Recovering a process from a hung debugger

One of the more annoying things that can happen while debugging processes that deal with network traffic is happening to attach to something that is in the “critical path” for accessing the debugger’s active symbol path.

In such a scenario, the debugger will usually deadlock itself trying to request symbols, which requires going through some code path that involves the debuggee, which being frozen by the debugger, never gets to run. Usually, one would think that this means a lost repro (and, depending on the criticality of the process attached to the debugger, possibly a forced reboot as well), neither of which happen to be particularly fun outcomes.

It turns out that you’re not actually necessarily hosed if this happens, though. If you can still start a new process on the computer, then there’s actually a way to steal the process back from the debugger (on Windows XP and later), with the new-fangled fancy kernel debug object-based debugging support. Here’s what you need to do:

  1. Attach a new debugger to the process, with symbol support disabled. (You will be attaching to the debuggee and not the debugger.)

    Normally, you can’t attach a debugger to process while it’s already being debugged. However, there’s an option in windbg (and ntsd/cdb as well) that allows you to do this: the “-pe” debugger command-line parameter (documented in the debugger documentation), which forcibly attaches to the target process despite the presence of the hung debugger.

    Of course, force-attaching to the process won’t do any good if the new debugger process will just deadlock right away. As a result, you should make sure that the debugger won’t try and do any symbol-loading activity that might engender a deadlock. This is the command line that I usually use for that purpose, which disables _NT_SYMBOL_PATH appending (“-sins“), disables CodeView pdb pointer following (“-sicv“), and resets the symbol path to a known good value (“-y .“):

    ntsd -sicv -sins -y . -pe -p hung_debuggee_pid

    I recommend using ntsd and not WinDbg for this purpose in order to reduce the chance of a symbol path that might be stored in a WinDbg workspace from causing the debugger to deadlock itself again.

  2. Kill the hung debugger.

    After successfully attaching with “-pe“, you can safely kill the hung debugger (by whatever means necessary) without causing the former debuggee to get terminated along with it.

  3. Resume all threads in the target.

    The suspend count of most threads in the target is likely to be wrong. You can correct this by issuing the “~*M” command set several times (which resumes all threads in the process with the “~M” command).

    To determine the suspend count of all processes in the thread, you can use the “~” command. For example, you might see the following:

    0:001> ~
       0  Id: 18c4.16f0 Suspend: 2 Teb: 7efdd000 Unfrozen
    .  1  Id: 18c4.1a04 Suspend: 2 Teb: 7efda000 Unfrozen
    

    You should issue the “~*M” command enough times to bring the suspend count of all threads down to zero. (Don’t worry if you need to resume a thread more times than it is suspended.) Typically, this would be two times, for the common case, but by checking the suspend count of active threads, you can be certain of the number of times that you need you need to resume all threads in the process.

  4. Detach the new debugger from the debuggee.

    After resuming all threads in the target, use the “qd” command to detach the debugger. Do not attempt to resume the debugger with the “g” command (as it will stay suspended), or quit the debugger without attaching (as that would cause the debuggee to get terminated).

    If you needed to keep a particular thread in the debuggee suspended so that you can re-attach the debugger without losing your place, you can leave that thread with its suspend count above zero.

Voila, the debuggee should return back to life. Now, you should be able to re-attach a debugger (hopefully, with a safe symbol path this time), or not, as desired.

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3 Responses to “Recovering a process from a hung debugger”

  1. Fist Sandwitch says:

    Isn’t there a good chance it’ll crash as soon as the code hits a breakpoint (a non hardware one) left behind by the previous debugger?

  2. Skywing says:

    Yes, if you had any outstanding software breakpoints (bu, bp, bm in WinDbg), you may need to manually restore the image contents.

    One way to do this would be to use “!chkimg -f imagename” in the debugger, where imagename refers to an image that had previously been breakpointed upon.

  3. Yuhong Bao says:

    It seems that there is a broken hyperlink in this article on your blog:
    http://www.nynaeve.net/?p=3

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