POISON));
  43   }
  44 }
  45
  46 /**
  47  * Initialize the poison value.
  48  * This should only be called once.
  49  */
  50 extern MFBT_API void mozPoisonValueInit();
  51
  52 /* Values annotated by CrashReporter */
  53 extern MFBT_DATA uintptr_t gMozillaPoisonBase;
  54 extern MFBT_DATA uintptr_t gMozillaPoisonSize;
  55
  56 MOZ_END_EXTERN_C
  57
  58 #if defined(__cplusplus)
  59
  60 namespace mozilla {
  61
  62 /**
  63  * A version of CorruptionCanary that is suitable as a member of objects that
  64  * are statically allocated.
  65  */
  66 class CorruptionCanaryForStatics {
  67  public:
  68   constexpr CorruptionCanaryForStatics() : mValue(kCanarySet) {}
  69
  70   // This is required to avoid static constructor bloat.
  71   ~CorruptionCanaryForStatics() = default;
  72
  73   void Check() const {
  74     if (mValue != kCanarySet) {
  75       MOZ_CRASH("Canary check failed, check lifetime");
  76     }
  77   }
  78
  79  protected:
  80   uintptr_t mValue;
  81
  82  private:
  83   static const uintptr_t kCanarySet = 0x0f0b0f0b;
  84 };
  85
  86 /**
  87  * This class is designed to cause crashes when various kinds of memory
  88  * corruption are observed. For instance, let's say we have a class C where we
  89  * suspect out-of-bounds writes to some members.  We can insert a member of type
  90  * Poison near the members we suspect are being corrupted by out-of-bounds
  91  * writes.  Or perhaps we have a class K we suspect is subject to use-after-free
  92  * violations, in which case it doesn't particularly matter where in the class
  93  * we add the member of type Poison.
  94  *
  95  * In either case, we then insert calls to Check() throughout the code.  Doing
  96  * so enables us to narrow down the location where the corruption is occurring.
  97  * A pleasant side-effect of these additional Check() calls is that crash
  98  * signatures may become more regular, as crashes will ideally occur
  99  * consolidated at the point of a Check(), rather than scattered about at
 100  * various uses of the corrupted memory.
 101  */
 102 class CorruptionCanary : public CorruptionCanaryForStatics {
 103  public:
 104   constexpr CorruptionCanary() = default;
 105
 106   ~CorruptionCanary() {
 107     Check();
 108     mValue = mozPoisonValue();
 109   }
 110 };
 111
 112 }  // namespace mozilla
 113
 114 #endif
 115
 116 #endif /* mozilla_Poison_h */