• Ntdebugging Blog

    Debugging Backwards: Proving root cause


    Matt Burrough here again.  On rare occasions when debugging, we'll actually know (or strongly suspect) what the root cause of a problem is at the beginning of our analysis - but we still need to investigate to confirm our assertion.  The following is a case study for an issue I worked on recently where the print spooler was hanging.


    This customer had recently upgraded their print servers from Windows 2003 to Windows 2008 R2.  After the upgrade, the spooler would frequently go unresponsive, and no jobs could be printed.  Rebooting the server provided some relief, but the problem would reoccur as the jobs started coming in again.


    One of my peers had completed an initial analysis of a user mode memory dump of the spooler process, and found that spooler seemed to be blocked waiting on PrintIsolationHost.exe.  For those not familiar with recent changes to the print spooler's design - Print Isolation Host was added in Windows 7/2008 R2 as a way to isolate print drivers from each other and from the spooler process so a crash in one driver doesn't take down the entire printing environment.  Given the large number of print drivers found on some print servers, this can be a great help for stability and availability of the spooler.  See http://blogs.technet.com/b/askperf/archive/2009/10/08/windows-7-windows-server-2008-r2-print-driver-isolation.aspx if you would like more details on Print Isolation Host.


    Unfortunately for my team mate, the data collected did not include dumps of Print Isolation Host, so he requested that the next time the problem happened that both spooler and PrintIsolationHost dumps would be gathered.  The customer had configured his server for the "Isolated" Print Isolation Host option during troubleshooting, which places each driver in its own process.  (The default is shared which places all drivers in one PrintIsolationHost.exe instance.  Driver isolation is configured using the Print Management Console.)


    The next morning, the newly requested data came in, and since the problem was urgent, I began looking at the new dumps immediately.  This dataset included both a spoolsv.exe dump, as well as nearly two dozen PrintIsolationHost dumps!  I knew from the past analysis that I should start with the PrintIsolationHost data - but where to begin?  In order to triage the dumps, I wrote a small batch file to open each dump (luckily, they were sequentially numbered), write the call stacks in each thread to a file, and close the log.  On every iteration, the script created a new cmd.txt file, which contained a set of commands that were passed to the debugger.  This allowed me to name the debugger output files sequentially with names that matched their dump (e.g. PrintIsolationHost1.txt contained output from PrintIsolationHost1.dmp).


    set x=1


    echo .reload > c:\data\cmd.txt

    echo .logopen c:\data\PrintIsolationHost%x%.txt  >> c:\data\cmd.txt

    echo ~*kc >> c:\data\cmd.txt

    echo .logclose >> c:\data\cmd.txt

    echo qq >> c:\data\cmd.txt

    c:\debuggers\kd.exe -cf c:\data\cmd.txt -z "c:\data\PrintIsolationHost%x%.DMP"

    set /A x+=1

    IF %x% LEQ 23 GOTO Top


    Now that I had a directory full of text files, I used my favorite differencing tool to compare the stacks in each text file.  I used the first PrintIsolationHost file as a reference.  It only had four stacks, and these were common to all of the other files:


    .  0  Id: 40c.1c28 Suspend: 0 Teb: 000007ff`fffde000 Unfrozen

    Call Site










       1  Id: 40c.23c0 Suspend: 0 Teb: 000007ff`fffdb000 Unfrozen

    Call Site






       2  Id: 40c.2598 Suspend: 0 Teb: 000007ff`fffd3000 Unfrozen

    Call Site








       3  Id: 40c.820 Suspend: 0 Teb: 000007ff`fffd9000 Unfrozen

    Call Site






    I was able to rule out a number of other PrintIsolationHost instances that were either identical to this one (aside from the process/thread IDs and Tebs), or which just had one or two additional idle worker threads (like thread 3 above).


    Things got interesting when I looked at two of the PrintIsolationHost dumps.  Both had these two stacks not found in any other the other dumps (note that I do not have symbols for the 3rd-party print processor ProseWarePrintProcessor):


       2  Id: 20a4.1328 Suspend: 0 Teb: 000007ff`fffda000 Unfrozen

    Call Site





    *** ERROR: Symbol file could not be found.  Defaulted to export symbols for ProseWarePrintProcessor.dll -





























       6  Id: 20a4.1668 Suspend: 0 Teb: 000007ff`fffac000 Unfrozen

    Call Site











    Interesting.  Thread 6 is running the DllMain code for the ProseWarePrintProcessor DLL, which holds the loader lock. It is waiting on a critical section.  Meanwhile, thread 2 is waiting on the loader lock.  So who holds the critical section that thread 6 wants?  First, let's find what lock thread 6 wants:


