My name is Ryan Mangipano (ryanman) and I am a Sr. Support Escalation Engineer at Microsoft. This two part blog will consist of a complete walkthrough of a bugcheck that occurred due to an overflowed stack condition. What is unique about this situation is the stack backtrace wasn’t being displayed. As we proceed with the walkthrough of the dump analysis, I will be providing demonstrations and background information relating to Task States and Double Faults.
I began my review with the command !analyze –v
!analyze -v
UNEXPECTED_KERNEL_MODE_TRAP (7f)
Arg1: 00000008, EXCEPTION_DOUBLE_FAULT
You can see from the output above that an unexpected kernel mode trap has taken place. Arg1 reported by the output from !analyze –v indicates the type of trap that occurred was a double fault. A double fault indicates that an exception occurred during a call to the handler for a prior exception. Although a double fault can be caused by other reasons (hardware or a corrupt stack pointer value), we most commonly observe this bugcheck when the drivers executing on the system have caused all of the available 12k of Kernel mode stack space to become exhausted.
Threads on a 32-bit system are given 12k of kernel-mode stack space. 16k of kernel virtual address space will actually be consumed due to the occupation of 4K of virtual address space by an invalid PTE. This guard PTE is used to guard the virtual address range before the kernel stack limit (The stack grows toward lower memory addresses). This 4k guard page is placed in this location to catch stack overflows. The 12k stack size is not configurable because it is hard coded into the kernel. For more information, please refer to “Windows Internals, Fifth Edition, page 786, Kernel Stacks”.
If the 12k of kernel stack space is all used up and drivers attempt to use stack space beyond the valid range, a page fault exception will occur as the invalid virtual addresses related to the guard PTE are referenced.
When this page fault exception occurs, the CPU will automatically attempt to push some data onto the stack before transferring control to the page fault handler (Thank you to one of our reader's for correcting this information). More details on what data is pushed to the stack is available in the Intel Processor Family Developer’s Manual, Vol. 3 Chapter 14 (Protected-Mode Exceptions and Interrupts). However when the CPU tries to push this data, another fault will occur due to the stack pointer still providing an invalid address. This causes a double exception (AKA EXCEPTION_DOUBLE_FAULT).
So how can the OS handle this type of situation in order to write out the dump file? The code associated with TRAP 0x8 (EXCEPTION_DOUBLE_FAULT) will perform a task state segment switch and obtain a new stack pointer which is valid. Task State Segment switching is a CPU provided mechanism that allows us to switch to a new task state and store a link to the previously executed task state. The information that is needed to restore a task is stored in a task-state segment (TSS). The debugger command .tss can later be used to switch back to the previous task state to examine the context at the time of failure. More information regarding Task-State Segment (TSS) is available in the Intel Processor Manual Set (volume 3, Chapter 6).
In addition to the bugcheck data listed above, the output from the command !analyze -v has also provided me with the .tss command that I needed to type into the debugger.:
TSS: 00000028 -- (.tss 0x28)
You can type .tss 0x28 in the command window but I simply clicked the DML (debugger Markup Language) hyperlink which entered the .tss command for me. As discussed above, this command accepts the address of the saved Task State Segment (TSS) information for the current processor. This command will set the appropriate context just like the .trap or .cxr commands.
The processor provides a Task Register which contains a 16-bit segment selector. The register is actually larger. There is other data stored in this register, however it is only viewable by the processor for caching the segment descriptor. Windbg’s r command can be used to dump out the usable portion of this register.
3: kd> rtr
tr=00000050
So the task register was pointing to a different task (.tss 0x50) at the time of the second exception. But where did !analyze –v get this .tss 0x28 value from?
Let’s do some digging. You can get the address of the TSS for the current processor by using the !pcr command
3: kd> !pcr
KPCR for Processor 3 at f7737000:
..
(omitted several fields for this blog)
TSS: f773a2e0
Extensions like !pcr are great, but I also like to understand how the values were obtained. So instead of just getting the value from !pcr, How else can we find it?
The fs register points to the memory segment that the _KPCR for the current processor is stored. This structure is stored at the base, offset 0x0.
3: kd> rfs
Last set context:
fs=00000030 ßpoints to the segment where the nt!_KPCR is stored at the base of.
Let’s see where the _KTSS pointer is stored within the KPCR structure.
3: kd> dt nt!*PCR*
ntkrpamp!_KPCR
3: kd> dt ntkrpamp!_KPCR TSS
+0x040 TSS : Ptr32 _KTSS 0x40 is the offset that the pointer to the TSS is stored.
Let’s use those two values to dump this out. The 0030: represents the memory segment. Note that I have added 0x40 from the base and dumped out this location
3: kd> dd 0030:00000040 L1
0030:00000040 f773a2e0 ßpointer to the nt!_KTSS
3: kd> dt nt!_KTSS f773a2e0 Backlink
+0x000 Backlink : 0x28 ß And here is our pointer to the previous task state.
