In our previous article we discussed how to identify a pool leak using perfmon. Although it may be interesting to know that you have a pool leak, most customers are interested in identifying the cause of the leak so that it can be corrected. In this article we will begin the process of identifying what kernel mode driver is leaking pool, and possibly identify why.
Often when we are collecting data for a poor performance scenario there are two pieces of data that we collect. Perfmon log data is one, as we discussed in our previous article. The other piece of data is poolmon logs. The memory manager tracks pool usage according to the tag associated with the pool allocations, using a technique called pool tagging. Poolmon gathers this data and displays it in an easy to use format. Poolmon can also be configured to dump data to a log, and in some scenarios it is beneficial to schedule poolmon to periodically collect such logs. There are several available techniques to schedule poolmon, however that is beyond the scope of this article.
Poolmon has shipped with many different packages over the years; it is currently available with the Windows Driver Kit. If you install the WDK to the default folder, poolmon will be in “C:\Program Files (x86)\Windows Kits\8.0\Tools\x64”. Poolmon does not have dependencies on other modules in this folder; you can copy it to your other computers when you need to investigate pool usage.
How does pool tagging work? When a driver allocates pool it calls the ExAllocatePoolWithTag API. This API accepts a tag - a four-letter string - that will be used to label the allocation. It is up to a driver developer to choose this tag. Ideally each developer will choose a tag that is unique to their driver and use a different tag for each code path which calls ExAllocatePoolWithTag. Because each tag should be unique to each driver, if we can identify the tag whose usage corresponds with the leak we can then begin to identify the driver which is leaking the memory. The tag may also give the driver developer clues as to why the memory is being leaked, if they use a unique tag for each code path.
To view the pool usage associated with each tag run “poolmon -b” from a command prompt. This will sort by the number of bytes associated with each tag. If you are tracking pool usage over a period of time, you can log the data to a file with “poolmon -b -n poolmonlog1.txt”, replacing 1 with increasing numbers to obtain a series of logs. Once you have a series of logs you may be able to view usage increasing for a specific tag, in a corresponding fashion to what you see in perfmon.
When analyzing poolmon the important data is at the top. Typically the tag with the largest usage in bytes is the cause of the leak.
In the above data we can see that the tag with the most pool usage is “Leak”. Now that we know what tag is leaking we need to identify what driver is using this tag. Techniques for associating a leak with a tag vary, but findstr is often effective. Most drivers are located in c:\windows\system32\drivers, so that is a good starting point when looking for the driver. If you don’t find a result in that folder, go up a folder and try again, repeating until you get to the root of the drive.
C:\>findstr /s Leak *.sys
·∟ §£♂ Θ─☺ A╗☻ E☼"├Θ╡☺ Hï♣╔♂ ╞ $Θª☺ Hï♣:Hc┴ ┴ê\♦@ë
└δ_Aï ╞♣@∟ ☺ë♣▓← Aï@♦ë♣¼← δCAâ∙♦u╓AïAìI♦A╕Leak;┴☼B┴3╔ï╨ §
In the above output we can see that “Leak” is used in myfault.sys. If we hadn’t forced this leak with notmyfault, the next step in troubleshooting would be an internet search for the tag and the driver. Often such a search will allow you to identify a specific fault within the driver and a solution.
Don’t panic if findstr doesn’t find your tag, or if you find the tag but it is not unique to one driver. In future articles we will cover additional techniques for associating drivers with tags, and for associating allocations with specific code within a driver.
Over the years the NTDebugging Blog has published several articles about pool memory and pool leaks. However, we haven’t taken a comprehensive approach to understanding and troubleshooting pool memory usage. This upcoming series of articles is going to tackle pool leaks from the basics to advanced troubleshooting techniques. Most of the examples will use the Windows Sysinternals tool NotMyFault to generate a leak so our readers will be able to reproduce the described behavior and repeat the troubleshooting steps.
We need to start by understanding what pool is and how it is used. Pool is virtual memory that is used by drivers in much the same way user mode applications use heap. A driver developer calls ExAllocatePoolWithTag to get a block of memory that can be used in much the same way a user mode programmer would use memory returned by HeapAlloc or malloc. The memory manager, which is responsible for managing pool, is able to efficiently handle small allocations by taking a page of virtual memory (typically 4KB) and breaking it up into smaller blocks. The memory manager is also able to allocate pool in blocks larger than a page. There are two types of pool a developer can request from ExAllocatePoolWithTag, paged pool and nonpaged pool. As the names suggest one type of pool memory can be paged out, and the other cannot be paged. Paged pool is used for most allocations, nonpagedpool is used for memory that will be written or read at an IRQL of DISPATCH_LEVEL or above.
