逃跑陷阱/单步

Question

我在写一个跟踪其他程序执行的程序。我正在使用动态指令工具来跟踪x86的CMP指令的行为。

我使用Windows调试API来控制调试程序的行为。我使用“只调试此进程”标志启动程序，然后在主线程上设置陷阱标志。

然后我进入主调试循环：

bool cDebugger::ProcessNextDebugEvent(bool Verbose) 
{ 
    bool Result = true; 
    DEBUG_EVENT Event = { 0 }; 

    DWORD Status = DBG_CONTINUE; 

    if (!WaitForDebugEvent(&Event, INFINITE)) 
    { 
     _Reporter("Error: WaitForDebugEvent: " + to_string(GetLastError())); 
     return Result; 
    } 
    else 
    { 
     if (Event.dwDebugEventCode == CREATE_PROCESS_DEBUG_EVENT) 
     { 
      if (Verbose) 
       _Reporter("Created process: " + GetFilenameFromHandle(Event.u.CreateProcessInfo.hFile)); 
     } 
     else if (Event.dwDebugEventCode == LOAD_DLL_DEBUG_EVENT) 
     { 
      if (Verbose) 
       _Reporter("Dll: " + GetFilenameFromHandle(Event.u.LoadDll.hFile) + " loaded at: " + to_string((unsigned int)Event.u.LoadDll.lpBaseOfDll)); 

      _Dlls.insert(make_pair((unsigned int)Event.u.LoadDll.lpBaseOfDll, GetFilenameFromHandle(Event.u.LoadDll.hFile))); 
     } 
     else if (Event.dwDebugEventCode == CREATE_THREAD_DEBUG_EVENT) 
     { 
      if (Verbose) 
       _Reporter("Thread[" + to_string(Event.dwThreadId) + "] created at: " + to_string((unsigned int)Event.u.CreateThread.lpStartAddress)); 

      _Threads.push_back(Event.dwThreadId); 
     } 
     else if (Event.dwDebugEventCode == EXIT_THREAD_DEBUG_EVENT) 
     { 
      if (Verbose) 
       _Reporter("Thread[" + to_string(Event.dwThreadId) + "] exited with: " + to_string(Event.u.ExitThread.dwExitCode)); 

      auto It = std::find(_Threads.begin(), _Threads.end(), Event.dwThreadId); 

      if (It != _Threads.end()) 
       _Threads.erase(It); 
     } 
     else if (Event.dwDebugEventCode == UNLOAD_DLL_DEBUG_EVENT) 
     { 
      if (Verbose) 
       _Reporter("Dll " + _Dlls[(unsigned int)Event.u.UnloadDll.lpBaseOfDll] + " unloaded at : " + to_string((unsigned int)Event.u.UnloadDll.lpBaseOfDll)); 
     } 
     else if (Event.dwDebugEventCode == EXIT_PROCESS_DEBUG_EVENT) 
     { 
      if (Verbose) 
       _Reporter("Process exited with: " + to_string(Event.u.ExitProcess.dwExitCode)); 

      Result = false; 

      _Threads.clear(); 
     } 
     else if (Event.dwDebugEventCode == EXCEPTION_DEBUG_EVENT) 
     { 
      if (Event.u.Exception.ExceptionRecord.ExceptionCode == EXCEPTION_SINGLE_STEP) 
      { 
       Status = DBG_EXCEPTION_HANDLED; 
      } 
      else 
      { 
       Status = DBG_EXCEPTION_NOT_HANDLED; 
      } 
     } 

     for (size_t i = 0; i < _Threads.size(); i++) 
     { 
      HANDLE hThread = OpenThread(THREAD_ALL_ACCESS, FALSE, _Threads[i]); 

      if (hThread == NULL) 
      { 
       _Reporter("Error: Failed to open thread: " + to_string(GetLastError())); 
      } 
      else 
      { 
       CONTEXT ThreadContext = GetThreadContext(hThread); 

       ProcessStep(ThreadContext, hThread); 

       ThreadContext.EFlags |= 0x100; // Set trap flag. 
       SetThreadContext(hThread, ThreadContext); 