This is why !analyze –v has directed us to type in .tss 0x28
But where did !pcr get the address of the KPCR itself? !pcr is listing f7737000. We can find that out also.
3: kd> dt ntkrpamp!_KPCR SelfPcr
+0x01c SelfPcr : Ptr32 _KPCR ßso the pointer is stored at 0x1c
This command demonstrates the use of fs: instead of 0030: (BTW - I then provided the offset of 1c to get the pointer)
3: kd> dd fs:0x1c L1
0030:0000001c f7737000 there it is, we found it
To demonstrate that both addresses reference the same data, let’s dump it out using the size given below.
3: kd> dd f7737000 L0x54
f7737000 b8ae60dc 00000000 00000000 f7737fe0
f7737010 19d5c42c 00000008 7ff9c000 f7737000
f7737020 f7737120 0000001f 00000000 00000000
f7737030 ffffffff 00000000 f773d800 f773d400
f7737040 f773a2e0 00010001 00000008 00000e56
f7737050 08000300 00000000 00000000 00000000
f7737060 00000000 00000000 00000000 00000000
f7737070 00000000 00000000 00000000 00000000
f7737080 00000000 00000000 00000000 00000000
f7737090 00100000 00000003 09f15190 00000000
f77370a0 09f15190 dabc6620 00000000 334e730f
f77370b0 00000000 00000000 00000000 00000000
f77370c0 00000000 00000000 00000000 00000000
f77370d0 00000000 00000000 00000000 00000000
f77370e0 00000000 00000000 00000000 00000000
f77370f0 00000000 00000000 00000000 00000000
f7737100 00000000 00000000 00000000 00000000
f7737110 00000000 00000000 00000000 00000000
f7737120 00010001 87d68438 00000000 f773a090
f7737130 00000003 00000008 0401010f 00000000
f7737140 00000000 00000000 00000000 00000000
3: kd> dd fs:0 L0x54
0030:00000000 b8ae60dc 00000000 00000000 f7737fe0
0030:00000010 19d5c42c 00000008 7ff9c000 f7737000
0030:00000020 f7737120 0000001f 00000000 00000000
0030:00000030 ffffffff 00000000 f773d800 f773d400
0030:00000040 f773a2e0 00010001 00000008 00000e56
0030:00000050 08000300 00000000 00000000 00000000
0030:00000060 00000000 00000000 00000000 00000000
0030:00000070 00000000 00000000 00000000 00000000
0030:00000080 00000000 00000000 00000000 00000000
0030:00000090 00100000 00000003 09f15190 00000000
0030:000000a0 09f15190 dabc6620 00000000 334e730f
0030:000000b0 00000000 00000000 00000000 00000000
0030:000000c0 00000000 00000000 00000000 00000000
0030:000000d0 00000000 00000000 00000000 00000000
0030:000000e0 00000000 00000000 00000000 00000000
0030:000000f0 00000000 00000000 00000000 00000000
0030:00000100 00000000 00000000 00000000 00000000
0030:00000110 00000000 00000000 00000000 00000000
0030:00000120 00010001 87d68438 00000000 f773a090
0030:00000130 00000003 00000008 0401010f 00000000
0030:00000140 00000000 00000000 00000000 00000000
Now that you have an idea of what a state is, let’s examine the stack output of the two states. First, we shall use .tss 0x50 to examine the stack backtrace associated with this state. We shall use the kC command to dump the stack after the .tss command. Notice that we have used the ; command to enter multiple commands on each line.
3: kd> .tss 0x50;kC
eax=00000000 ebx=00000000 ecx=00000000 edx=00000000 esi=00000000 edi=00000000
eip=8088b702 esp=f773d3c0 ebp=00000000 iopl=0 nv up di pl nz na po nc
cs=0008 ss=0010 ds=0023 es=0023 fs=0030 gs=0000 efl=00000000
nt!_KiTrap08:
8088b702 fa cli
*** Stack trace for last set context - .thread/.cxr resets it
nt!_KiTrap08
nt!_KiTrap0E
The stack backtrace shows two trap handlers and nothing else. According to this stack output, we were first attempting to handle a Trap 0x0E which is a page fault. The page fault handler was invoked in an attempt to handle the invalid address that we accessed in the guard page when we overflowed the stack. You can see that after the page fault, we encountered another exception represented by the KiTrap08. This is a result of the EXCEPTION_DOUBLE_FAULT indicating that the page fault handler has also encountered an exception. This matches what is listed as arg1 in the bugcheck data that !analyze –v has output. So, the stack backtrace for .tss 0x50 shows that we were first executing the task referenced by 0x28 for Trap0E/Page Fault, when a task state switch occurred and we switched to .tss 0x50 to handle the Trap08/DoubleFault.