Pool leaks happen when a driver calls ExAllocatePoolWithTag but never calls the corresponding ExFreePool or ExFreePoolWithTag routine. A leak is different than just high memory utilization, which may happen in normal conditions as load increases. For example, the srv.sys driver creates work items for incoming requests, and when there is a large amount of SMB traffic to a server the pool usage from srv.sys may increase to handle this traffic. Typically the differentiation between a leak and high memory usage due to load is that a leak never decreases. Memory usage that is load related should decrease when the load is reduced. Monitoring is required to differentiate between these two scenarios. Performance Monitor (aka perfmon) is typically the most effective tool to begin such an investigation.
The symptom of a pool leak is often poor system performance when the system runs out of pool, or on 64-bit systems the pool may begin to consume a large amount of the available memory. This symptom makes perfmon an ideal tool to begin troubleshooting as it can be used to identify a wide variety of potential causes of poor performance. Perfmon is most useful when it is started before a system enters a state of poor performance so that trend data can be analyzed leading up to the problem.
You can use the below commands from an elevated command prompt to collect perfmon data from such a scenario.
First create the data collector. This command collects data from a variety of counters at a 5 minute interval and is designed to be run for several hours prior to and during a the time a system experiences poor performance (shorter intervals can be used for leaks that happen faster than several hours). We often recommend collecting these counters to perform general performance troubleshooting because we usually don’t know that there is a memory leak until after this data is collected and analyzed.
Logman.exe create counter PerfLog-Long -o "c:\perflogs\\%computername%_PerfLog-Long.blg" -f bincirc -v mmddhhmm -max 300 -c "\LogicalDisk(*)\*" "\Memory\*" "\Cache\*" "\Network Interface(*)\*" "\Paging File(*)\*" "\PhysicalDisk(*)\*" "\Processor(*)\*" "\Processor Information(*)\*" "\Process(*)\*" "\Redirector\*" "\Server\*" "\System\*" "\Server Work Queues(*)\*" "\Terminal Services\*" –si 00:05:00
Then start collecting data:
Logman.exe start PerfLog-Long
When the performance problem is being experienced, stop collecting data:
Logman.exe stop PerfLog-Long
After you have collected the data, open the .blg file in the Performance Monitor MMC snap-in. Browse to the Memory object, and add the counters “Pool Nonpaged Bytes” and “Pool Paged Bytes”. This should leave you with a view similar to the below screenshot.
The steadily increasing line in the above screenshot, without a substantial decrease in the line, is an indicator that nonpaged pool memory is being leaked. If we look at the maximum count we see that nonpaged pool has consumed 540MB. The significance of this value depends on the amount of RAM in the system. In this instance the system has 1GB of RAM so nonpaged pool is consuming 54% of the available memory. We can now conclude that the cause of the performance problem is a nonpaged pool memory leak, which is consuming a large amount of RAM and preventing other components from using this RAM.
Next we need to start investigating which driver has allocated the most pool. We will begin that in part 2.
Some APIs should come with a warning in big red letters saying “DANGER!”, or perhaps more subtly “PROCEED WITH CAUTION”. One such API is ExSetResourceOwnerPointer. Although the documentation contains an explanation of what limited activity you can do with the resource after making this call, its warning is not very strongly worded.
You may see evidence of a call to ExSetResourceOwnerPointer in a debug. A lock in !locks will have an unusual owner field, such as the one shown below:
2: kd> !locks **** DUMP OF ALL RESOURCE OBJECTS **** KD: Scanning for held locks...
Resource @ 0xfffffa8011efede8 Exclusively owned Contention Count = 20 NumberOfSharedWaiters = 16 Threads: fffff88007fab7f3-02<*> *** Unknown owner, possibly FileSystem fffffa80169538a0-01 fffffa801ea69b60-01 fffffa8017dfd430-01 fffffa800cd76b60-01 fffffa801512a410-01 fffffa801279b340-01 fffffa8016d079a0-01 fffffa8015452aa0-01 fffffa801607bb60-01 fffffa8012f79b60-01 fffffa8013b4e040-01 fffffa801b03e300-01 fffffa800cd77040-01 fffffa8013a8e040-01 fffffa800cd76040-01 fffffa80172d7490-01
The error “*** Unknown owner, possibly FileSystem” is an indicator that the owner field of this eresource has likely been modified by ExSetResourceOwnerPointer. Fortunately for us debuggers, programmers often point the owner field to a location on the thread stack. You can pass an address to the !thread command and it will interpret the address as a stack value.
