如何用windbg调试因需加载的DLL

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参考技术A 点击文件就可,如果不能运行
1、打开电脑管家找到工具箱;
2、打开工具箱找到“电脑诊所”
3、打开电脑诊所后在右上角的搜索内输入dlL找到需要修复的文件后点击一键修复或者可以点击详情查看原因并点击立即修复。

[原]调试实战——使用windbg调试DLL卸载时的死锁

前言

最近我们的程序在退出时会卡住,调查发现是在卸载dll时死锁了。大概流程是这样的:我们的dll在加载的时候会创建一个工作线程,在卸载的时候,会设置退出标志并等待之前开启的工作线程结束。为了研究这个经典的死锁问题,写了一个模拟程序,用到的dump文件及示例代码参考附件。

{% note info %}

这也是几年前在项目中遇到的一个问题,我对之前的笔记进行了整理重新发布于此。

{% endnote %}

关键代码

主程序 WaitDllUnloadExe

//WaitDllUnloadExe.cpp
#include "stdafx.h"
#include "windows.h"

int _tmain(int argc, _TCHAR* argv[])
{
    HMODULE module = LoadLibraryA(".\DllUnload.dll");

    Sleep(5000);

    FreeLibrary(module);

    return 0;
}

DLL程序 DllUnload

// dllmain.cpp
#include "stdafx.h"
#include "process.h"
HANDLE g_hThread;
bool g_quit = false;
unsigned __stdcall procThread(void *)
{
    while ( !g_quit )
    {
        OutputDebugStringA("procThread running.
");
        Sleep(100);
    }

    OutputDebugStringA("==========================procThread quitting.
");
    return 0;
}

unsigned __stdcall quitDemoProc(void *)
{
    int idx = 0;
    while ( idx++ < 5 )
    {
        OutputDebugStringA("quitDemoProc running!!!!!!!!.
");
        Sleep(100);
    }

    OutputDebugStringA("--------------------------------------------------quitDemoProc quitting.
");
    return 0;
}


BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
                     )
{
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
        {
            g_hThread = (HANDLE)_beginthreadex(NULL, 0, &procThread, NULL, 0, NULL);
            CloseHandle((HANDLE)_beginthreadex(NULL, 0, &quitDemoProc, NULL, 0, NULL));
        }		
        break;
    case DLL_THREAD_ATTACH:
    case DLL_THREAD_DETACH:
        {
            OutputDebugStringA("------------DLL_THREAD_DETACH called.
");
        }
        break;
    case DLL_PROCESS_DETACH:
        {
            OutputDebugStringA("------------DLL_PROCESS_DETACH begin wait...
");
            g_quit = true;
            WaitForSingleObject(g_hThread, INFINITE);
            OutputDebugStringA("------------DLL_PROCESS_DETACH end wait...
");
        }
        
