逃跑陷阱/單步

Question

我在寫一個跟蹤其他程序執行的程序。我正在使用動態指令工具來跟蹤x86的CMP指令的行爲。

我使用Windows調試API來控制調試程序的行爲。我使用「只調試此進程」標誌啓動程序，然後在主線程上設置陷阱標誌。

然後我進入主調試循環：

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歷在函數結束時，所有的線程，以確保單步異常將觸發每一個指令的每一個線程。但是有時執行似乎會「逃避」這個陷阱，通常會在下一次調試事件再次捕獲之前執行數百萬條指令。

我寫了另一個小應用程序來測試我的程序的行爲：

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

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

    return 0; 
} 

示蹤劑的預期輸出應該是：

0xDEADBEEF CMP 1 41234123 

然而不知何故示蹤劑不記錄該指令（表明沒有調試事件發生，並且沒有設置陷阱標誌）。

任何人都可以看到，如果我在我的調試循環中做錯了什麼？或者測試程序的哪種行爲（加載dll）可能會對此負責？

Answer 1

這個問題與調用windows apis時代碼進入內核空間有關。我的解決辦法是設置測試程序的可執行部分來PAGE_GUARD頁面保護：

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); 
    } 
} 

這樣，我重新獲得控制權，因爲系統會觸發一個EXCEPTION_GUARD_PAGE當執行返回到有人看守的頁面。

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; 
      } 

此解決方案並不完美。仍然有一些情況下執行仍然可以逃脫「陷阱」。但它解決了我最直接的問題（能夠在我的測試程序中看到比較結果）。