2: kd> !thread fffff88007fab7f3 e fffff88007fab7f3 is not a thread object, interpreting as stack value... THREAD fffffa80169538a0 Cid 0004.0638 Teb: 0000000000000000 Win32Thread: 0000000000000000 WAIT: (WrResource) KernelMode Non-Alertable fffffa80139ea3e0 Semaphore Limit 0x7fffffff IRP List: fffffa8016fd1010: (0006,0310) Flags: 00000884 Mdl: 00000000 Not impersonating DeviceMap fffff8a000008aa0 Owning Process fffffa800cd6a5f0 Image: System Attached Process N/A Image: N/A Wait Start TickCount 27606952 Ticks: 141 (0:00:00:02.199) Context Switch Count 90787 UserTime 00:00:00.000 KernelTime 00:00:02.496 Win32 Start Address nt!ExpWorkerThread (0xfffff80002293a50) Stack Init fffff88007fabdb0 Current fffff88007fab3a0 Base fffff88007fac000 Limit fffff88007fa6000 Call 0 Priority 14 BasePriority 13 UnusualBoost 1 ForegroundBoost 0 IoPriority 2 PagePriority 5 Child-SP RetAddr Call Site fffff880`07fab3e0 fffff800`0228da52 nt!KiSwapContext+0x7a fffff880`07fab520 fffff800`0228fbaf nt!KiCommitThreadWait+0x1d2 fffff880`07fab5b0 fffff800`0224ec9e nt!KeWaitForSingleObject+0x19f fffff880`07fab650 fffff800`022ad98c nt!ExpWaitForResource+0xae fffff880`07fab6c0 fffff880`0140fc10 nt!ExAcquireSharedStarveExclusive+0x1bc fffff880`07fab720 fffff880`0140f8e2 sis!SipDereferenceCSFile+0x40 fffff880`07fab750 fffff880`0140f608 sis!SipDereferencePerLink+0x62 fffff880`07fab780 fffff880`014102e7 sis!SipDereferenceScb+0x184 fffff880`07fab7c0 fffff800`025796e6 sis!SiFilterContextFreedCallback+0xaf fffff880`07fab7f0 fffff880`016b9bcc nt!FsRtlTeardownPerStreamContexts+0xe2 fffff880`07fab840 fffff880`016b98d5 Ntfs!NtfsDeleteScb+0x108 fffff880`07fab880 fffff880`0162ccb4 Ntfs!NtfsRemoveScb+0x61 fffff880`07fab8c0 fffff880`016b72dc Ntfs!NtfsPrepareFcbForRemoval+0x50 fffff880`07fab8f0 fffff880`01635882 Ntfs!NtfsTeardownStructures+0xdc fffff880`07fab970 fffff880`016ce813 Ntfs!NtfsDecrementCloseCounts+0xa2 fffff880`07fab9b0 fffff880`016a838f Ntfs!NtfsCommonClose+0x353 fffff880`07faba80 fffff880`016cd7ef Ntfs!NtfsFspClose+0x15f fffff880`07fabb50 fffff880`01635c0d Ntfs!NtfsCommonCreate+0x193f fffff880`07fabd30 fffff800`0227e787 Ntfs!NtfsCommonCreateCallout+0x1d fffff880`07fabd60 fffff800`0227e741 nt!KySwitchKernelStackCallout+0x27 (TrapFrame @ fffff880`07fabc20) fffff880`085fffe0 fffff800`0229620a nt!KiSwitchKernelStackContinue fffff880`08600000 fffff880`01635b2f nt!KeExpandKernelStackAndCalloutEx+0x29a fffff880`086000e0 fffff880`016d29c0 Ntfs!NtfsCommonCreateOnNewStack+0x4f fffff880`08600140 fffff880`013330b6 Ntfs!NtfsFsdCreate+0x1b0 fffff880`086002f0 fffff800`0258d717 fltmgr!FltpCreate+0xa6 fffff880`086003a0 fffff800`0258379f nt!IopParseDevice+0x5a7 fffff880`08600530 fffff800`02588b16 nt!ObpLookupObjectName+0x32f fffff880`08600630 fffff800`0258f827 nt!ObOpenObjectByName+0x306 fffff880`08600700 fffff800`02599438 nt!IopCreateFile+0x2b7 fffff880`086007a0 fffff880`01405bcf nt!NtCreateFile+0x78 fffff880`08600830 fffff880`01405fbf sis!SipOpenBackpointerStream+0x10b fffff880`086008f0 fffff880`0140657d sis!SipOpenCSFileWork+0x3bf fffff880`08600c70 fffff800`02293b61 sis!SipOpenCSFile+0x21 fffff880`08600cb0 fffff800`0252ea26 nt!ExpWorkerThread+0x111 fffff880`08600d40 fffff800`02264866 nt!PspSystemThreadStartup+0x5a fffff880`08600d80 00000000`00000000 nt!KxStartSystemThread+0x16