Next, we will use the command .tss 0x28 and dump the stack backtrace associated with that task state
3: kd> .tss 0x28;kC
eax=b8ae0023 ebx=b8ae60ec ecx=87d68438 edx=87758bd8 esi=b8ae6068 edi=808813d8
eip=8088c718 esp=b8ae5fe4 ebp=b8ae5fe4 iopl=0 nv up di pl zr na pe nc
cs=0008 ss=0010 ds=0023 es=0023 fs=0030 gs=0000 efl=00010046
nt!_KiTrap0E+0x5c:
8088c718 89550c mov dword ptr [ebp+0Ch],edx ss:0010:b8ae5ff0=????????
You can see in the output above that the stack backtrace has only displayed KiTrap0xE. We should see multiple stack frames listed. I’m a bit concerned about the fact that I do not see a valid stack backtrace listed in the output above. Nevertheless, let's proceed with our examination.
Now that we have set the proper task state using .tss 0x28 which loaded the registers with the appropriate context, our next step will be to determine where the stack related registers are pointing and how they relate to the 12k range of valid stack addresses for the current thread. This will help us to validate that we did in fact overflow the stack. The easiest way to examine the stack range that this thread was given is to use !thread
!thread
Owning Process 874c6800 Image: StackHog.exe
...
Base b8ae9000 Limit b8ae6000
3: kd> resp;rebp
esp=b8ae5fe4 ßNotice that this is outside of the Base and Limit ranges listed above.
ebp=b8ae5fe4
Since the stack grows toward lower addresses, an overflow of the b8ae6000 limit will result in a value that is below the address of the limit, you can see that the address of esp has fallen out of the valid range of stack space.
3: kd> dd b8ae5fe4 L1
b8ae5fe4 ????????
Let’s look at this memory range in more detail. The invalid ranges displayed by ????? represent the guard page. The range of valid stack addresses starts (or ends depending on how you look at it) at b8ae6000.
3: kd> dd b8ae5fe0 L10
b8ae5fe0 ???????? ???????? ???????? ????????
b8ae5ff0 ???????? ???????? ???????? ????????
b8ae6000 00000000 00000000 00000000 00000000
b8ae6010 00000000 b8ae0000 b8ae0023 00000023
Also, note that we are running in trap handler 0x0E. This is the page fault handler on x86 (refer to your Intel Processor Manuals for more details).
3: kd> u . L1
nt!_KiTrap0E+0x5c
8088c718 89550c mov dword ptr [ebp+0Ch],edx
The address we we're attempting to access may be in cr2. Let’s dump it out.
3: kd> rcr2
cr2=b8ae5fe0 ßThis address is just beyond the stack limit for this thread
What is the present instruction in the trap handler doing?
Ok, so were dereferencing ebp plus an offset of 0x0C. What does that add up to be?
3: kd> ? ebp+0x0c
Evaluate expression: -1196531728 = b8ae5ff0
3: kd> dd b8ae5ff0 L1
b8ae5ff0 ????????
Once the stack overflowed, we can see there were many access attempts to addresses which are not in the valid stack range. This led us to the 7f bugcheck with the double fault parameter.
When the system bugchecks because the entire 12k range of a thread’s kernel-mode stack space has been filled up, there can be a few causes. Drivers on the stack may have made very large allocations on the stack instead of using other methods of obtaining memory such as calling ExAllocatePoolWithTag(). Sometimes this is done since it is quicker to use the stack instead of making calls to allocate and free memory from the operating system pools. Other times a driver will have made calls in a manner that causes too many other calls to be made filling up the stack. It is possible for nested functions that never encounter an exit condition to continuously call themselves to exhaust the stack. Often a system will have software from many different vendors that all install heavy stack consuming drivers into the I/O path. Each driver will use a portion of stack space that will add up to a lot since there are so many drivers installed. For example, if a system has too many file system filter drivers installed in the file system stack and they use more than the minimum amount of stack space possible, it’s not uncommon for all of them put together to cause a stack overflow.
Sometimes when dealing with this error, we need to realize that there may not be any one product to blame. A stop 7f sometimes isn’t about identifying the faulting component as it often is in other areas of troubleshooting. It is more about understanding that stack space isn’t an unlimited resource and developing a clear picture of what lead up to the stack space filling up. Sometimes this will result in the need to engage multiple vendors for assistance when there are a combination of drivers on the stack that are all using a large amount of stack space. Sometimes vendors will provide newer updated drivers that have been optimized to use less stack space. Other times, we simply have too much I/O related software installed and the only answer is to simply remove some of the drivers by uninstalling the product.
NTFS and some 3rd party file system filter drivers employ a technique to avoid a stack overflow. What they do is probe the stack by calling IoGetRemainingStackSize() and if there is not enough stack space left, they will offload the remainder of the work to a dedicated kernel thread that they created just for that purpose. On Vista or Later (or 2003 x64), developers can call KeExpandKernelStackAndCallout, which will allow chaining to another 16k stack. For more information, see http://msdn.microsoft.com/en-us/library/aa906738.aspx.