        break;
    }
    return TRUE;
}

点我下载测试工程

分析

使用windbg打开dump文件。然后使用~kvn 列出所有线程的调用栈。

0  Id: 1918.1924 Suspend: 1 Teb: 7efdd000 Unfrozen
 # ChildEBP RetAddr  Args to Child              
00 004af6f4 76150816 00000038 00000000 00000000 ntdll!NtWaitForSingleObject+0x15 (FPO: [3,0,0])
01 004af760 76781194 00000038 ffffffff 00000000 KERNELBASE!WaitForSingleObjectEx+0x98 (FPO: [Non-Fpo])
02 004af778 76781148 00000038 ffffffff 00000000 kernel32!WaitForSingleObjectExImplementation+0x75 (FPO: [Non-Fpo])
*** WARNING: Unable to verify checksum for DllUnload.dll
03 004af78c 6d0c15eb 00000038 ffffffff 00000000 kernel32!WaitForSingleObject+0x12 (FPO: [Non-Fpo])
04 004af86c 6d0c1e2b 6d0b0000 00000000 00000000 DllUnload!DllMain+0xdb (FPO: [Non-Fpo]) (CONV: stdcall) [c:usersianchengnandocumentsvisual studio 2012projectswaitdllunloadexedllunloaddllmain.cpp @ 55]
05 004af8b0 6d0c1d4f 6d0b0000 00000000 00000000 DllUnload!__DllMainCRTStartup+0xcb (FPO: [Non-Fpo]) (CONV: cdecl) [f:ddvctoolscrt_bldself_x86crtsrccrtdll.c @ 508]
06 004af8c4 77139930 6d0b0000 00000000 00000000 DllUnload!_DllMainCRTStartup+0x1f (FPO: [Non-Fpo]) (CONV: stdcall) [f:ddvctoolscrt_bldself_x86crtsrccrtdll.c @ 472]
07 004af8e4 77160000 6d0c10f0 6d0b0000 00000000 ntdll!LdrpCallInitRoutine+0x14
08 004af96c 77141221 6d0b0000 004af990 750227be ntdll!LdrpUnloadDll+0x375 (FPO: [Non-Fpo])
09 004af9b0 76151da7 6d0b0000 7efde000 004afaa4 ntdll!LdrUnloadDll+0x4a (FPO: [Non-Fpo])
*** WARNING: Unable to verify checksum for WaitDllUnloadExe.exe
0a 004af9c0 003a1425 6d0b0000 00000000 00000000 KERNELBASE!FreeLibrary+0x15 (FPO: [Non-Fpo])
0b 004afaa4 003a1989 00000001 0059a650 0059cf30 WaitDllUnloadExe!wmain+0x55 (FPO: [Non-Fpo]) (CONV: cdecl) [c:usersianchengnandocumentsvisual studio 2012projectswaitdllunloadexewaitdllunloadexewaitdllunloadexe.cpp @ 13]
0c 004afaf4 003a1b7d 004afb08 767833ca 7efde000 WaitDllUnloadExe!__tmainCRTStartup+0x199 (FPO: [Non-Fpo]) (CONV: cdecl) [f:ddvctoolscrt_bldself_x86crtsrccrtexe.c @ 533]
0d 004afafc 767833ca 7efde000 004afb48 77139ed2 WaitDllUnloadExe!wmainCRTStartup+0xd (FPO: [Non-Fpo]) (CONV: cdecl) [f:ddvctoolscrt_bldself_x86crtsrccrtexe.c @ 377]
0e 004afb08 77139ed2 7efde000 75022546 00000000 kernel32!BaseThreadInitThunk+0xe (FPO: [Non-Fpo])
0f 004afb48 77139ea5 003a107d 7efde000 00000000 ntdll!__RtlUserThreadStart+0x70 (FPO: [Non-Fpo])
10 004afb60 00000000 003a107d 7efde000 00000000 ntdll!_RtlUserThreadStart+0x1b (FPO: [Non-Fpo])

   1  Id: 1918.594 Suspend: 1 Teb: 7efda000 Unfrozen
 # ChildEBP RetAddr  Args to Child              
00 0090fc68 77138dd4 00000040 00000000 00000000 ntdll!NtWaitForSingleObject+0x15 (FPO: [3,0,0])
01 0090fccc 77138cb8 00000000 00000000 0059c5b8 ntdll!RtlpWaitOnCriticalSection+0x13e (FPO: [Non-Fpo])
02 0090fcf4 7715d349 772020c0 75d82382 00000000 ntdll!RtlEnterCriticalSection+0x150 (FPO: [Non-Fpo])
03 0090fd8c 7715d5c2 00000000 00000000 0090fdac ntdll!LdrShutdownThread+0x50 (FPO: [Non-Fpo])
04 0090fd9c 0f78e099 00000000 0059ec48 0090fde8 ntdll!RtlExitUserThread+0x2a (FPO: [Non-Fpo])
05 0090fdac 0f78e007 00000000 d910e7ee 00000000 MSVCR110D!_endthreadex+0x39 (FPO: [Non-Fpo])
06 0090fde8 0f78e1d1 0059ec48 0090fe00 767833ca MSVCR110D!_beginthreadex+0x1a7 (FPO: [Non-Fpo])
07 0090fdf4 767833ca 0059ec48 0090fe40 77139ed2 MSVCR110D!_endthreadex+0x171 (FPO: [Non-Fpo])
08 0090fe00 77139ed2 0059c5b8 75d8204e 00000000 kernel32!BaseThreadInitThunk+0xe (FPO: [Non-Fpo])
09 0090fe40 77139ea5 0f78e120 0059c5b8 00000000 ntdll!__RtlUserThreadStart+0x70 (FPO: [Non-Fpo])
0a 0090fe58 00000000 0f78e120 0059c5b8 00000000 ntdll!_RtlUserThreadStart+0x1b (FPO: [Non-Fpo])

