转义陷阱标志/单步
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【中文标题】转义陷阱标志/单步【英文标题】:Escaping trapflag/single step 【发布时间】:2014-09-05 20:11:34 【问题描述】:我正在编写一个跟踪其他程序执行的程序。我正在使用动态指令检测来跟踪 x86 的 CMP 指令的行为。
我正在使用 windows 调试 api 来控制被调试程序的行为。我使用“仅调试此进程”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;
如您所见,我在函数末尾循环遍历所有线程,以确保单步异常将在每个线程中的每条下一条指令上触发。 然而,有时执行似乎“逃脱”了这个陷阱,通常在被下一个调试事件再次捕获之前执行数百万条指令。
我编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;
跟踪器的预期输出应该是:
0xDEADBEEF CMP 1 41234123
但是不知何故,跟踪器没有记录这个指令(表明没有引发调试事件,并且没有设置陷阱标志)。
谁能看看我在调试循环中是否做错了什么?或者测试程序的哪种行为(加载 dll)可能对此负责?
【问题讨论】:
您确定编译器生成了CMP
指令,并为您期望的指令使用了编码?您在哪里打印您期望的输出的代码?
是的,我用反汇编程序仔细检查了一遍,以确保确实有一条 CMP 指令,而不是 TEST 或其他指令。它就在那里,它会被执行,只是我的追踪器看不到它。
【参考方案1】:
问题与调用windows api时代码进入内核空间有关。我的解决方案是将测试程序的可执行部分的页面保护设置为PAGE_GUARD:
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);
这样我可以重新获得控制权,因为当执行返回到受保护的页面时系统将触发 EXCEPTION_GUARD_PAGE。
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;
这个解决方案并不完美。在某些情况下,执行仍然可能逃脱“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)。
【讨论】:
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