C++ EH Exception（0xe06d7363）----抛出过程

C++ EH Exception是Windows系统VC++里对c++语言的throw的分类和定义，它的代码就是0xe06d7363。在VC++里其本质也是SEH结构化异常机制。在我们分析用户崩溃的例子中经常会遇到它。一般情况下，遇到它，是我们代码里用throw抛出异常后没有处理导致程序崩溃。下面分析一下它的原理。

我们借助一段代码来跟踪和分析

class MyException
{
public:
    int nErr;
    char *szMessage;
public:
    MyException(void)
        :nErr()
        , szMessage(NULL)
    {

    }

    MyException(int nerr,char *szMess)
        :nErr(nerr)
        , szMessage(szMess)
    {

    }

    ~MyException(void)
    {

    }
};
int _tmain(int argc, _TCHAR* argv[])
{
    try
    {
        MyException me(1, "test exception");
        throw me;
    }
    catch (MyException me1)
    {
        printf("err=%s\n",me1.szMessage);
    }
}

将上述代码在VS2013里编译调试，转到汇编

:     try
    :     {
  mov         dword ptr [ebp-],
    :         MyException me(, "test exception");
0136179E  push        1367858h
013617A3  push
013617A5  lea         ecx,[ebp-1Ch]
013617A8  call        MyException::MyException (013610D2h)
013617AD  mov         dword ptr [ebp-104h],eax
013617B3  mov         byte ptr [ebp-],
    :         throw me;
013617B7  mov         eax,dword ptr [ebp-1Ch]
013617BA  mov         dword ptr [ebp-0FCh],eax
013617C0  mov         ecx,dword ptr [ebp-18h]
013617C3  mov         dword ptr [ebp-0F8h],ecx
013617C9  push        1369084h
013617CE  lea         edx,[ebp-0FCh]
013617D4  push        edx
013617D5  call        __CxxThrowException@8 (0136111Dh)
    :     }

我们可以看到，throw 首先通过下面的代码

013617B7  mov         eax,dword ptr [ebp-1Ch]
013617BA  mov         dword ptr [ebp-0FCh],eax
013617C0  mov         ecx,dword ptr [ebp-18h]
013617C3  mov         dword ptr [ebp-0F8h],ecx  

复制了一份异常对象MyException me，然后取了这份拷贝的地址作为参数传给了__CxxThrowException函数，同时将异常类型信息也传递了过去

013617CE  lea         edx,[ebp-0FCh]
013617D4  push        edx

接着调用了__CxxThrowException函数，我们进入看看

下面是改函数的代码

/////////////////////////////////////////////////////////////////////////////
//
// _CxxThrowException - implementation of 'throw'
//
// Description:
//      Builds the NT Exception record, and calls the NT runtime to initiate
//      exception processing.
//
//      Why is pThrowInfo defined as _ThrowInfo?  Because _ThrowInfo is secretly
//      snuck into the compiler, as is the prototype for _CxxThrowException, so
//      we have to use the same type to keep the compiler happy.
//
//      Another result of this is that _CRTIMP can't be used here.  Instead, we
//      synthesisze the -export directive below.
//
// Returns:
//      NEVER.  (until we implement resumable exceptions, that is)
//

// We want double underscore for CxxThrowException for ARM CE only
__declspec(noreturn) extern "C" void __stdcall
#if !defined(_M_ARM) || defined(_M_ARM_NT)
_CxxThrowException(
#else
__CxxThrowException(
#endif
        void*           pExceptionObject,   // The object thrown
        _ThrowInfo*     pThrowInfo          // Everything we need to know about it
) {
        EHTRACE_ENTER_FMT1("Throwing object @ 0x%p", pExceptionObject);

        static const EHExceptionRecord ExceptionTemplate = { // A generic exception record
            EH_EXCEPTION_NUMBER,            // Exception number
            EXCEPTION_NONCONTINUABLE,       // Exception flags (we don't do resume)
            NULL,                           // Additional record (none)
            NULL,                           // Address of exception (OS fills in)
            EH_EXCEPTION_PARAMETERS,        // Number of parameters
            {   EH_MAGIC_NUMBER1,           // Our version control magic number
                NULL,                       // pExceptionObject
                NULL,
#if _EH_RELATIVE_OFFSETS
                NULL                        // Image base of thrown object
#endif
            }                      // pThrowInfo
        };
        EHExceptionRecord ThisException = ExceptionTemplate;    // This exception