Looking at the call stack for the above thread we can see that sis.sys is trying to acquire the eresource shared. Ordinarily, if a thread already owns an eresource exclusive, it can obtain it shared without first releasing the exclusive ownership. In this scenario the kernel will compare the eresource’s owner field to the current thread and if they match the thread will be allowed to take shared ownership of the eresource. This is where the danger of ExSetResourceOwnerPointer comes into play. If you change the owner field with ExSetResourceOwnerPointer then this check fails because the owner field doesn’t match the current thread.
The result of this scenario is that the thread waits for the exclusive owner to release the lock so this thread can get shared access. Unfortunately this thread is the exclusive owner, and it is the shared waiter. The thread has deadlocked on itself.
Even if you are careful in your handling of the resource after calling ExSetResourceOwnerPointer, there is often a risk that your driver may be re-entered in the same thread and you may end up in a scenario you didn’t initially anticipate. This is why using this API is dangerous, and should be avoided when not absolutely necessary.
This issue demonstrated in this article was addressed in KB2608658 (issue 3), which is available for download from the Microsoft Download Center.
In a previous article I showed how to track an I/O request from the filesystem, through the class driver, and to the storage driver. In that article I concluded with "From this data we can usually assume that the request has been sent to the disk drive and we are waiting for the disk to respond" and "There may be conditions where the request is stuck in storport, or in the miniport". In this article I want to show what happens to the I/O request when gets to storport.sys.
You will find that the structures needed to investigate a request inside of storport can be dumped with the dt command using publicly available storport.sys symbols. However, until now those structures have not been useful to most debuggers because we have not described how an IRP goes through storport, or how to find an IRP once it has been sent to storport.
In the previous article I showed how to find the IRP that was sent to the storport miniport driver. Below is the example IRP we will be using today.
48: kd> !irp fffffa81`4135a010
Irp is active with 5 stacks 3 is current (= 0xfffffa814135a170)
Mdl=fffffa8232ad1100: No System Buffer: Thread 00000000: Irpstack trace.
cmd flg cl Device File Completion-Context
[ 0, 0] 0 0 00000000 00000000 00000000-00000000
Args: 00000000 00000000 00000000 00000000
>[ f, 0] 0 e1 fffffa813ba0c060 00000000 fffff880011651a0-fffffa8141609cb0 Success Error Cancel pending
Args: fffffa8231a15c10 00000000 00000000 fffffa813ba0c1b0
[ f, 0] 0 e1 fffffa80c329f400 00000000 fffff88001487f8c-fffffa81414bc9c0 Success Error Cancel pending
Args: fffffa8231a15c10 00000000 00000000 fffffa8141609cb0
[ f, 0] 0 e1 fffffa80c34ab550 00000000 fffff88001dbaa00-fffffa8231a15af0 Success Error Cancel pending
Args: fffffa8231a15c10 00000000 00000000 00000000
Although the device object in the IRP is named \Driver\stormini, the dispatch routines for the driver all point to storport. Storport is the port driver that handles queuing the request, stormini is the miniport.