#  2  Id: 1918.1960 Suspend: 1 Teb: 7efd7000 Unfrozen
 # ChildEBP RetAddr  Args to Child              
00 00a4f904 7719f826 75ec273a 00000000 00000000 ntdll!DbgBreakPoint (FPO: [0,0,0])
01 00a4f934 767833ca 00000000 00a4f980 77139ed2 ntdll!DbgUiRemoteBreakin+0x3c (FPO: [Non-Fpo])
02 00a4f940 77139ed2 00000000 75ec278e 00000000 kernel32!BaseThreadInitThunk+0xe (FPO: [Non-Fpo])
03 00a4f980 77139ea5 7719f7ea 00000000 00000000 ntdll!__RtlUserThreadStart+0x70 (FPO: [Non-Fpo])
04 00a4f998 00000000 7719f7ea 00000000 00000000 ntdll!_RtlUserThreadStart+0x1b (FPO: [Non-Fpo])

0号线程是主线程(线程id1924),1号线程是子线程(线程id594),2号线程线程id1960)是windbg插入的远程线程,用来中断到调试器。

0号线程在调用WaitForSingleObject时陷入了等待,我们来看它等什么。输入!handle 0x38 f

!handle 0x38 f 
Handle 00000038
  Type         Thread
  Attributes   0
  GrantedAccess 0x1fffff:
         Delete,ReadControl,WriteDac,WriteOwner,Synch
         Terminate,Suspend,Alert,GetContext,SetContext,SetInfo,QueryInfo,SetToken,Impersonate,DirectImpersonate
  HandleCount   5
  PointerCount 8
  Name         <none>
  Object specific information
    Thread Id   1918.594
    Priority    10
    Base Priority 0

原来0号线程在等线程id594的线程 。我们代码里确实有WaitForSingleObject(g_hThread, INFINITE); ,我们再来看看1号线程。从调用栈看来,1号线程已经在调用_endthreadex()准备关闭了,在关闭的过程中进入了一个关键段,并调用ntdll!NtWaitForSingleObject()进入等待。等待的句柄为0x40。输入!handle 0x40 f查看句柄的相关信息。

0:002> !handle 0x40 f 
Handle 00000040
  Type          Event
  Attributes    0
  GrantedAccess 0x100003:
         Synch
         QueryState,ModifyState
  HandleCount   2
  PointerCount  4
  Name          <none>
  Object specific information
    Event Type Auto Reset
    Event is Waiting

我们发现句柄0x40对应的对象是Event,暂时先不管。使用万能死锁调试命令!cs -l看看(因为从调用堆栈来看1号线程是调用RtlEnterCriticalSection而死锁的。)

0:002> !cs -l
-----------------------------------------
DebugInfo          = 0x77204360
Critical section   = 0x772020c0 (ntdll!LdrpLoaderLock+0x0)
LOCKED
LockCount          = 0x1
WaiterWoken        = No
OwningThread       = 0x00001924
RecursionCount     = 0x1
LockSemaphore      = 0x40
SpinCount          = 0x00000000

从输出结果可知,有一个锁住的关键段,被0号线程线程id0x00001924)拥有。而且这个死锁的关键段的成员LockSemaphore正是1号线程正在等待的句柄值。突然想起来《windows核心编程》上讲过关键段的结构,其中的LockSemaphoreEvent类型的,具体参考第八章8.4节。

至此,终于真相大白了,0号线程DllMain()内(ul_reason_for_callDLL_PROCESS_DETACH)等待1号线程结束,而1号线程在结束的时候同样要调用DllMain(),并且ul_reason_for_call参数为DLL_THREAD_DETACH。由于对DllMain()的调用需要序列化,需要等待0号线程释放锁后,其它线程才能调用。而0号线程又在无限等待1号线程结束,故死锁。

{% note warning %}

注意:即使在DllMain()里调用DisableThreadLibraryCalls(hModule);也不管用,具体参考《windows核心编程》中的相关分析。

{% endnote %}

winnt.h里找到了CriticalSection的定义,如下

typedef struct _RTL_CRITICAL_SECTION {
    PRTL_CRITICAL_SECTION_DEBUG DebugInfo;

    //
    //  The following three fields control entering and exiting the critical
    //  section for the resource
    //

    LONG LockCount;
    LONG RecursionCount;
    HANDLE OwningThread;        // from the thread‘s ClientId->UniqueThread
    HANDLE LockSemaphore;
    ULONG_PTR SpinCount;        // force size on 64-bit systems when packed
} RTL_CRITICAL_SECTION, *PRTL_CRITICAL_SECTION;

总结

  • 不要在DllMain()里等待线程结束。

  • 使用!cs -l调试关键段死锁,真香。

参考资料

  • 《格蠹汇编》
  • 《windows核心编程》第八章(CriticalSection相关知识,尤其是8.4.1节) 第二十章(dll相关知识,尤其是20.2.5节)的相关内容。
  • Dynamic-Link Library Best Practices (Windows)

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