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       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             : 
      20             : MOZ_BEGIN_EXTERN_C
      21             : 
      22             : extern MFBT_DATA uintptr_t gMozillaPoisonValue;
      23             : 
      24             : /**
      25             :  * @return the poison value.
      26             :  */
      27      106219 : inline uintptr_t mozPoisonValue()
      28             : {
      29      106219 :   return gMozillaPoisonValue;
      30             : }
      31             : 
      32             : /**
      33             :  * Overwrite the memory block of aSize bytes at aPtr with the poison value.
      34             :  * aPtr MUST be aligned at a sizeof(uintptr_t) boundary.
      35             :  * Only an even number of sizeof(uintptr_t) bytes are overwritten, the last
      36             :  * few bytes (if any) is not overwritten.
      37             :  */
      38        6867 : inline void mozWritePoison(void* aPtr, size_t aSize)
      39             : {
      40        6867 :   const uintptr_t POISON = mozPoisonValue();
      41        6867 :   char* p = (char*)aPtr;
      42        6867 :   char* limit = p + aSize;
      43        6867 :   MOZ_ASSERT((uintptr_t)aPtr % sizeof(uintptr_t) == 0, "bad alignment");
      44        6867 :   MOZ_ASSERT(aSize >= sizeof(uintptr_t), "poisoning this object has no effect");
      45      228419 :   for (; p < limit; p += sizeof(uintptr_t)) {
      46      110776 :     *((uintptr_t*)p) = POISON;
      47             :   }
      48        6867 : }
      49             : 
      50             : /**
      51             :  * Initialize the poison value.
      52             :  * This should only be called once.
      53             :  */
      54             : extern MFBT_API void mozPoisonValueInit();
      55             : 
      56             : /* Values annotated by CrashReporter */
      57             : extern MFBT_DATA uintptr_t gMozillaPoisonBase;
      58             : extern MFBT_DATA uintptr_t gMozillaPoisonSize;
      59             : 
      60             : MOZ_END_EXTERN_C
      61             : 
      62             : #if defined(__cplusplus)
      63             : 
      64             : namespace mozilla {
      65             : 
      66             : /**
      67             :  * This class is designed to cause crashes when various kinds of memory
      68             :  * corruption are observed. For instance, let's say we have a class C where we
      69             :  * suspect out-of-bounds writes to some members.  We can insert a member of type
      70             :  * Poison near the members we suspect are being corrupted by out-of-bounds
      71             :  * writes.  Or perhaps we have a class K we suspect is subject to use-after-free
      72             :  * violations, in which case it doesn't particularly matter where in the class
      73             :  * we add the member of type Poison.
      74             :  *
      75             :  * In either case, we then insert calls to Check() throughout the code.  Doing
      76             :  * so enables us to narrow down the location where the corruption is occurring.
      77             :  * A pleasant side-effect of these additional Check() calls is that crash
      78             :  * signatures may become more regular, as crashes will ideally occur
      79             :  * consolidated at the point of a Check(), rather than scattered about at
      80             :  * various uses of the corrupted memory.
      81             :  */
      82             : class CorruptionCanary {
      83             : public:
      84        1189 :   CorruptionCanary() {
      85        1189 :     mValue = kCanarySet;
      86        1189 :   }
      87             : 
      88        2302 :   ~CorruptionCanary() {
      89        1151 :     Check();
      90        1151 :     mValue = mozPoisonValue();
      91        1151 :   }
      92             : 
      93        7038 :   void Check() const {
      94        7038 :     if (mValue != kCanarySet) {
      95           0 :       MOZ_CRASH("Canary check failed, check lifetime");
      96             :     }
      97        7038 :   }
      98             : 
      99             : private:
     100             :   static const uintptr_t kCanarySet = 0x0f0b0f0b;
     101             :   uintptr_t mValue;
     102             : };
     103             : 
     104             : } // mozilla
     105             : 
     106             : #endif
     107             : 
     108             : #endif /* mozilla_Poison_h */