    0:006> kn

     # Child-SP          RetAddr           Call Site

    00 00000000`0104eb18 00000000`777fe4e8 ntdll!ZwWaitForSingleObject+0xa

    01 00000000`0104eb20 00000000`777fe3db ntdll!RtlpWaitOnCriticalSection+0xe8

    02 00000000`0104ebd0 00000000`750c5d6b ntdll!RtlEnterCriticalSection+0xd1

    03 00000000`0104ec00 00000000`750c6256 ProseWarePrintProcessor+0xabf

    04 00000000`0104ec30 00000000`750c7015 ProseWarePrintProcessor+0xfaa

    05 00000000`0104f090 00000000`777dc76c ProseWarePrintProcessor+0x1d69

    06 00000000`0104f1f0 00000000`777dc42f ntdll!LdrpInitializeThread+0x17c

    07 00000000`0104f2f0 00000000`777dc32e ntdll!LdrpInitialize+0x9f

    08 00000000`0104f360 00000000`00000000 ntdll!LdrInitializeThunk+0xe

    0:006> .frame /c /r 2

    02 00000000`0104ebd0 00000000`750c5d6b ntdll!RtlEnterCriticalSection+0xd1

    rax=0000000300d1001a rbx=00000000750ca330 rcx=00000000001d0000

    rdx=0000000000000040 rsi=0000000000000001 rdi=0000000000000004

    rip=00000000777fe3db rsp=000000000104ebd0 rbp=00000000ff000000

     r8=00000000002c6a00  r9=00000000002c6a10 r10=0000000000000073

    r11=0000000000000001 r12=000007fffffd6000 r13=00000000778e2660

    r14=0000000000000001 r15=000000007780a280

    iopl=0         nv up ei pl zr na po nc

    cs=0033  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000246


    00000000`777fe3db 83f801          cmp     eax,1

    0:006> ub 00000000`750c5d6b      <<<< Let's see what gets passed to RtlEnterCriticalSection


    00000000`750c5d4e f9              stc

    00000000`750c5d4f ff743f33        push    qword ptr [rdi+rdi+33h]

    00000000`750c5d53 d2ff            sar     bh,cl

    00000000`750c5d55 15aeb4ffff      adc     eax,0FFFFB4AEh

    00000000`750c5d5a 85c0            test    eax,eax

    00000000`750c5d5c 7533            jne     ProseWarePrintProcessor+0xae5 (00000000`750c5d91)

    00000000`750c5d5e 488d0dcb450000  lea     rcx,[ProseWarePrintProcessor+0x5084 (00000000`750ca330)] <<<< Critical section is in rcx

    00000000`750c5d65 ff15bdb3ffff    call    qword ptr [ProseWarePrintProcessor+0x1128 (00000000`750c1128)]

    0:006> u ntdll!RtlEnterCriticalSection


    00000000`77802fc0 fff3            push    rbx

    00000000`77802fc2 4883ec20        sub     rsp,20h

    00000000`77802fc6 f00fba710800    lock btr dword ptr [rcx+8],0

    00000000`77802fcc 488bd9          mov     rbx,rcx   <<<< Critical section is saved in RBX.  RBX isn't modified between here and our current position

    00000000`77802fcf 0f83e9b1ffff    jae     ntdll!RtlEnterCriticalSection+0x31 (00000000`777fe1be)

    00000000`77802fd5 65488b042530000000 mov   rax,qword ptr gs:[30h]

    00000000`77802fde 488b4848        mov     rcx,qword ptr [rax+48h]

    00000000`77802fe2 c7430c01000000  mov     dword ptr [rbx+0Ch],1

    0:006> r rbx

    Last set context:

    rbx=00000000750ca330  <<<< This is the address of the critical section thread 6 is waiting for


    Now let's look at the threads in this process, and the held locks:


    0:002> ~

    #  0  Id: 20a4.180c Suspend: 0 Teb: 000007ff`fffde000 Unfrozen

       1  Id: 20a4.2294 Suspend: 0 Teb: 000007ff`fffdc000 Unfrozen

    .  2  Id: 20a4.1328 Suspend: 0 Teb: 000007ff`fffda000 Unfrozen

       3  Id: 20a4.1c34 Suspend: 0 Teb: 000007ff`fffd8000 Unfrozen

       4  Id: 20a4.5bc Suspend: 0 Teb: 000007ff`fffd4000 Unfrozen

       5  Id: 20a4.3c4 Suspend: 0 Teb: 000007ff`fffae000 Unfrozen

       6  Id: 20a4.1668 Suspend: 0 Teb: 000007ff`fffac000 Unfrozen

    0:002> !locks


    CritSec ntdll!LdrpLoaderLock+0 at 00000000778e7490

    WaiterWoken        No

    LockCount          2

    RecursionCount     1

    OwningThread       1668

    EntryCount         0

    ContentionCount    2

    *** Locked


    CritSec ProseWarePrintProcessor+5084 at 00000000750ca330

    WaiterWoken        No

    LockCount          1

    RecursionCount     2

    OwningThread       1328

    EntryCount         0

    ContentionCount    1

    *** Locked


    Scanned 201 critical sections


    That's not good!  We can see that thread 6 indeed owns the loader lock, which thread 2 is waiting for.  But thread 2 owns the ProseWarePrintProcessor lock - and thread 6 is waiting for it!  This is a classic deadlock.  In fact, Raymond Chen even described this on his blog.  More information about LdrInitialize can be found here.



    So we know that there is a deadlock in the Print Isolation Host, but why exactly does this cause spooler to hang?  Here's where we work backwards.  We know that thread 6 is handling DLL initialization, but what is thread 2 doing?  From the stack we can see it is handling an RPC request that called into ProseWarePrintProcessor.  Let's determine who called into this thread.


