• Ntdebugging Blog

    Windows 7 XP Mode – Auto Publish ‘Not Available’


    Hello All, Ron Riddle here again to share another mystery relating to the XP Mode Auto Publish feature for Windows 7 that had an unexpected root cause.  As you may know, the Auto Publish feature allows applications installed within the guest to be automagically available from the start menu of the Windows 7 host.  Of course, one requirement for this to work properly is that the feature must be enabled; however, I have encountered two such instances where the feature is listed as ‘Not Available’ within the Virtual PC Settings UI once the guest is started.




    Debugging the Issue


    The Virtual PC Integration Components Services Application (1-vmsrvc) service, which runs within a vmsrvc.exe instance on the guest, decides whether the Auto Publish feature should be enabled.  For non-Windows 7 guests, the first thing 1-vmsrvc does is issue a WMI query to determine whether KB961742 (for an XP guest) or KB961741 (for a Vista guest), which provide Remote Applications Integrated Locally (RAIL) support, have been applied.


    Root Cause Analysis


    Before we issue the WMI query, we must first activate the CLSID_WbemLevel1Login component.  Here’s an excerpt below:


    0:009> k

    ChildEBP RetAddr 

    00b1f730 74ef186e wbemprox!CDCOMTrans::DoActualCCI+0x3d

    00b1f774 74ef15db wbemprox!CDCOMTrans::DoCCI+0x12d

    00b1f830 74ef17e4 wbemprox!CDCOMTrans::DoActualConnection+0x25c

    00b1f85c 74ef1ee1 wbemprox!CDCOMTrans::DoConnection+0x25

    00b1f89c 01018283 wbemprox!CLocator::ConnectServer+0x7c

    00b1fae8 010182da vmsrvc!VPCRAILUpdates::Connect+0xa8

    00b1faf4 0101842b vmsrvc!VPCRAILUpdates::QueryInstalledFixes+0xb

    00b1fe34 0100b61f vmsrvc!VPCRAILUpdates::CheckIfUpdatesArePresent+0x91

    00b1ffb4 7c80b729 vmsrvc!Win32VPCAppPublisherService::AllowListNotificationThreadProc+0x90

    00b1ffec 00000000 kernel32!BaseThreadStart+0x37


    However, I noticed that the activation attempt failed with WBEM_E_CRITICAL_ERROR(0x8004100a).


    0:009> r eax



    So, I proceeded to debug the activation attempt from within the Windows Management Instrumentation(winmgmt) service, since it provides the class factory for this component.  Notice the call to LoadLibraryExW passing a relative path for the lpFileName parameter.  This means that a search strategy must be applied which leverages the PATH environment variable.


    0:002> k

    ChildEBP RetAddr             

    0086f438 594976e2 kernel32!LoadLibraryExW

    0086f494 7751d8a7 wmisvc!CForwardFactory::CreateInstance+0xf8

    0086f4b8 7751daac ole32!GetInstanceHelperMulti+0x20

    0086f578 77e799f4 ole32!CObjServer::CreateInstance+0x251

    0086f59c 77ef421a RPCRT4!Invoke+0x30

    0086f9a8 77ef4bf3 RPCRT4!NdrStubCall2+0x297

    0086fa00 77600c15 RPCRT4!CStdStubBuffer_Invoke+0xc6

    0086fa40 77600bbf ole32!SyncStubInvoke+0x33

    0086fa88 7752ad31 ole32!StubInvoke+0xa7

    0086fb60 7752ac56 ole32!CCtxComChnl::ContextInvoke+0xe3

    0086fb7c 776007f5 ole32!MTAInvoke+0x1a

    0086fbac 77602df3 ole32!AppInvoke+0x9c

    0086fc80 77600715 ole32!ComInvokeWithLockAndIPID+0x2c2

    0086fccc 77e794bd ole32!ThreadInvoke+0x1cd

    0086fd00 77e79422 RPCRT4!DispatchToStubInC+0x38

    0086fd54 77e7934e RPCRT4!RPC_INTERFACE::DispatchToStubWorker+0x113

    0086fd78 77e8a384 RPCRT4!RPC_INTERFACE::DispatchToStub+0x84

    0086fdb8 77e8a3c5 RPCRT4!RPC_INTERFACE::DispatchToStubWithObject+0xc0

    0086fdf8 77e7bcc1 RPCRT4!LRPC_SCALL::DealWithRequestMessage+0x2cd

    0086fe1c 77e7bc05 RPCRT4!LRPC_ADDRESS::DealWithLRPCRequest+0x16d


    0:002> du poi(esp+4)

    59491668  "wbemcore.dll"


    Surprisingly, I found that the requested library could not be found using the standard search strategy.