The easiest way to figure out why we have overflowed the stack is to dump it out and examine the stack backtrace. Therefore, this is typically the first and sometimes the only step necessary to perform when reviewing an EXCEPTION_DOUBLE_FAULT memory dump. We will now proceed to dump out the stack and examine the stack usage of the different drivers and the calls that they made to determine if further investigation is needed. So, let’s do that now. I will use the L200 option; otherwise the debugger will only display the default number of frames which won’t display the entire stack. It doesn’t make much sense to review only the top of the stack since the entire stack is full. I dumped the stack and only got one stack frame listed.
3: kd> kfL200
Memory ChildEBP RetAddr
b8ae5fe4 00000000 nt!_KiTrap0E+0x5c
This is not what I was hoping to see. We don’t have a stack. Let me try using kv to see if there is a trap frame
3: kd> kvL200
ChildEBP RetAddr Args to Child
b8ae5fe4 00000000 00000000 00000000 00000000 nt!_KiTrap0E+0x5c (FPO: [0,0] TrapFrame-EDITED @ 00000000)
So, I don’t see a valid trap frame either. I went back to my !analyze –v output and verified that it had also displayed this one frame only. How will we see what filled up the stack to provide recommendations to the customer or analysis on what happened without the stack? In part two of this blog, we will review how to manually reconstruct the stack and pass values into the kf command in order to get a useful stack backtrace to display.
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Hi All. Recently I had a Windows 2000 case where the machine wouldn’t shut down. After initiating the shutdown process, we saw the user get logged off, and on the console we watched the services shut down. The final “Windows is shutting down…” message was displayed on the screen, and then the screen would go blank. At that point we expected the machine to power down, but it continued running. Even after waiting 10 minutes the machine stayed powered up. The same behavior was exhibited after a restart as well. We tried the same thing in safe mode, but the machine still wouldn’t shut down.
We setup the machine for a full memory dump, and crashed the box once the screen went blank. The customer uploaded the dump, and I started digging.
The first thing I noticed was a bunch of processes still running. I expected csrss.exe, smss.exe, winlogon.exe and the System process , but to my surprise I found other processes such as svchost.exe and services.exe still lingering. Why were they still running?
0: kd> !process 0 0
**** NT ACTIVE PROCESS DUMP ****
PROCESS f95ed480 SessionId: 0 Cid: 0008 Peb: 00000000 ParentCid: 0000
DirBase: 00030000 ObjectTable: f95edf68 TableSize: 62.
Image: System
PROCESS f94de520 SessionId: 0 Cid: 00fc Peb: 7ffff000 ParentCid: 0008
DirBase: 2465c000 ObjectTable: f94def08 TableSize: 34.
Image: SMSS.EXE
PROCESS f94b01c0 SessionId: 0 Cid: 0118 Peb: 7ffff000 ParentCid: 00fc
DirBase: 26515000 ObjectTable: f94b29e8 TableSize: 103.
Image: CSRSS.EXE
PROCESS f94a5d60 SessionId: 0 Cid: 0130 Peb: 7ffff000 ParentCid: 00fc
DirBase: 2671a000 ObjectTable: f94b18a8 TableSize: 114.
Image: WINLOGON.EXE
PROCESS f9495a60 SessionId: 0 Cid: 014c Peb: 7ffff000 ParentCid: 0130
DirBase: 26814000 ObjectTable: f94b2848 TableSize: 204.
Image: SERVICES.EXE
PROCESS f9494860 SessionId: 0 Cid: 0158 Peb: bffdf000 ParentCid: 0130
DirBase: 2681d000 ObjectTable: f9499608 TableSize: 247.
Image: LSASS.EXE
PROCESS f9476500 SessionId: 0 Cid: 01e8 Peb: 7ffff000 ParentCid: 014c
DirBase: 272a1000 ObjectTable: f94833a8 TableSize: 136.
Image: svchost.exe
Let’s find out.
How is Shutdown Performed
At first I thought some service was stuck and not shutting down, causing the entire system to stall. I pulled out my copy of Windows Internals and thumbed through the Startup and Shutdown chapter. I found that to initiate a shutdown, Csrss sends a shutdown message to Winlogon. Winlogon then calls ExitWindowsEx, which in turn sends a message to each session’s Csrss. Inside each session, Csrss then sends messages to all the threads that own a Windows message loop to shut down. If the threads do not respond, Csrss waits for a timeout specified by HKCU\Control Panel\Desktop\HungAppTimeout. Once all the threads that own windows exit, Csrss then terminates the processes as well. If Csrss finds a console application, it sends it a message to shut down. It will wait for a timeout specified by HKCU\Control Panel\Desktop\WaitToKillAppTimeout, and then displays a message on the desktop.
If you want Windows to terminate the processes immediately after timeout period, you can set the HKCU\Control Panel\Desktop\AutoEndTasks to 1.