    0:002> kn

     # Child-SP          RetAddr           Call Site

    00 00000000`0085dd88 00000000`777fe4e8 ntdll!ZwWaitForSingleObject+0xa

    01 00000000`0085dd90 00000000`777fe3db ntdll!RtlpWaitOnCriticalSection+0xe8

    02 00000000`0085de40 00000000`777db9e7 ntdll!RtlEnterCriticalSection+0xd1

    03 00000000`0085de70 000007fe`fd963706 ntdll!LdrLockLoaderLock+0x6d

    04 00000000`0085deb0 00000000`750c58f3 KERNELBASE!GetModuleFileNameW+0x96

    05 00000000`0085df10 00000000`750c5d77 ProseWarePrintProcessor!InstallPrintProcessor+0x647

    06 00000000`0085e380 00000000`750c51d9 ProseWarePrintProcessor!InstallPrintProcessor+0xacb

    07 00000000`0085e3b0 000007fe`f96766b4 ProseWarePrintProcessor!ControlPrintProcessor+0x25

    08 00000000`0085e3e0 000007fe`fe2f23d5 PrintIsolationProxy!sandbox::PrintProcessor::ControlPrintProcessor+0x34

    09 00000000`0085e420 000007fe`fe39b68e rpcrt4!Invoke+0x65

    0a 00000000`0085e470 000007fe`fe2f48d6 rpcrt4!Ndr64StubWorker+0x61b

    0b 00000000`0085ea30 000007fe`fdd50883 rpcrt4!NdrStubCall3+0xb5

    0c 00000000`0085ea90 000007fe`fdd50ccd ole32!CStdStubBuffer_Invoke+0x5b

    0d 00000000`0085eac0 000007fe`fdd50c43 ole32!SyncStubInvoke+0x5d

    0e 00000000`0085eb30 000007fe`fdc0a4f0 ole32!StubInvoke+0xdb

    0f 00000000`0085ebe0 000007fe`fdd514d6 ole32!CCtxComChnl::ContextInvoke+0x190

    10 00000000`0085ed70 000007fe`fdd5122b ole32!AppInvoke+0xc2

    11 00000000`0085ede0 000007fe`fdd4fd6d ole32!ComInvokeWithLockAndIPID+0x52b

    12 00000000`0085ef70 000007fe`fe2e50f4 ole32!ThreadInvoke+0x30d

    13 00000000`0085f010 000007fe`fe2e4f56 rpcrt4!DispatchToStubInCNoAvrf+0x14

    14 00000000`0085f040 000007fe`fe2e775b rpcrt4!RPC_INTERFACE::DispatchToStubWorker+0x146

    15 00000000`0085f160 000007fe`fe2e769b rpcrt4!RPC_INTERFACE::DispatchToStub+0x9b

    16 00000000`0085f1a0 000007fe`fe2e7632 rpcrt4!RPC_INTERFACE::DispatchToStubWithObject+0x5b

    17 00000000`0085f220 000007fe`fe2e532d rpcrt4!LRPC_SCALL::DispatchRequest+0x422

    18 00000000`0085f300 000007fe`fe302e7f rpcrt4!LRPC_SCALL::HandleRequest+0x20d

    19 00000000`0085f430 000007fe`fe302a35 rpcrt4!LRPC_ADDRESS::ProcessIO+0x3bf

    1a 00000000`0085f570 00000000`777cb68b rpcrt4!LrpcIoComplete+0xa5

    1b 00000000`0085f600 00000000`777cfeff ntdll!TppAlpcpExecuteCallback+0x26b

    1c 00000000`0085f690 00000000`776a652d ntdll!TppWorkerThread+0x3f8

    1d 00000000`0085f990 00000000`777dc521 kernel32!BaseThreadInitThunk+0xd

    1e 00000000`0085f9c0 00000000`00000000 ntdll!RtlUserThreadStart+0x1d


    I know that the code in frame 19 deals with processing the RPC and has a record of the calling process' PID and TID.  In fact, from a bit of code review, I know that at this portion of the code, we get back a value that contains a ntdll!_CLIENT_ID structure at offset 8:



    000007fe`fe302ba6 ff151c050a00    call    qword ptr [rpcrt4!_imp_AlpcGetMessageFromCompletionList (000007fe`fe3a30c8)]

    000007fe`fe302bac 4885c0          test    rax,rax

    000007fe`fe302baf 0f84d1020000    je      rpcrt4!LRPC_ADDRESS::ProcessIO+0x3c6 (000007fe`fe302e86)

    000007fe`fe302bb5 4c8bac2488000000 mov     r13,qword ptr [rsp+88h]

    000007fe`fe302bbd 41bc01000000    mov     r12d,1

    000007fe`fe302bc3 33d2            xor     edx,edx

    000007fe`fe302bc5 488bf8          mov     rdi,rax

    000007fe`fe302bc8 41bfff000000    mov     r15d,0FFh


    Reviewing the assembly, from ProcessIO+0xe6 to where we are now (ProcessIO+0x3bf), we don't modify rdi again - and rdi is nonvolatile - so if we switch to that frame and check out rdi+8, we'll know who called this thread!