48: kd> !drvobj \Driver\stormini 2
Driver object (fffffa80c19d5bf0) is for:
DriverEntry: fffff88001729008 stormini!StateHandler
DriverUnload: fffff8800168f28c stormini
AddDevice: fffff88001416dc0 storport!DriverAddDevice
 IRP_MJ_CREATE fffff880014582a0 storport!RaDriverCreateIrp
 IRP_MJ_CREATE_NAMED_PIPE fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_CLOSE fffff880014581c0 storport!RaDriverCloseIrp
 IRP_MJ_READ fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_WRITE fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_QUERY_INFORMATION fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_SET_INFORMATION fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_QUERY_EA fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_SET_EA fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_FLUSH_BUFFERS fffff80001a721d4 nt!IopInvalidDeviceRequest
[0a] IRP_MJ_QUERY_VOLUME_INFORMATION fffff80001a721d4 nt!IopInvalidDeviceRequest
[0b] IRP_MJ_SET_VOLUME_INFORMATION fffff80001a721d4 nt!IopInvalidDeviceRequest
[0c] IRP_MJ_DIRECTORY_CONTROL fffff80001a721d4 nt!IopInvalidDeviceRequest
[0d] IRP_MJ_FILE_SYSTEM_CONTROL fffff80001a721d4 nt!IopInvalidDeviceRequest
[0e] IRP_MJ_DEVICE_CONTROL fffff88001453010 storport!RaDriverDeviceControlIrp
[0f] IRP_MJ_INTERNAL_DEVICE_CONTROL fffff880014016c0 storport!RaDriverScsiIrp
 IRP_MJ_SHUTDOWN fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_LOCK_CONTROL fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_CLEANUP fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_CREATE_MAILSLOT fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_QUERY_SECURITY fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_SET_SECURITY fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_POWER fffff8800141a9d0 storport!RaDriverPowerIrp
 IRP_MJ_SYSTEM_CONTROL fffff88001458f40 storport!RaDriverSystemControlIrp
 IRP_MJ_DEVICE_CHANGE fffff80001a721d4 nt!IopInvalidDeviceRequest
 IRP_MJ_QUERY_QUOTA fffff80001a721d4 nt!IopInvalidDeviceRequest
[1a] IRP_MJ_SET_QUOTA fffff80001a721d4 nt!IopInvalidDeviceRequest
[1b] IRP_MJ_PNP fffff88001459b00 storport!RaDriverPnpIrp
To find what storport has done with the IRP we must look at the unit. The device extension contains the information regarding the state of the unit.
48: kd> !devobj fffffa813ba0c060
Device object (fffffa813ba0c060) is for:
000000aa \Driver\stormini DriverObject fffffa80c19d5bf0
Current Irp 00000000 RefCount 0 Type 00000007 Flags 00001050
Dacl fffff9a10049abb0 DevExt fffffa813ba0c1b0 DevObjExt fffffa813ba0c630 DevNode fffffa80c19dad90
Unknown flags 0x00000800
AttachedDevice (Upper) fffffa80c31ef9d0 \Driver\Disk
Device queue is not busy.
48: kd> dt storport!_RAID_UNIT_EXTENSION fffffa813ba0c1b0
+0x000 ObjectType : 1 ( RaidUnitObject )
+0x008 DeviceObject : 0xfffffa81`3ba0c060 _DEVICE_OBJECT
+0x010 Adapter : 0xfffffa80`c25a61b0 _RAID_ADAPTER_EXTENSION
+0x018 SlowLock : 0
+0x020 DeviceState : 1 ( DeviceStateWorking )
+0x028 NextUnit : _LIST_ENTRY [ 0xfffffa80`c25941d8 - 0xfffffa80`c25a6228 ]
+0x038 UnitTableLink : _LIST_ENTRY [ 0xfffffa80`c25a3320 - 0xfffffa80`c25e7b48 ]
+0x048 Address : _STOR_SCSI_ADDRESS
+0x050 Identity : _STOR_SCSI_IDENTITY
+0x070 AlignmentInfo : _STOR_ALIGNMENT_INFO
+0x080 Flags : <unnamed-tag>
+0x088 ZeroOutstandingEvent : _KEVENT
+0x0a0 OutstandingCount : 0n30
+0x0a4 PagingPathCount : 0
+0x0a8 CrashDumpPathCount : 0
+0x0ac HiberPathCount : 0
+0x0b0 TagList : _QUEUE_TAG_LIST
+0x0d0 IoQueue : _IO_QUEUE
+0x140 MaxQueueDepth : 0xff
+0x144 Power : _RAID_POWER_STATE
+0x150 PendingQueue : _STOR_EVENT_QUEUE