At this point all the processes in the interactive user process have been shut down. Csrss in session 0 then sends messages to the Service Control Manager(SCM) to shut down all the services. When a service is registered, it also specifies a wait hint. During shutdown, this hint is used by the SCM to wait for services to terminate. Prior to Vista, Csrss waits for the SCM to shut down, but if that does not happen by the timeout value in HKLM\SYSTEM\CurrentControlSet\Control\WaitToKillServiceTimeout, Csrss would just kill the SCM and proceed with the shutdown. Since this could lead to services not completing their shutdown routines, going forward in Vista and beyond Windows now implements preshutdown notification routines. Services that want to get these notifications can register themselves by using the SetServiceStatus API.
Once Csrss has finished its pass notifying system processes that the system is shutting down, Winlogon in session 0 finishes the shutdown process by calling the NtShutdownSystem. Note, many system processes such as Csrss, Smss, WinInit, Services, Lsass are still running when the machine actually powers down. They are ready to be shut down, but the processes still exist. NtShutdownSystem calls PoSetSystemPowerState to shut down all the drivers and the rest of the executive subsystems.
One of the things that PoSetSystemPowerState does is call the I/O manager to send notifications to all the drivers that have requested the shutdown notification. Once the drivers receive these notifications, they can then clean up and perform any special steps that might be needed for their devices. Other subsystems also perform clean up, and the page file is cleared by the Memory Manager if that option was turned on. The I/O manager is called a second time, and then the file system drivers are notified that the system is shutting down. The System actually shuts down in the power manager. The power manager tells the BIOS to turn the power off to the machine.
The Memory Dump Analysis
I started by looking at the Winlogon process in session 0 and found the main thread had called NtShutdownSystem as we expected. I always start with this thread when troubleshooting shutdown problems.
0: kd> !process f94a5d60 17
VadRoot f93eb748 Clone 0 Private 515. Modified 396. Locked 0.
DeviceMap f95c8448
Token e246cdf0
ElapsedTime 0:34:42.0937
UserTime 0:00:00.0109
KernelTime 0:00:01.0015
QuotaPoolUsage[PagedPool] 30112
QuotaPoolUsage[NonPagedPool] 31264
Working Set Sizes (now,min,max) (1172, 50, 345) (4688KB, 200KB, 1380KB)
PeakWorkingSetSize 1866
VirtualSize 29 Mb
PeakVirtualSize 31 Mb
PageFaultCount 3603
MemoryPriority FOREGROUND
BasePriority 13
CommitCharge 888
THREAD f94a5ae0 Cid 130.114 Teb: 7fffe000 Win32Thread: e002e328 WAIT: (Suspended) KernelMode Non-Alertable
f93c2010 SynchronizationEvent
f94a5bc8 NotificationTimer
Not impersonating
Owning Process f94a5d60
Wait Start TickCount 138381 Elapsed Ticks: 842
Context Switch Count 1914 LargeStack
UserTime 0:00:00.0078
KernelTime 0:00:00.0875
Start Address winlogon!_setargv (0x0100ae2c)
Stack Init f1d00000 Current f1cffa60 Base f1d00000 Limit f1cfb000 Call 0
Priority 15 BasePriority 15 PriorityDecrement 0 DecrementCount 0
f1cffa78 dd42d893 dd0655c0 f93c2008 dd0654f0 nt!KiSwapThread+0x1b1
f1cffaa0 dd490be9 f93c2010 00000005 00000000 nt!KeWaitForSingleObject+0x1a3
f1cffae0 dd4908c5 f93c2000 00000001 00000001 nt!PopWaitForSystemPowerIrp+0xf2
f1cffb04 dd4903db 00000000 f1cffbf4 f1cffc78 nt!PopSetDevicesSystemState+0xfb
f1cffbe0 dd468389 00000005 00000004 c0000004 nt!NtSetSystemPowerState+0x329
f1cffbe0 dd432197 00000005 00000004 c0000004 nt!_KiSystemService+0xc9
f1cffc64 dd490122 00000005 00000004 c0000004 nt!ZwSetSystemPowerState+0xb
f1cffd48 dd494db4 00000005 00000004 c0000004 nt!NtSetSystemPowerState+0x70
f1cffd58 dd468389 00000001 00000000 00000000 nt!NtShutdownSystem+0x2e
f1cffd58 77f88e07 00000001 00000000 00000000 nt!_KiSystemService+0xc9
0006fe90 01014c3d 00000001 00000001 000767b8 ntdll!NtShutdownSystem+0xb
0006fea8 01014f6c 00000060 0000000b 0000000b winlogon!ShutdownMachine+0x165
0006ff04 0100e20f 000767b8 0000000b 5ffa0000 winlogon!Logoff+0x216
0006ff20 01007e8c 000767b8 00000005 000735dc winlogon!MainLoop+0x1fb
0006ff58 0100af70 00071fc8 00000000 000735dc winlogon!WinMain+0x37a
0006fff4 00000000 7ffff000 000000c8 00000100 winlogon!WinMainCRTStartup+0x156
Looks like the I/O Manager has sent shutdown notifications to all the devices and this winlogon thread is waiting for it to complete. Dumping out the IRP, I found it had been completed, but for some reason the completion routine had never been called. That is why we are waiting forever. Digging further, it looked like someone had messed up the current IO stack location on the IRP, which resulted in the IRP completion routine never being called.