       CloseHandle(hThread); 
      } 
     } 

     if (!ContinueDebugEvent(Event.dwProcessId, Event.dwThreadId, Status)) 
     { 
      _Reporter("Error: ContinueDebugEvent: " + to_string(GetLastError())); 
     } 
    } 

    return Result; 
} 

正如你可以看到我遍历在函数结束时，所有的线程，以确保单步异常将触发每一个指令的每一个线程。但是有时执行似乎会“逃避”这个陷阱，通常会在下一次调试事件再次捕获之前执行数百万条指令。

我写了另一个小应用程序来测试我的程序的行为：

int main(int argc, char* argv[]) 
{ 

    //__asm int 3h 
    if (argc == 41234123) 
    { 
     printf("Got one\n"); 
    } 

    return 0; 
} 

示踪剂的预期输出应该是：

0xDEADBEEF CMP 1 41234123 

然而不知何故示踪剂不记录该指令（表明没有调试事件发生，并且没有设置陷阱标志）。

任何人都可以看到，如果我在我的调试循环中做错了什么？或者测试程序的哪种行为（加载dll）可能会对此负责？

Answer 1

这个问题与调用windows apis时代码进入内核空间有关。我的解决办法是设置测试程序的可执行部分来PAGE_GUARD页面保护：

SYSTEM_INFO Info; 
    GetSystemInfo(&Info); 

    DWORD StartAddress = (DWORD)Info.lpMinimumApplicationAddress; 
    DWORD StopAddress = (DWORD)Info.lpMaximumApplicationAddress; 
    DWORD PageSize = 0; 

    PageSize = Info.dwPageSize; 

    _Sections.clear(); 

    for (DWORD AddressPointer = StartAddress; AddressPointer < StopAddress; AddressPointer += PageSize) 
    { 
     MEMORY_BASIC_INFORMATION Buffer; 
     VirtualQueryEx(_Process.GetHandle(), (LPCVOID)AddressPointer, &Buffer, sizeof(Buffer)); 

     if (CheckBit(Buffer.Protect, 4) || CheckBit(Buffer.Protect, 5) || CheckBit(Buffer.Protect, 6) || CheckBit(Buffer.Protect, 7)) 
     { 
      if (Buffer.State == MEM_COMMIT) 
      { 
       _Sections.push_back(make_pair((unsigned int)Buffer.BaseAddress, (unsigned int)Buffer.RegionSize)); 
       AddressPointer = (unsigned int)Buffer.BaseAddress + (unsigned int)Buffer.RegionSize; 
      } 
     } 
    } 


void cDebugger::SetPageGuard() 
{ 
    for (size_t i = 0; i < _Sections.size(); i++) 
    { 
     DWORD Dummy; 
     VirtualProtectEx(_Process.GetHandle(), (LPVOID)_Sections[i].first, _Sections[i].second, PAGE_GUARD | PAGE_EXECUTE_READWRITE, &Dummy); 
    } 
} 

这样，我重新获得控制权，因为系统会触发一个EXCEPTION_GUARD_PAGE当执行返回到有人看守的页面。

if (Event.u.Exception.ExceptionRecord.ExceptionCode == EXCEPTION_SINGLE_STEP) 
      { 
       Status = DBG_CONTINUE; 
       if (!_Tracing) 
       { 
        HANDLE hThread = OpenThread(THREAD_ALL_ACCESS, FALSE, Event.dwThreadId); 
        CONTEXT ThreadContext = GetThreadContext(hThread); 

        if (ThreadContext.Eip == _EntryAddress) 
        { 
         ClearHardwareBreakpoint(0, hThread); 
         _Tracing = true; 
        } 

        CloseHandle(hThread); 
       } 

       SetPageGuard(); 

       _Guarded = true; 
      } 
      else if (Event.u.Exception.ExceptionRecord.ExceptionCode == EXCEPTION_BREAKPOINT) 
      { 
       Status = DBG_CONTINUE; 
      } 
      else if (Event.u.Exception.ExceptionRecord.ExceptionCode == EXCEPTION_GUARD_PAGE) 
      { 
       Status = DBG_CONTINUE; // fires when processor lands on guarded pages 
      } 
      else 
      { 
       Status = DBG_EXCEPTION_NOT_HANDLED; 
      } 

此解决方案并不完美。仍然有一些情况下执行仍然可以逃脱“陷阱”。但它解决了我最直接的问题（能够在我的测试程序中看到比较结果）。