    0:002> !gle

    LastErrorValue: (Win32) 0x7e (126) - The specified module could not be found.

    LastStatusValue: (NTSTATUS) 0xc0000135 - {Unable To Locate Component}  This application has failed to start because %hs was not found. Re-installing the application may fix this problem.


    I then decided to enable boot logging within the Process Monitor tool to catch a glimpse into why the load for wbemcore.dll was failing.  Here’s an excerpt from the Process Monitor log:


    8:18:11.1652951 PM      svchost2.exe      1628  QueryOpen   C:\WINDOWS\system32\%SystemRoot%\system32\wbemcore.dll  PATH NOT FOUND    

    8:18:11.1653627 PM      svchost2.exe      1628  QueryOpen   C:\WINDOWS\system32\%SystemRoot%\wbemcore.dll        PATH NOT FOUND 

    8:18:11.1654161 PM      svchost2.exe      1628  QueryOpen   C:\WINDOWS\system32\%SystemRoot%\System32\Wbem\wbemcore.dll  PATH NOT FOUND     


    At this point, it became clear to me that something must be wrong with the PATH environment variable configuration because the SystemRoot environment variable was not being properly expanded.  Sure enough, the registry showed that the Path value was of type REG_SZ rather than REG_EXPAND_SZ.




    Once I saved off the data for the Path value and recreated it specifying the proper type(REG_EXPAND_SZ), the issue was resolved!


    Sidenote on the Debugging Strategy


    I chose to debug the services by attaching ntsd.exe and redirecting the session to the kernel debugger.  Alternatively, I could have chosen to use a remote debugging session since it’s much more natural than redirecting to the kernel debugger, in my opinion.  However, this issue was further complicated by the fact that introducing a debugger in the mix had potential to change the timing enough such that I struggled to reproduce the issue using a remote session.  The reason for this is that if we slow down the 1-vmsrvc execution enough, the winmgmt service itself will attempt to load wbemcore.dll, albeit through an activation request where an absolute path is specified, thereby avoiding application of a search strategy, which will succeed.  Thus, when 1-vmsrvc attempts to activate CLSID_WbemLevel1Login, it will now succeed because wbemcore.dll is already loaded and the Auto Publish feature will now be ‘Enabled’!




    I leveraged the Image File Execution Options key, creating a new entry for vmsrvc.exe and configuring the Debugger value with the following command line:


    [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\vmsrvc.exe]

    "Debugger"="c:\\debuggers\\ntsd.exe -d -y srv*c:\\vmsrvc -c \"bu vmsrvc!Win32VPCAppPublisherService::AllowListNotificationThreadProc;bu wbemprox!CDCOMTrans::DoActualCCI;g\""


    I also isolated the winmgmt service into its own svchost.exe, copied %systemroot%\system32\svchost.exe to %systemroot%\system32\svchost2.exe, and then created a new key for svchost2.exe with the following command line:


    [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\svchost2.exe]

    "Debugger"="c:\\debuggers\\ntsd.exe -d -y srv*c:\\winmgmt -c \"bu wmisvc!CForwardFactory::CreateInstance;g\""


    Although redirecting ntsd.exe to the kernel debugger was cumbersome from a usability perspective, I found the ability to debug both processes from a central facility very appealing and worthwhile.




    For issues like these that ultimately resolve to a misconfiguration of the OS, I can’t help but think how unfortunate it was that I didn’t stumble onto root cause sooner via the routine task of launching some executable from a command shell.  Surely this would have been a red flag and could have saved me a lot of time debugging!




    While this was a rather extreme example of how a misconfiguration of the OS can affect other seemingly unrelated parts such as the Auto Publish feature of XP Mode, the take-away here is when you detect that the Auto Publish feature is ‘Not Available’, you should begin with standard WMI troubleshooting; and, as we’ve just seen here, a quick sanity check of the environment might not be a bad idea either! :)


    Until next time, happy debugging!

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