    0:002> .frame /c /r 19

    19 00000000`0085f430 000007fe`fe302a35 rpcrt4!LRPC_ADDRESS::ProcessIO+0x3bf

    rax=0000e5a47e7646ad rbx=0000000000000001 rcx=00000000750ca330

    rdx=0000000000000000 rsi=00000000002b8aa0 rdi=00000000002c4a80

    rip=000007fefe302e7f rsp=000000000085f430 rbp=0000000000ecff90

     r8=000000000085e2d8  r9=0000000000000002 r10=0000000000000000

    r11=0000000000000246 r12=0000000000ec8bf0 r13=0000000000000000

    r14=0000000000000000 r15=0000000000ec8080

    iopl=0         nv up ei pl zr na po nc

    cs=0033  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000246


    000007fe`fe302e7f 4c8d357ad1fbff  lea     r14,[rpcrt4!COMMON_ResubmitListen <PERF> (rpcrt4+0x0) (000007fe`fe2c0000)]

    0:002> dt _CLIENT_ID rdi+8


       +0x000 UniqueProcess    : 0x00000000`00002694 Void

       +0x008 UniqueThread     : 0x00000000`000005e8 Void

    0:002> ? 0x00000000`00002694

    Evaluate expression: 9876 = 00000000`00002694

    0:002> ? 0x00000000`000005e8

    Evaluate expression: 1512 = 00000000`000005e8


    So now we know that the caller was process 0x2694 and thread 0x5e8, or 9876 and 1512 in decimal, respectively.  Our current process (PrintIsolationHost.exe) is PID 20a4 (see above ~ output), or 8356 decimal.  So who is 9876?  I happen to have a process listing from the data collection:


    [0]  0 64 9876 spoolsv.exe     Svcs:  Spooler

         Command Line: C:\Windows\System32\spoolsv.exe

    [0]  0 64 8356 PrintIsolationHost.exe

         Command Line: C:\Windows\system32\PrintIsolationHost.exe -Embedding


    Okay, so I know the caller is thread 5e8 in spooler.  Loading up the spooler dump, what is that thread doing?


    0:000> ~~[5e8]s


    00000000`77801b6a c3              ret

    # Child-SP          RetAddr           Call Site

    00 00000000`0649d898 000007fe`fe2fa776 ntdll!NtAlpcSendWaitReceivePort+0xa

    01 00000000`0649d8a0 000007fe`fe2f4e42 rpcrt4!LRPC_CCALL::SendReceive+0x156

    02 00000000`0649d960 000007fe`fdd528c0 rpcrt4!I_RpcSendReceive+0x42

    03 00000000`0649d990 000007fe`fdd5282f ole32!ThreadSendReceive+0x40

    04 00000000`0649d9e0 000007fe`fdd5265b ole32!CRpcChannelBuffer::SwitchAptAndDispatchCall+0xa3

    05 00000000`0649da80 000007fe`fdc0daaa ole32!CRpcChannelBuffer::SendReceive2+0x11b

    06 00000000`0649dc40 000007fe`fdc0da0c ole32!CAptRpcChnl::SendReceive+0x52

    07 00000000`0649dd10 000007fe`fdd5205d ole32!CCtxComChnl::SendReceive+0x68

    08 00000000`0649ddc0 000007fe`fe39b949 ole32!NdrExtpProxySendReceive+0x45

    09 00000000`0649ddf0 000007fe`fdd521d0 rpcrt4!NdrpClientCall3+0x2e2

    0a 00000000`0649e0b0 000007fe`fdc0d8a2 ole32!ObjectStublessClient+0x11d

    0b 00000000`0649e440 000007fe`fb059070 ole32!ObjectStubless+0x42

    0c 00000000`0649e490 000007fe`fb057967 localspl!sandbox::PrintProcessorExecuteObserver::ControlPrintProcessor+0x10

    0d 00000000`0649e4c0 000007fe`fb055e27 localspl!sandbox::PrintProcessorAdapterImpl::ControlPrintProcessor+0x27

    0e 00000000`0649e4f0 000007fe`faff7392 localspl!sandbox::PrintProcessorAdapter::ControlPrintProcessor+0x1b

    0f 00000000`0649e520 000007fe`faff8a0a localspl!DeleteJob+0x1ce

    10 00000000`0649eae0 00000000`ff41fe25 localspl!SplSetJob+0x49e

    11 00000000`0649eb80 000007fe`f9683603 spoolsv!SetJobW+0x25

    12 00000000`0649ebc0 00000000`61001ce1 spoolss!SetJobW+0x1f

    13 00000000`0649ec00 00000000`61001d7d Contoso!InitializePrintMonitor+0x781

    14 00000000`0649ec40 000007fe`faffa674 Contoso!InitializePrintMonitor+0x81d

    15 00000000`0649ec70 00000000`ff41c9c7 localspl!SplEndDocPrinter+0x214

    16 00000000`0649ecd0 00000000`ff403ba6 spoolsv!EndDocPrinter+0x1f

    17 00000000`0649ed00 00000000`ff3fe772 spoolsv!YEndDocPrinter+0x22

    18 00000000`0649ed30 000007fe`fe2f23d5 spoolsv!RpcEndDocPrinter+0x3e

    19 00000000`0649ed60 000007fe`fe39b68e rpcrt4!Invoke+0x65

    1a 00000000`0649edb0 000007fe`fe2dac40 rpcrt4!Ndr64StubWorker+0x61b

    1b 00000000`0649f370 000007fe`fe2e50f4 rpcrt4!NdrServerCallAll+0x40

    1c 00000000`0649f3c0 000007fe`fe2e4f56 rpcrt4!DispatchToStubInCNoAvrf+0x14

    1d 00000000`0649f3f0 000007fe`fe2e5679 rpcrt4!RPC_INTERFACE::DispatchToStubWorker+0x146