+0x170 PendingTimer : _KTIMER
+0x1b0 PendingDpc : _KDPC
+0x1f0 PauseTimer : _KTIMER
+0x230 PauseTimerDpc : _KDPC
+0x270 RestartDpc : _KDPC
+0x2b0 CommonBufferVAs : (null)
+0x2b8 CommonBufferSize : 0
+0x2bc CommonBufferBlocks : 0
+0x2c0 UnitExtension : 0xfffffa80`c185dda0 Void
+0x2c8 DefaultTimeout : 0xa
+0x2d0 DeferredList : <unnamed-tag>
+0x410 ResetCount : 0n1
+0x418 ResetResources : _RAID_IO_RESOURCES
+0x438 ResetResourcesAcquired : 0n1
+0x43c SenseInfoSize : 0x14 ''
+0x440 IdleCounter : (null)
+0x448 PowerUpRequired : 0n0
+0x450 HierarchicalResetWorkItem : (null)
+0x458 HierarchicalResetWorkItemTimeoutCountdown : 0n0
+0x45c HierarchicalResetAbortCount : 1
+0x460 HierarchicalResetWorkItemSpinLock : 0
+0x468 RegistryParameters : _RAID_UNIT_REGISTRY_PARAMETERS
Two parts of the device extension are relevant to finding the I/O request, the PendingQueue and the IoQueue. Under most conditions, when a request is sent to storport it will be sent to the miniport driver and put on the PendingQueue. The PendingQueue has a list of outstanding requests and a timer that is decremented every second. The timer is refreshed when the request on the head of the queue is completed. A request is completed when the miniport calls StorPortNotification with RequestComplete. If the timer is decremented to 0, the request has timed out and storport will do a hierarchical reset to attempt to correct the problem.
In the below PendingQueue we can see that there is one entry in the queue, and the Timeout is set to -2. Under most conditions the Timeout will be a value between 0 and 0x3c (0n60), indicating the number of seconds left in the timer. A Timeout value of -2 indicates that there was a timeout and it is being handled. A Timeout value of -1 indicates that the queue is empty.
48: kd> dt storport!_RAID_UNIT_EXTENSION fffffa813ba0c1b0 PendingQueue.
+0x150 PendingQueue :
+0x000 List : _LIST_ENTRY [ 0xfffff880`05906040 - 0xfffff880`05906040]
+0x010 Lock : 0
+0x018 Timeout : 0xfffffffe
Items in the queue will be in the form of an XRB, or Extended Request Block. There is one request in the pending queue (we know there is one request because the Flink and Blink point to the same address). The Irp for this XRB is not our IRP, so we need to go look somewhere else for our request.
48: kd> dt storport!_EXTENDED_REQUEST_BLOCK 0xfffff880`05906040-30
+0x000 Signature : 0x1f2e3d4c
+0x008 Pool : (null)
+0x010 OwnedMdl : 0y0
+0x010 RemoveFromEventQueue : 0y1
+0x010 State : 0y011
+0x010 RemappedSenseInfo : 0y1
+0x012 InitiatingProcessor : _PROCESSOR_NUMBER
+0x018 InitiatingToken : (null)
+0x020 CompletedLink : _SLIST_ENTRY
+0x030 PendingLink : _STOR_EVENT_QUEUE_ENTRY
+0x048 Mdl : (null)
+0x050 SgList : (null)
+0x058 RemappedSgListMdl : (null)
+0x060 RemappedSgList : (null)
+0x068 Irp : 0xfffffa82`35142ee0 _IRP
+0x070 Srb : 0xfffffa81`41cf54a0 _SCSI_REQUEST_BLOCK
+0x078 SrbData : <unnamed-tag>
+0x098 Adapter : 0xfffffa80`c25a61b0 _RAID_ADAPTER_EXTENSION
+0x0a0 Unit : 0xfffffa81`3ba0c1b0 _RAID_UNIT_EXTENSION
+0x0a8 ScatterGatherBuffer :  ""
+0x250 CompletionRoutine : 0xfffff880`014199d0 void storport!RaidUnitCompleteResetRequest+0
+0x258 u : <unnamed-tag>
+0x270 RequestStartTimeStamp : 0
If the unit is in a state where the request cannot be sent to the miniport, the requests will be queued on the DeviceQueue in the IoQueue. Storport may not be able to send the request to the miniport because the queue is full, the queue is paused, or because an untagged request is outstanding and tagged requests cannot be sent until the untagged request is completed.