Let’s take a look at the IRP.
Based on the thread stack you can see Winlogon sent an IRP down to Plug and Play and is waiting for the action to complete. Since there is no obvious way to get a pointer to the IRP from the stack we’ll need to do some digging. We see that nt!PopWaitForSystemPowerIrp is waiting for some type of dispatch object to signal. According to MSDN the first parameter passed to KeWaitForSingleObject() is the object. Here I’m dumping the raw memory for the Object parameter.
0: kd> dc f93c2010
f93c2010 00040001 00000000 f94a5b4c f94a5b4c ........L[J.L[J.
f93c2020 00000000 f94a5ae0 00000000 00000063 .....ZJ.....c...
f93c2030 00000028 00000005 dd47bd20 f93c203c (....... .G.< <.
f93c2040 f93c203c f93c2044 f93c2044 f938fea8 < <.D <.D <...8.
f93c2050 00000000 00000000 01010000 00000100 ................
f93c2060 f946c860 f946c860 f93c23ac 00000000 `.F.`.F..#<.....
f93c2070 00000000 f93c2360 f93c2360 f93c207c ....`#<.`#<.| <.
f93c2080 f93c207c f93c2084 f93c2084 f93c208c | <.. <.. <.. <.
0: kd> !pool f93c2010 2
*f93c2000 size: 420 previous size: 0 (Allocated) *PDss
You may ask yourself, “are any of these values in the PDss pool pointers to IRP pool blocks?” If you wanted to know that, you could iterate over each one of them passing them to !pool or !address. That sounds tedious to do manually. But thankfully, the debugger has some nice command tokens such as .foreach that make this a breeze. If you want to pass each of the above values to !pool, you can perform the following in the debugger:
0: kd> .foreach /pS 1 /ps 1 ( value { dp /c 1 f93c2000 L 420/@$ptrsize } ) { .if( value != 0 ) {.printf "**** %p ****\n", ${value}; !pool ${value} 0x2} }
**** 21018000 ****
21018000: Unable to get contents of pool block
**** 73734450 ****
73734000: Unable to get contents of pool block
**** 00000002 ****
00000000: Unable to get contents of pool block
**** 00000006 ****
.
. (I truncated this output for the blog. There were many more entries)
**** f946c860 ****
*f946c800 size: 1e0 previous size: 40 (Allocated) *Irp
**** f93c23ac ****
**** f93c207c ****
Bingo! There is a pointer to an IRP block in our PDss pool. Let’s take a look at f946c800. I’m dumping it’s pool.
0: kd> !pool f946c800
f946c000 size: 80 previous size: 0 (Allocated) MmCa
f946c080 size: 40 previous size: 80 (Free) ....
f946c0c0 size: 40 previous size: 40 (Allocated) Ntfn
f946c100 size: a0 previous size: 40 (Allocated) File (Protected)
f946c1a0 size: 40 previous size: a0 (Allocated) Ntfr.
Here you see the IRP tag in this raw memory output. The first two DWORDs contain the POOL_HEADER.
0: kd> dc f946c800
f946c800 0f018002 20707249 01d80006 00000000 ....Irp ........
f946c810 00000000 00000000 f946c818 f946c818 ..........F...F.
f946c820 c00000bb 00000000 0b0a0000 04000000 ................
f946c830 00000000 00000000 00000000 00000000 ................
f946c840 00000000 00000000 00000000 00000000 ................
f946c850 00000000 00000000 00000000 00000000 ................
f946c860 f93c2060 f93c2060 f946c9e0 00000000 ` <.` <...F.....
f946c870 00000000 00000000 00000000 00000000 ................
Since the pool header is 8 bytes long we need to add 8 to f946c800 to skip over the header and get to the actual IRP.
0: kd> ? f946c800 + 8
Evaluate expression: -112801784 = f946c808
0: kd> !irp f946c808
Irp is active with 10 stacks 11 is current (= 0xf946c9e0)
No Mdl Thread 00000000: Irp is completed.
cmd flg cl Device File Completion-Context
[ 0, 0] 0 0 00000000 00000000 00000000-00000000
Args: 00000000 00000000 00000000 00000000
[ 16, 2] 0 e1 f9533ba0 00000000 dd491028-f93c2354 Success Error Cancel pending
\Driver\Diskperf nt!PopCompleteSystemPowerIrp
Args: 00000000 00000000 00000006 00000005
The Major function is 16 (IRP_MJ_POWER) which synchs up with the power management activity we saw in the stack. You can find all the major function codes for IRPs in the standard header files such as WDM.H or NTDDK.H available with the WDK. There is an excellent chance this is the IRP Winlogon sent down to Plug and Play. Now that we have the IRP in question with the IO Stack locations we see that the pending status is set.
It appears to be stuck in \Driver\Diskperf which is the device stack for the device associated with Diskperf. At this point I determined it was most likely due to one of the devices in it’s devstack.