    1e 00000000`0649f510 000007fe`fe2e532d rpcrt4!LRPC_SCALL::DispatchRequest+0x149

    1f 00000000`0649f5f0 000007fe`fe302e7f rpcrt4!LRPC_SCALL::HandleRequest+0x20d

    20 00000000`0649f720 000007fe`fe302a35 rpcrt4!LRPC_ADDRESS::ProcessIO+0x3bf

    21 00000000`0649f860 00000000`777cb68b rpcrt4!LrpcIoComplete+0xa5

    22 00000000`0649f8f0 00000000`777cfeff ntdll!TppAlpcpExecuteCallback+0x26b

    23 00000000`0649f980 00000000`776a652d ntdll!TppWorkerThread+0x3f8

    24 00000000`0649fc80 00000000`777dc521 kernel32!BaseThreadInitThunk+0xd

    25 00000000`0649fcb0 00000000`00000000 ntdll!RtlUserThreadStart+0x1d


    It's calling into the print isolation host as we expect.  It looks like it is doing that to end a print job, based on an RPC call it received.  Using our same method as last time, let's pull out the PID and TID he is responding to:


    0:048> .frame /c /r 20

    20 00000000`0649f720 000007fe`fe302a35 rpcrt4!LRPC_ADDRESS::ProcessIO+0x3bf

    rax=000000000622986c rbx=0000000000000000 rcx=000000000622985c

    rdx=000007fefe3a3c40 rsi=00000000027e5150 rdi=0000000008e6abd0

    rip=000007fefe302e7f rsp=000000000649f720 rbp=0000000008e43480

     r8=0000000000000010  r9=0000000000000000 r10=000007fefe2c0000

    r11=0000000000000002 r12=000000000512d8c0 r13=0000000000000000

    r14=0000000000000000 r15=00000000102e3710

    iopl=0         nv up ei pl zr na po nc

    cs=0033  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000246


    000007fe`fe302e7f 4c8d357ad1fbff  lea     r14,[rpcrt4!COMMON_ResubmitListen <PERF> (rpcrt4+0x0) (000007fe`fe2c0000)]

    0:048> dt _CLIENT_ID rdi+8


       +0x000 UniqueProcess    : 0x00000000`000020a4 Void

       +0x008 UniqueThread     : 0x00000000`00001c34 Void


    Look at that, it's doing work for a different thread back in our Print Isolation Host.  Switching back to that dump, what is thread 1c34 doing?


    0:002> ~~[1c34]s


    00000000`77801b6a c3 

    0:003> kn

     # Child-SP          RetAddr           Call Site

    00 00000000`00ebc098 000007fe`fe2fa776 ntdll!NtAlpcSendWaitReceivePort+0xa

    01 00000000`00ebc0a0 000007fe`fe39cc74 rpcrt4!LRPC_CCALL::SendReceive+0x156

    02 00000000`00ebc160 000007fe`fe39cf25 rpcrt4!NdrpClientCall3+0x244

    03 00000000`00ebc420 000007fe`f9bfd878 rpcrt4!NdrClientCall3+0xf2

    04 00000000`00ebc7b0 000007fe`f96845bf winspool!EndDocPrinter+0x15c

    05 00000000`00ebc7f0 00000000`750c4102 spoolss!EndDocPrinter+0x2f

    06 00000000`00ebc820 00000000`750c5013 ProseWarePrintProcessor+0x4102

    07 00000000`00ebe620 000007fe`f9676be2 ProseWarePrintProcessor!PrintDocumentOnPrintProcessor+0xb3

    08 00000000`00ebe650 000007fe`fe2f23d5 PrintIsolationProxy!sandbox::PrintProcessor::PrintDocThroughPrintProcessor+0x82

    09 00000000`00ebe6b0 000007fe`fe39b68e rpcrt4!Invoke+0x65

    0a 00000000`00ebe710 000007fe`fe2f48d6 rpcrt4!Ndr64StubWorker+0x61b

    0b 00000000`00ebecd0 000007fe`fdd50883 rpcrt4!NdrStubCall3+0xb5

    0c 00000000`00ebed30 000007fe`fdd50ccd ole32!CStdStubBuffer_Invoke+0x5b

    0d 00000000`00ebed60 000007fe`fdd50c43 ole32!SyncStubInvoke+0x5d

    0e 00000000`00ebedd0 000007fe`fdc0a4f0 ole32!StubInvoke+0xdb

    0f 00000000`00ebee80 000007fe`fdd514d6 ole32!CCtxComChnl::ContextInvoke+0x190

    10 00000000`00ebf010 000007fe`fdd5122b ole32!AppInvoke+0xc2

    11 00000000`00ebf080 000007fe`fdd4fd6d ole32!ComInvokeWithLockAndIPID+0x52b

    12 00000000`00ebf210 000007fe`fe2e50f4 ole32!ThreadInvoke+0x30d

    13 00000000`00ebf2b0 000007fe`fe2e4f56 rpcrt4!DispatchToStubInCNoAvrf+0x14

    14 00000000`00ebf2e0 000007fe`fe2e775b rpcrt4!RPC_INTERFACE::DispatchToStubWorker+0x146