        ThrowInfo* pTI = (ThrowInfo*)pThrowInfo;
        if (pTI && (THROW_ISWINRT( (*pTI) ) ) )
        {
            ULONG_PTR *exceptionInfoPointer = *reinterpret_cast<ULONG_PTR**>(pExceptionObject);
            exceptionInfoPointer--; // The pointer to the ExceptionInfo structure is stored sizeof(void*) infront of each WinRT Exception Info.

            WINRTEXCEPTIONINFO** ppWei = reinterpret_cast<WINRTEXCEPTIONINFO**>(exceptionInfoPointer);
            pTI = (*ppWei)->throwInfo;

            (*ppWei)->PrepareThrow( ppWei );
        }

        //
        // Fill in the blanks:
        //
        ThisException.params.pExceptionObject = pExceptionObject;
        ThisException.params.pThrowInfo = pTI;
#if _EH_RELATIVE_OFFSETS
        PVOID ThrowImageBase = RtlPcToFileHeader((PVOID)pTI, &ThrowImageBase);
        ThisException.params.pThrowImageBase = ThrowImageBase;
#endif

        //
        // If the throw info indicates this throw is from a pure region,
        // set the magic number to the Pure one, so only a pure-region
        // catch will see it.
        //
        // Also use the Pure magic number on Win64 if we were unable to
        // determine an image base, since that was the old way to determine
        // a pure throw, before the TI_IsPure bit was added to the FuncInfo
        // attributes field.
        //
        if (pTI != NULL)
        {
            if (THROW_ISPURE(*pTI))
            {
                ThisException.params.magicNumber = EH_PURE_MAGIC_NUMBER1;
            }
#if _EH_RELATIVE_OFFSETS
            else if (ThrowImageBase == NULL)
            {
                ThisException.params.magicNumber = EH_PURE_MAGIC_NUMBER1;
            }
#endif
        }

        //
        // Hand it off to the OS:
        //

        EHTRACE_EXIT;
#if defined(_M_X64) && defined(_NTSUBSET_)
        RtlRaiseException( (PEXCEPTION_RECORD) &ThisException );
#else
        RaiseException( ThisException.ExceptionCode,
                        ThisException.ExceptionFlags,
                        ThisException.NumberParameters,
                        (PULONG_PTR)&ThisException.params );
#endif
}

可以看到，这个函数首先是创建了一个EHExceptionRecord 对象，其实对应的就是 SEH里的结构EXCEPTION_RECORD，并且给这个结构成员赋值。

在这里通过如下代码，0xe06d7363就赋值给ThisException.ExceptionCode

还有就是将ThrowInfo赋值给EHExceptionRecordThisException.params.pThrowInfo。_ThrowInfo 结构体定义如下：

typedef const struct _s__ThrowInfo
{
unsigned int attributes;
_PMFN pmfnUnwind;
int (__cdecl*pForwardCompat)(...);
_CatchableTypeArray *pCatachableTypeArray;
} _ThrowInfo;

结构体中重要的成员是_CatchableTypeArray。它包含了程序运行时抛出对象的类新信息(RTTI).
如果你的程序运行时抛出一个my_exception类型的对象，那么抛出的数据参数pCatchableTypeArray包含了两个重要子数据信息。一个是typeid(my_exception)，另外一个是typeid(std::exception)。

在我们的例子里

紧接着就调用RaiseException函数进入了异常的分发过程。

综上，在C++ EH Exception 的异常里，EXCEPTION_RECORD结构填充如下：

ExceptionAddress: 异常地址 76d018a2 (KERNELBASE!RaiseException+0x00000062)
   ExceptionCode: 异常代码 e06d7363 (C++ EH exception)
  ExceptionFlags: 标志 00000001
NumberParameters: 3 or 4 64位时是4
   Parameter[0]: 19930520//魔数
   Parameter[1]: 00d5f690// 抛出的异常对象指针
   Parameter[2]: 00989084// 异常对象类型信息
   Parameter[3]: 0000000010000000 // 64位下模块句柄HINSTANCE