0: kd> !devstack f9533ba0
!DevObj !DrvObj !DevExt ObjectName
f94d5660 \FileSystem\Foo f94d5718
f95338a0 \Driver\VSP f9533958
> f9533ba0 \Driver\Diskperf f9533c58
f9539150 \Driver\Ftdisk f9539208 HarddiskVolume1
!DevNode f953aba8 :
DeviceInst is "STORAGE\Volume\1&30a96598&0&Signature80Offset7E00Length2730C00"
ServiceName is "DiskPerf"
Doing a bing search for the drivers in the list above (and shutdown), I found a file system filter driver known to cause this issue in the past. Updating to the latest version of the driver fixed the issue, and the machine would shutdown normally.
BTW - For more information on how Windows performs a shutdown check out this past article from Ntdebugging - http://blogs.msdn.com/ntdebugging/archive/2007/06/09/how-windows-shuts-down.aspx
Today I want to talk about tracking down leaking pool. Back with Server 2003 and before, leaking pool was a major issue because it was a limited resource. In Vista and beyond, it isn’t as much of an issue since pool is allocated dynamically, but it can still cause system performance issues if a component uses too much of it. Since all nonpaged pool allocations are locked in RAM, it leaves the system with a smaller amount of RAM for anything else.
I recently worked a case involving leaking nonpaged pool and it was easy to troubleshoot. I thought I would walk through the steps I used to narrow down the issue in this article. So let's get started. Note - The following can be done on a live machine or with a kernel memory dump as well.
Running the !vm command, you can get statistics about the virtual memory usage on the system.
Virtual Memory Usage
Physical Memory: 655219 ( 2620876 Kb)
Page File: \??\C:\pagefile.sys
Current: 4190208Kb Free Space: 4115148Kb
Minimum: 4190208Kb Maximum: 4190208Kb
Available Pages: 303760 ( 1215040 Kb)
ResAvail Pages: 571159 ( 2284636 Kb)
Modified Pages: 677 ( 2708 Kb)
NonPagedPool Usage: 59613 ( 238452 Kb)
NonPagedPool Max: 69377 ( 277508 Kb)
NonPaged Pool Usage : 85%
********** Excessive NonPaged Pool Usage *****
PagedPool 0 Usage: 31684 ( 126736 Kb)
PagedPool 1 Usage: 1824 ( 7296 Kb)
PagedPool 2 Usage: 1821 ( 7284 Kb)
PagedPool 3 Usage: 1824 ( 7296 Kb)
PagedPool 4 Usage: 1827 ( 7308 Kb)
PagedPool Usage: 38980 ( 155920 Kb)
PagedPool Maximum: 86016 ( 344064 Kb)
Paged Pool Usage : 45%
Hmm….looks like something is using a lot of nonpaged pool. To determine what is using the maximum amount of nonpaged pool, run the !poolused command.
0: kd>!poolused 2
Sorting by NonPaged Pool Consumed
Pool Used:
NonPaged Paged
Tag Allocs Used Allocs Used
Thre 326514 208968960 0 0
MmCm 647 2830624 0 0
LSwi 1 2293760 0 0
Npfs 1681 1276672 1202 208320
File 5863 945472 0 0
Dmio 131 525760 0 0
Irp 1158 508800 0 0
Adding a value of 2 to end of the command instructs the debugger to sort by nonpaged pool used. For more information on the command, please review the help file.
According to the output, the “Thre” tag is the highest user of nonpaged pool. To get a rough idea of all the modules that might be allocating pool using this tag, we can search all the modules to see if they contain the string. This can help narrow down the problem to a few drivers. To search all the modules, just use the for_each_module command.
!for_each_module s -a @#Base @#End "Thre"
Look at David Butler’s blog post if you want to narrow it down even further by looking at each driver’s import table to see if they are importing the ExAllocatePool or ExAllocatePoolWithTag functions. Though, in this case I already know the “Thre” tag is used by the kernel to allocate thread objects. Could the kernel be leaking thread objects? Most likely not, otherwise we would have seen a lot of customers reporting this issue. I am not ruling it out, but to start off, I am going to focus on the third party software installed on the machine.
Let’s dump out a few sample pool allocations, to see what exactly they are. I was sure there were going to be a ton of them, so I just hit the Break button once the output started flying by.