In the below DeviceQueue we can see that the unit is paused (the PauseCount is 1). This would explain why our request is not in the PendingQueue. It should be in the DeviceQueue.
48: kd> dt storport!_RAID_UNIT_EXTENSION fffffa813ba0c1b0 IoQueue.DeviceQueue.
+0x0d0 IoQueue :
+0x010 DeviceQueue :
+0x000 Type : 0n258
+0x002 Size : 0n88
+0x008 Gateway : 0xfffffa80`c25a6470_STOR_IO_GATEWAY
+0x010 DeviceList : _LIST_ENTRY [ 0xfffffa81`4232d3a8 - 0xfffffa81`4172c768 ]
+0x020 ByPassList : _LIST_ENTRY [ 0xfffffa81`3ba0c2b0 - 0xfffffa81`3ba0c2b0 ]
+0x030 Lock : 0
+0x038 Depth : 0n32
+0x03c OutstandingCount : 0n0
+0x040 DeviceCount : 0n27
+0x044 ByPassCount : 0n0
+0x048 ByPassPowerCount : 0n0
+0x04c PauseCount : 0n1
+0x050 BusyCount : 0n0
+0x054 Frozen : 0 ''
+0x055 Locked : 0 ''
+0x056 Untagged : 0 ''
+0x057 PowerLocked : 0 ''
Requests from the DeviceQueue are in the DeviceList. We can dump the DeviceList with !list and look for our IRP.
48: kd> !list "-t nt!_IRP.Tail.Overlay.DeviceQueueEntry.DeviceListEntry.Flink -x\"??@$extret\" 0xfffffa814232d3a8-78"
unsigned int64 0xfffffa81`4232d330
unsigned int64 0xfffffa81`bea59630
unsigned int64 0xfffffa81`41dad420
unsigned int64 0xfffffa82`32e90770
unsigned int64 0xfffffa81`4135a010
<snip the end of the list>
Now that we have found our request in the DeviceQueue's DeviceList we may want to look into why the request has not been sent to the disk and put on the PendingQueue. Earlier I mentioned that the Timeout value in the PendingQueue is -2, indicating that a request in the PendingQueue has timed out. If we take another look at the XRB from the PendingQueue, we see that the CompletionRoutine is RaidUnitCompleteResetRequest. This indicates that a LUN reset request was sent to the disk, and it timed out. When a LUN reset times out storport calls the miniport’s bus reset function. When the bus reset function is called the miniport is required to complete all requests issued to it. In this instance the reset was issued, but for some reason the reset has not yet completed and so all of the outstanding requests are stuck. For more information on storport timeouts, see our article on Event 129 Errors.
Greetings ntdebugging community, Bob here again and today I would like to let everyone know about a new feature implemented in Windows Server 2008 R2’s kernel and filter manager binaries released in knowledge base article 2666390.
Beginning with this update, a minifilter that is adversely affecting system performance can be identified in Windows 2008 R2. These measurements are taken within the filter manager and measure the execution time of the minifilter’s pre and post operations. On every file access file system minifilters are given the chance to perform pre and post modifications to IO operations within the driver stack.
To start recording the operations and to view the results, the latest version of the Windows Performance Toolkit from the Windows 8 beta ADK needs to be installed on the system. How to install the package can be found in the KB article.
The command to start recording and to record stack traces is (note that these commands may wrap depending on your screen resolution, they are intended to be typed in one line):
xperf -on PROC_THREAD+LOADER+FLT_IO_INIT+FLT_IO+FLT_FASTIO+FLT_IO_FAILURE+FILENAME -stackwalk MiniFilterPreOpInit+MiniFilterPostOpInit
The command above starts a trace with most of the buffering done in memory. The command below specifies a file used as a backing store to use less memory:
xperf -start -f <traceFile> -on PROC_THREAD+LOADER+FLT_IO_INIT+FLT_IO+FLT_FASTIO+FLT_IO_FAILURE+FILENAME -stackwalk MiniFilterPreOpInit+MiniFilterPostOpInit
Note: <traceFile> is the path of the file to store the backing file (i.e. trace.etl). It is recommended that the backing file be placed on a drive other than the one experiencing the problem, to avoid xperf operations interfering with or contributing to the scenario you are measuring. You can add the +FILENAME switch if you want to log names of the files that the minifilter is operating on.