    15 00000000`00ebf400 000007fe`fe2e769b rpcrt4!RPC_INTERFACE::DispatchToStub+0x9b

    16 00000000`00ebf440 000007fe`fe2e7632 rpcrt4!RPC_INTERFACE::DispatchToStubWithObject+0x5b

    17 00000000`00ebf4c0 000007fe`fe2e532d rpcrt4!LRPC_SCALL::DispatchRequest+0x422

    18 00000000`00ebf5a0 000007fe`fe302e7f rpcrt4!LRPC_SCALL::HandleRequest+0x20d

    19 00000000`00ebf6d0 000007fe`fe302a35 rpcrt4!LRPC_ADDRESS::ProcessIO+0x3bf

    1a 00000000`00ebf810 00000000`777cb68b rpcrt4!LrpcIoComplete+0xa5

    1b 00000000`00ebf8a0 00000000`777cfeff ntdll!TppAlpcpExecuteCallback+0x26b

    1c 00000000`00ebf930 00000000`776a652d ntdll!TppWorkerThread+0x3f8

    1d 00000000`00ebfc30 00000000`777dc521 kernel32!BaseThreadInitThunk+0xd

    1e 00000000`00ebfc60 00000000`00000000 ntdll!RtlUserThreadStart+0x1d


    This thread called into spooler to end a document based on yet another RPC!  To be clear, this is what we're looking at so far:



    Again, who called into this thread?


    0:003> .frame /c /r 19

    19 00000000`00ebf6d0 000007fe`fe302a35 rpcrt4!LRPC_ADDRESS::ProcessIO+0x3bf

    rax=57633d3cd2c9c145 rbx=0000000000000000 rcx=0000000000861ff0

    rdx=ffffffffff88ffe0 rsi=00000000002b8aa0 rdi=0000000000ec6bc0

    rip=000007fefe302e7f rsp=0000000000ebf6d0 rbp=00000000002cdef0

     r8=000000000003dbf0  r9=00000000000000fe r10=6dc0575d3d3cc4c8

    r11=0000000000860020 r12=0000000000ec82b0 r13=0000000000000000

    r14=0000000000000000 r15=0000000000ec8080

    iopl=0         nv up ei pl zr na po nc

    cs=0033  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000246


    000007fe`fe302e7f 4c8d357ad1fbff  lea     r14,[rpcrt4!COMMON_ResubmitListen <PERF> (rpcrt4+0x0) (000007fe`fe2c0000)]

    0:003> dt _CLIENT_ID rdi+8


       +0x000 UniqueProcess    : 0x00000000`00002694 Void

       +0x008 UniqueThread     : 0x00000000`000014cc Void


    So another call from spooler (note the same PID as earlier) - let's go back to spoolsv.dmp.  Here is this thread:

    0:048> ~~[14cc]s


    00000000`77801b6a c3              ret

    0:049> k

    Child-SP          RetAddr           Call Site

    00000000`0351e538 000007fe`fe2fa776 ntdll!NtAlpcSendWaitReceivePort+0xa

    00000000`0351e540 000007fe`fe2f4e42 rpcrt4!LRPC_CCALL::SendReceive+0x156

    00000000`0351e600 000007fe`fdd528c0 rpcrt4!I_RpcSendReceive+0x42

    00000000`0351e630 000007fe`fdd5282f ole32!ThreadSendReceive+0x40

    00000000`0351e680 000007fe`fdd5265b ole32!CRpcChannelBuffer::SwitchAptAndDispatchCall+0xa3

    00000000`0351e720 000007fe`fdc0daaa ole32!CRpcChannelBuffer::SendReceive2+0x11b

    00000000`0351e8e0 000007fe`fdc0da0c ole32!CAptRpcChnl::SendReceive+0x52

    00000000`0351e9b0 000007fe`fdd5205d ole32!CCtxComChnl::SendReceive+0x68

    00000000`0351ea60 000007fe`fe39b949 ole32!NdrExtpProxySendReceive+0x45

    00000000`0351ea90 000007fe`fdd521d0 rpcrt4!NdrpClientCall3+0x2e2

    00000000`0351ed50 000007fe`fdc0d8a2 ole32!ObjectStublessClient+0x11d

    00000000`0351f0e0 000007fe`fb0591ac ole32!ObjectStubless+0x42

    00000000`0351f130 000007fe`fb057882 localspl!sandbox::PrintProcessorExecuteObserver::PrintDocThroughPrintProcessor+0x124

    00000000`0351f1f0 000007fe`fb05601d localspl!sandbox::PrintProcessorAdapterImpl::PrintDocumentOnPrintProcessor+0x3a

    00000000`0351f220 000007fe`fb013b70 localspl!sandbox::PrintProcessorAdapter::PrintDocumentOnPrintProcessor+0x9d

    00000000`0351f270 000007fe`fb014c7c localspl!PrintDocumentThruPrintProcessor+0x46c

    00000000`0351fa70 00000000`776a652d localspl!PortThread+0x4d0

    00000000`0351fd80 00000000`777dc521 kernel32!BaseThreadInitThunk+0xd

    00000000`0351fdb0 00000000`00000000 ntdll!RtlUserThreadStart+0x1d


    Perfect.  So now we know that localspl was printing a document, which resulted in all of these RPC calls between spooler and Print Isolation Host, and ultimately the deadlock in Print Isolation Host is holding up this thread.  Just out of curiosity, are there any other threads blocked on this wait chain?


