Archive for October 20th, 2006

Debugging the Debugger

Friday, October 20th, 2006

Have you ever tried to debug a debugger when it debugs a debuggee? Is it possible? Good question. I never asked it to myself until today. And tried. And it works! First I tried to attach WinDbg.exe to an instance of WinDbg.exe executing ‘!analyze -v’ command and got these stacks:

0:002> ~*kL 100
0 Id: 1ff0.104c Suspend: 1 Teb: 7ffdf000 Unfrozen
ChildEBP RetAddr
0006df38 7739d02f ntdll!KiFastSystemCallRet
0006ff7c 01055e36 USER32!NtUserWaitMessage+0xc
0006ffc0 77e523e5 windbg!_initterm_e+0x170
0006fff0 00000000 kernel32!BaseProcessStart+0x23

1 Id: 1ff0.1af8 Suspend: 1 Teb: 7ffde000 Unfrozen
ChildEBP RetAddr
00ac3448 030a5677 dbghelp!CAllPubNameTrav::next+0x1b
00ac345c 0301e16e
dbghelp!CDiaEnumTraversalCSymRow::Next+0x48
00ac44fc 0301e452 dbghelp!diaGetGlobals+0x8fe
00ac4524 0304967a dbghelp!diaGetSymbols+0x42
00ac453c 03045ca3 dbghelp!diaEnumSymbols+0x1a
00ac4554 03031e5a dbghelp!modEnumSymbols+0x43
00ac459c 030338a5 dbghelp!ModLoop+0x10a
00ac6570 030391d8 dbghelp!EnumSymbols+0x155
00ac65a0 0220947b dbghelp!SymEnumSymbolsW+0x48
00ac7600 0220a53d dbgeng!FindTypeInfoInMod+0x18b
00aca5cc 0220caa2 dbgeng!TypeInfoFound+0xced
00acb62c 0220c95f dbgeng!SymbolTypeDumpNew+0xa2
00acb654 0220d36b dbgeng!FastSymbolTypeDump+0xef
00acb700 0213c753 dbgeng!SymbolTypeDump+0xbb
00acc25c 0147d632 dbgeng!ExtIoctl+0x1073
00acc2f4 0150e10e ext!GetFieldData+0xe2
00accc14 014f9f00 ext!UaThread::_Extract_UIThread+0x34e
00accc24 014fa1f9 ext!UaThread::CallExtractors+0x20
00accc34 01511126 ext!UaThread::ExtractAttributes+0x99
00accd78 015212e2
ext!UserAnalyze::ExtractAttributes+0x376
00acd02c 01521467 ext!UeFillAnalysis+0x462
00acd10c 01521650 ext!UeAnalyze+0x147
00acd208 0147c90c ext!AnalyzeUserException+0x1a0
00acd23c 02141299 ext!analyze+0x28c
00acd2c8 021414d9 dbgeng!ExtensionInfo::CallA+0x2e9
00acd458 021415a2 dbgeng!ExtensionInfo::Call+0x129
00acd474 0213feb1 dbgeng!ExtensionInfo::CallAny+0x72
00acd8ec 02181698 dbgeng!ParseBangCmd+0x661
00acd9dc 02182b29 dbgeng!ProcessCommands+0x508
00acda20 020c9049 dbgeng!ProcessCommandsAndCatch+0x49
00acdeb8 020c92aa dbgeng!Execute+0x2b9
00acdee8 010283bf dbgeng!DebugClient::ExecuteWide+0x6a
00acdf88 0102883b windbg!ProcessCommand+0xff
00acffa4 0102aabc windbg!ProcessEngineCommands+0x8b
00acffb8 77e6608b windbg!EngineLoop+0x3dc
00acffec 00000000 kernel32!BaseThreadStart+0x34

# 2 Id: 1ff0.116c Suspend: 1 Teb: 7ffdd000 Unfrozen
ChildEBP RetAddr
00fdffc8 7c845ea0 ntdll!DbgBreakPoint
00fdfff4 00000000 ntdll!DbgUiRemoteBreakin+0x36

Next I thought, wait a moment, we are debugging dump analysis session. Can we debug a debugger debugging a running process? So I attached WinDbg.exe to an instance of WinDbg.exe attached to an instance of notepad.exe and got these stacks:

0:002> ~*kL
0 Id: 11f0.164c Suspend: 1 Teb: 7ffde000 Unfrozen
ChildEBP RetAddr
0006df38 7739d02f ntdll!KiFastSystemCallRet
0006ff7c 01055e36 USER32!NtUserWaitMessage+0xc
0006ffc0 77e523e5 windbg!_initterm_e+0x170
0006fff0 00000000 kernel32!BaseProcessStart+0x23

1 Id: 11f0.1bb0 Suspend: 1 Teb: 7ffdd000 Unfrozen
ChildEBP RetAddr
00adff0c 7c822124 ntdll!KiFastSystemCallRet
00adff10 77e6bad8 ntdll!NtWaitForSingleObject+0xc
00adff80 020bf8aa kernel32!WaitForSingleObjectEx+0xac
00adffa0 0102aa42
dbgeng!DebugClient::DispatchCallbacks+0x4a
00adffb8 77e6608b windbg!EngineLoop+0x362
00adffec 00000000 kernel32!BaseThreadStart+0x34

# 2 Id: 11f0.100c Suspend: 1 Teb: 7ffdc000 Unfrozen
ChildEBP RetAddr
00beffc8 7c845ea0 ntdll!DbgBreakPoint
00befff4 00000000 ntdll!DbgUiRemoteBreakin+0x36

Given that many functions from dbghelp.dll and dbgeng.dll are described in WinDbg help you can quickly reverse engineer WinDbg.exe and its extensions code.

- Dmitry Vostokov -

Posted in Crash Dump Analysis, Debugging | 3 Comments »