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0006f7e8 0006f804 0006f824 0006f844 0006f94c 0006fbf8 0006fea0 0006ff08 0006ff1c 0006ff38 0006ff44 0006ffc0 0006fff0
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kernel32!WaitForSingleObject+0x12 10modDll!DllMain+0x4a 10modDll!_DllMainCRTStartup+0x52 ntdll!LdrpCallInitRoutine+0x14 ntdll!LdrpRunInitializeRoutines+0x344 ntdll!LdrpLoadDll+0x3e5 ntdll!LdrLoadDll+0x230 kernel32!LoadLibraryExW+0x18e kernel32!LoadLibraryExA+0x1f kernel32!LoadLibraryA+0x94 10dllmain!main+0x10 10dllmain!mainCRTStartup+0x12f kernel32!BaseProcessStart+0x23
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The first thread in Listing 1016 makes perfect sense It is our main application thread, where we call LoadLibrary on our sample DLL The call goes through a myriad of system calls before ending up in our DllMain implementation, as per expectations The DllMain function finally calls WaitForSingleObject to wait for our worker thread to finish This wait, however, never seems to be satisfied, implying that our worker thread either did not execute or is hung Judging from the other threads in the process, we can t see a trace of our worker thread; hence, we draw the conclusion that it never started So far, the debug session does not seem to make any sense The last two threads look very strange as well What we have are two threads that are trying to enter a critical section Additionally, the bottommost frames of those threads look a bit strange:
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002cfca0 7c927357 7c97c0d8 002cfd2c 00000004 ntdll!RtlEnterCriticalSection+0x46 002cfd18 7c90eac7 002cfd2c 7c900000 00000000 ntdll!_LdrpInitialize+0xf0 00000000 00000000 00000000 00000000 00000000 ntdll!KiUserApcDispatcher+0x7
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If we dump the critical section that is being acquired, we can see that it is referred to as a loader lock:
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0:000> !cs 7c97c0d8 --------------------Critical section = 0x7c97c0d8 (ntdll!LdrpLoaderLock+0x0) DebugInfo = 0x7c97c100 LOCKED LockCount = 0x3 OwningThread = 0x0000027c RecursionCount = 0x2 LockSemaphore = 0x7E8 SpinCount = 0x00000000
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The critical section is in a locked state, and the thread owning the critical section has a thread ID of 0x27c The thread corresponding to that thread ID happens to be our main thread that is in the process of initializing the DLL we loaded Why do we have two threads waiting for the loader lock to be released Furthermore, why do the thread stacks look so strange Typically, a user mode thread begins with a frame that resembles the following:
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What we are seeing is an example of how a user mode thread actually comes to life in Windows Figure 103 illustrates the high-level process of creating a new thread Note that many of the details surrounding thread creations are largely undocumented, and, as such, Figure 103 should serve as a close high-level approximation When a thread is created in Windows, it does not auto-magically start executing from the kernel32!BaseThreadStart function, but rather it always begins its life as a user mode APC (Asynchronous Procedure Call) At a high level, when Windows is notified to create a new thread, a user mode APC is queued to the APC queue of the new thread The APC dispatcher dispatches an APC to the new thread, which in return performs a bunch of initialization work prior to transitioning into a state that we typically associate with being a user mode thread (that is, we see the kernel32!BaseThreadStart in the stack trace) This is exactly what we are seeing with our two threads They are still in an APC state, performing initialization work