0: kd> !poolfind Thre 0
Scanning large pool allocation table for Tag: Thre
Searching NonPaged pool (81093000 : 89093000) for Tag: Thre
81098000 size: 280 previous size: 0 (Allocated) Thre (Protected)
81098380 size: 280 previous size: 100 (Allocated) Thre (Protected)
81098600 size: 280 previous size: 280 (Allocated) Thre (Protected)
81098880 size: 280 previous size: 280 (Allocated) Thre (Protected)
81098b00 size: 280 previous size: 280 (Allocated) Thre (Protected)
81098d80 size: 280 previous size: 280 (Allocated) Thre (Protected)
81099000 size: 280 previous size: 0 (Allocated) Thre (Protected)
810992c0 size: 280 previous size: 40 (Allocated) Thre (Protected)
810996e0 size: 280 previous size: 1a0 (Allocated) Thre (Protected)
81099960 size: 280 previous size: 280 (Allocated) Thre (Protected)
81099d80 size: 280 previous size: 1a0 (Allocated) Thre (Protected)
8109a000 size: 280 previous size: 0 (Allocated) Thre (Protected)
8109a380 size: 280 previous size: 100 (Allocated) Thre (Protected)
8109a600 size: 280 previous size: 280 (Allocated) Thre (Protected)
8109a880 size: 280 previous size: 280 (Allocated) Thre (Protected)
It would be interesting to see the thread stacks for the KTHREAD structures in these pools. To dump out the thread using the !thread command, I needed to figure out where the base of the KTHREAD structure started in the pool. To figure out the offset I dumped out all the threads in the System process, picked a random thread, and then calculated the thread’s offset from the start of it’s pool block. You can use any thread in the entire system because the offset will always be the same.
0: kd> !thread 8904a180
THREAD 8904a180 Cid 8.18 Teb: 00000000 Win32Thread: 00000000 WAIT: (WrEventPairLow) KernelMode Non-Alertable
80478d40 Unknown
Owning Process 890754a0
Wait Start TickCount 3831270 Elapsed Ticks: 209711
Context Switch Count 28238
UserTime 0:00:00.0000
Start Address nt!ExpWorkerThread (0x80417a9a)
Stack Init f6444000 Current f6443d34 Base f6444000 Limit f6441000 Call 0
Priority 13 BasePriority 13 PriorityDecrement 0 DecrementCount 0
f6443d4c 8042ef5f bfec0068 80478d40 8904a180 nt!KiSwapThread+0x1b1
f6443d70 80417b0d 00000000 80442f00 00000000 nt!KeRemoveQueue+0x197
f6443da8 804578c2 00000000 00000000 00000000 nt!ExpWorkerThread+0x73
f6443ddc 8046c966 80417a9a 00000000 00000000 nt!PspSystemThreadStartup+0x54
00000000 00000000 00000000 00000000 00000000 nt!KiThreadStartup+0x16
0: kd> !pool 8904a180
…
8904a080 size: e0 previous size: 20 (Allocated) Dire (Protected)
*8904a160 size: 280 previous size: e0 (Allocated) *Thre (Protected)
8904a3e0 size: 280 previous size: 280 (Allocated) Thre (Protected)
0: kd> ?8904a180-8904a160
Evaluate expression: 32 = 00000020
So based on my calculations, the Kthread structure starts 32 bytes into the Thre pool.
Now I can use this offset to dump threads stacks from the pools returned by the poolfind command. Some of the threads are going to be valid, but most of them are going to be doing something fishy. Basically I need to use a large sample to figure out what would be causing the Thre tag to leak. After a few tries, I was able to find a lot of threads which looked like this -
kd> !Thread 81098620
THREAD 81098620Cid 8e8.a391c Teb: 00000000 Win32Thread: 00000000 TERMINATED
Owning Process 886f5020
Wait Start TickCount 1949088 Elapsed Ticks: 2091893
Context Switch Count 2
KernelTime 0:00:00.0000
Start Address 0x7c57b740
Win32 Start Address 0x20003474
Stack Init 0 Current bca4fc44 Base bca50000 Limit bca4d000 Call 0
Priority 16 BasePriority 8 PriorityDecrement 0 DecrementCount 0
kd> !PROCESS 886f5020 0
PROCESS 886f5020SessionId: 0 Cid: 08e8 Peb: 7ffdf000 ParentCid: 016c
DirBase: 694bc000 ObjectTable: 88b3ec68 TableSize: 391189.
Image: xxxxxx.EXE
Looks like the thread is terminated. But why is it still hanging around?
0: kd> !object 81098620
Object: 81098620 Type: (89075900) Thread
ObjectHeader: 81098608
HandleCount: 1 PointerCount: 1
Aha! Since the handle count and pointer count are not 0, the system has not freed this Thre allocation. Most likely something has an open handle to this object. Dumping out all the processes using the !process command, I found a particular process with 388,836 open handles. This is the culprit. Interesting enough, this is the same process which was listed in the thread’s process field.
1 Token
1 Desktop
1 Section
2 WindowStation
3 Directory
4 IoCompletion
5 Mutant
7 Port
9 Semaphore
15 File
73 Event
65110 Key
323155 Thread
388386 Handles in process
Running !handle 0 2 886f5020 and analyzing the output, I found a ton of thread handles. Most likely the process is leaking thread handles and preventing the system from cleaning up the thread allocations. This is resulting in high usage of nonpaged pool, which is causing the performance issues seen by the customer.
We contacted the software vendor responsible for the process, and it turned out to be a known bug in their software. An update was provided to the customer, and it fixed the issue immediately.