    0:049> !locks

    CritSec +5a00060 at 0000000005a00060

    WaiterWoken        No

    LockCount          105

    RecursionCount     1

    OwningThread       5e8

    EntryCount         0

    ContentionCount    84

    *** Locked


    Scanned 14148 critical sections


    Yes there are.  Thread 5e8, which we looked at earlier, is holding a lock that 104 other threads are waiting for!  This dump had 177 threads, so we know now that thread 5e8, 14cc, and 104 others in spooler are all hung on this deadlock. With about 60% of all the threads in spooler hung, the deadlock in ProseWarePrintProcessor is clearly the cause of our issue.  Here's the final wait chain diagram:



    To resolve the issue, ProseWarePrintProcessor needs to avoid calling GetModuleFileName while its DllMain routine is still running, since the former requires and the latter holds the Loader Lock.

  • Ntdebugging Blog

    What Should Never Happen... Did


    Hi, this is Bob Golding; I wanted to write a blog about an interesting hardware issue I ran into. Hardware problems can be tricky to isolate. I recently came across one that I thought was interesting and gave an example of how to trace code execution.  The machine executed the filler “int 3” instructions generated by the compiler.  Execution should never reach these filler instructions, so we needed to determine how the instruction pointer got there.


    What was the issue?

    The issue was a bug check 8E (unhandled exception).  The exception was a debug exception (80000003), because a filler INT 3 instruction was executed.


    1: kd> .bugcheck

    Bugcheck code 0000008E

    Arguments 80000003 8082e3e0 f78aec38 00000000


    Below is the trap frame (the trap frame is the third argument in the bugcheck code). Note that the actual trapping instruction is 8082e3e0, the instruction pointer is incremented before the INT 3 generates a trap. The correct EIP is reported in the bug check values.


    1: kd> .trap f78aec38

    ErrCode = 00000000

    eax=0e364a01 ebx=f78aed50 ecx=f78aecb8 edx=000000e1 esi=f7727ec0 edi=f75d9ca2

    eip=8082e3e1 esp=f78aecac ebp=f78aecc0 iopl=0         nv up ei pl nz na po nc

    cs=0008  ss=0010  ds=0023  es=0023  fs=0030  gs=0000             efl=00000202


    8082e3e1 cc              int     3


    The machine executed code at the end of a function that should never be executed.  This should never happen.


    1: kd> ub 8082e3e1


    8082e3c6 32c9            xor     cl,cl

    8082e3c8 ff2508118080    jmp     dword ptr [nt!_imp_KfLowerIrql (80801108)]

    8082e3ce 64a124010000    mov     eax,dword ptr fs:[00000124h]

    8082e3d4 c6403d01        mov     byte ptr [eax+3Dh],1

    8082e3d8 ff2598108080    jmp     dword ptr [nt!_imp_HalRequestSoftwareInterrupt (80801098)]

    8082e3de cc              int     3 <<< This is after the function.

    8082e3df cc              int     3

    8082e3e0 cc              int     3


    OK, So Now What?

    Now, we need to find the execution path that caused the machine to execute this INT 3. There are places to look to find clues that will tell us. The first place to start looking is the stack. If a “call” instruction was made, the return will be pushed on the stack. This way we can try to determine if we arrived at this bad instruction pointer from a call or a ret instruction.


    Using the value from esp in the above trap frame, let’s dump the stack.


    1: kd> dps f78aecac

    f78aecac  80a5d8fc hal!HalpIpiHandler+0xcc <<< Interesting?

    f78aecb0  f78aecc0

    f78aecb4  00000000

    f78aecb8  00000002

    f78aecbc  000000e1

    f78aecc0  f78aed50

    f78aecc4  f75d9ca2

    f78aecc8  badb0d00

    f78aeccc  000086a8


    In looking at the stack dump, we see that there may have been a call from HalpIpiHandler.  Let’s dump the code leading up to hal!HalpIpiHandler+0xcc to see what it did.


    1: kd> ub 80a5d8fc


    80a5d8e1 e8d82b0000      call    hal!HalBeginSystemInterrupt (80a604be)

    80a5d8e6 0ac0            or      al,al

    80a5d8e8 7509            jne     hal!HalpIpiHandler+0xc3 (80a5d8f3)

    80a5d8ea 83c408          add     esp,8

    80a5d8ed ff2510b0a580    jmp     dword ptr [hal!_imp_Kei386EoiHelper (80a5b010)]

    80a5d8f3 6a00            push    0

    80a5d8f5 55              push    ebp

    80a5d8f6 ff1520b0a580    call    dword ptr [hal!_imp__KiIpiServiceRoutine (80a5b020)]


    In the above assembly, we can see that there is a call made using a pointer in the import table.  Now, let’s have a look at that pointer.


    1: kd> dd 80a5b020 l 1

    80a5b020 8082e3e4


    The pointer is very close to the instruction we trapped on.  Is this a coincidence?


    It looks like due to an effective address calculation failure, the machine starting executing at 8082e3e0 instead of 8082e3e4.  Somewhere in the data path the processor executing this instruction stream dropped bit three, turning a 4 into a 0.


    1: kd> ?0y0100

    Evaluate expression: 4 = 00000000`00000004

    1: kd> ?0y0000

    Evaluate expression: 0 = 00000000`00000000


    What does all of this mean?

    There is some circumstantial evidence here that the machine was in the IPI handler.  The IPI Handler is used in multiprocessor systems so that one processor may interrupt another.  So, how can we further prove this is where we were?  Let’s try to match the trap frame registers to the assembly from HalpIpiHandler before it calls KiIpiServiceRoutine.


    1: kd> .trap f78aec38

    ErrCode = 00000000

    eax=0e364a01 ebx=f78aed50 ecx=f78aecb8 edx=000000e1 esi=f7727ec0 edi=f75d9ca2

    eip=8082e3e1 esp=f78aecac ebp=f78aecc0 iopl=0         nv up ei pl nz na po nc

    cs=0008  ss=0010  ds=0023  es=0023  fs=0030  gs=0000             efl=00000202


    8082e3e1 cc              int     3

    1: kd> ub 80a5d8fc l10


    80a5d8c4 c74508000ddbba  mov     dword ptr [ebp+8],0BADB0D00h <<< EBP+ 8 does equal badb0d00

    80a5d8cb 895d00          mov     dword ptr [ebp],ebx <<< ebp does equal ebx


    80a5d8ce 897d04          mov     dword ptr [ebp+4],edi <<< ebp+4 does equal edi

    80a5d8d1 68e1000000      push    0E1h

    80a5d8d6 83ec04          sub     esp,4

    80a5d8d9 54              push    esp

    80a5d8da 68e1000000      push    0E1h

    80a5d8df 6a1d            push    1Dh

    80a5d8e1 e8d82b0000      call    hal!HalBeginSystemInterrupt (80a604be)

    80a5d8e6 0ac0            or      al,al <<< al is equal to 1

    80a5d8e8 7509            jne     hal!HalpIpiHandler+0xc3 (80a5d8f3)

    80a5d8ea 83c408          add     esp,8

    80a5d8ed ff2510b0a580    jmp     dword ptr [hal!_imp_Kei386EoiHelper (80a5b010)]

    80a5d8f3 6a00            push    0

    80a5d8f5 55              push    ebp

    80a5d8f6 ff1520b0a580    call    dword ptr [hal!_imp__KiIpiServiceRoutine (80a5b020)]


    Below is the stack data that the instructions above produced (borrowed from the dps output above). It matches, so the machine did execute the above instructions.


    f78aecac  80a5d8fc  hal!HalpIpiHandler+0xcc <<< Return Pushed by call instruction

    f78aecb0  f78aecc0 <<< Pushed EBP

    f78aecb4  00000000 <<< Pushed a 0


    Finally, the last bit of evidence:

    Let's view the IPI state using the debugger command !ipi.  From this output we can see that processor 1 is the receiver of a cross interrupt from Processor 0.  This is consistent with the data we found on the stack.


    1: kd> !ipi

    IPI State for Processor 0


        As a sender, awaiting packet completion from processor 1.


        TargetSet             2  PacketBarrier         e  IpiFrozen      2 [Frozen]

        IpiFrame       8089a570  SignalDone     00000000  RequestSummary 0

        Packet State     Active                           WorkerRoutine  nt!KiFlushTargetMultipleTb

        Parameter[0]   00000000  Parameter[1]   80899f10  Parameter[2]   80899f04


    IPI State for Processor 1


        As a receiver, the following unhandled requests are pending: [Packet] [DPC]


        TargetSet             0  PacketBarrier         0  IpiFrozen      0 [Running]

        IpiFrame       b5ad7be8  SignalDone     ffdff120  RequestSummary 2 [DPC]

        Packet State      Stale                           WorkerRoutine  nt!KiFlushTargetMultipleTb

        Parameter[0]   00000000  Parameter[1]   b5ad7950  Parameter[2]   b5ad7948


    What went wrong?

    Based on the evidence in this dump it appears that call instruction transferred execution to the wrong address.  The machine ended up executing at address 8082e3e0 instead of 8082e3e4, a single bit difference.  This same bit was flipped in several crashes from this machine, so all the evidence pointed to a faulty processor.  After replacing the processor we were running on when we bugchecked, the issue did not occur again.


    Hardware can sometimes cause pretty specific failures, such as the flipped bit we see here.  To determine that this failure was a hardware issue, we had to reconstruct the execution path and trace how we ended up at the failing instruction.  We were able to match the register contents to what they would have been before the call to KiIpiServiceRoutine.  This demonstrated that the call should have been made to KiIpiServiceRoutine, but it unexpectedly went to the wrong address.

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