You may not know if a minifilter is causing the problem until after you begin data analysis, so you can add the filter manager tracing to the Driver Delays command presented in my previous blog, Using Xperf to investigate slow I/O issues.
xperf -on PROC_THREAD+LOADER+CSWITCH+FILENAME+FILE_IO+FILE_IO_INIT+DRIVERS+FLT_IO_INIT+FLT_IO+FLT_FASTIO+FLT_IO_FAILURE -f kernel.etl -stackwalk CSwitch+DiskReadInit+DiskWriteInit+DiskFlushInit+FileCreate+FileCleanup+FileClose+FileRead+FileWrite+MiniFilterPreOpInit+MiniFilterPostOpInit -BufferSize 1024 -MaxBuffers 1024 -MaxFile 1024 -FileMode Circular
To stop the trace and flush data to the trace file, type the following command:
xperf –stop –d mymergedtrace.etl
The file specified must be a different file name than the one provided with the –f switch if that option was used when starting the trace.
Before we begin looking at the trace, let’s talk a little about what we are tracing. We are tracing fltmgr calling into the minifilter. The fltmgr manages minifilters. When a file operation is done, each minifilter has a chance to process and optionally modify an operation on a file before and after the call goes into the file system. For example, in this case we are going to see results of the time it took for the minifilter to do a “pre create” operation. That is the function that is called before the “create file” function is sent to the file system. This tracing can also track the time it took for the minifilter to do a “post create” operation. That is the minifilter function that gets called after the create file is sent to the file system. So the minifilter is called before and after every file system operation. The minifilter may not have functions to process before and after, however it has the option.
Once the problem is traced, the data can be viewed by clicking on the merged etl file and you will see a similar presentation as below.
The Mini-Filter Delays are in the Storage display. So click on the triangle on the left side of Storage and you will see:
To get more detail, right click on the “Mini-Filter Delays” section and select “Add To New Analysis View”and that will bring up minifilter delays detail as below.
The Duration is the time in micro seconds of the total time delay in the minifilter. So in the example above the scanner minifilter did 4,068 requests and it took 30 seconds total.
We can expand the display by clicking on the triangle next to scanner.sys to get a more detailed view, I chose to investigate scanner.sys because it has the longest Duration. I dragged the “Major Function Name (Count)” and “File Name” columns to the left of the yellow bar to get detail and used the triangles to expand the ProcessID with the longest Duration, and then I expanded the File Name with the longest Duration. To get more usable space on your monitor, you can click the X to close the Graph Explorer (to get it back, from the Window menu choose Graph Explorer).
On the display above it shows the scanner.sys filter was operating on the file “C:\users\Administrator\Desktop\scanner\test2.txt” The green 6 is the total operations. So there were three cleanups and three creates. So the display is telling us that the filter did 3 “create” operations that took close to 15 seconds and 3 cleanup operations that took 5 milliseconds on the file.
If you prefer operating from a command line rather than a GUI, this same data can be extracted using the below command. Note that -min specifies the time in ns, so this command will show delays longer than 100 ms.
xperf -i mymergedtrace.etl -a minifilterdelay -min 100000
This will give you output similar to the below information:
There are 3 MiniFilter Delay completion events in given time range.
Process Name, Module Name, Call (us), Return (us), Time (us), Major Func Name, Filename
explorer.exe, scanner.sys, 18649145, 23645502, 4996357, CREATE, C:\Users\Administrator\Desktop\scanner\test2.txt
explorer.exe, scanner.sys, 23646355, 28644638, 4998282, CREATE, C:\Users\Administrator\Desktop\scanner\test2.txt
explorer.exe, scanner.sys, 28645194, 33645158, 4999964, CREATE, C:\Users\Administrator\Desktop\scanner\test2.txt
It seems there is a problem in the create file handling of the scanner.sys minifilter. For some reason the create file function in the minifiltertook 5 seconds for each operation. The vendor of this filter would need to do additional investigation to identify why the filter has this delay.
Hi, I'm Manish from Global Escalation Services. I would like to present a multiple random bug check issue, which was caused by malicious code (trojan) running on the machine. This is the walkthrough of how we found the virus on the server.
In this particular dump, the machine crashed with Bugcheck 0xA (IRQL_NOT_LESS_OR_EQUAL) because we got a Page Fault at dispatch level, also known as DPC Level (IRQL 2). Windows system architecture governs that we cannot have a page fault at dispatch level because paging requires I/O, I/O requires a wait, and we cannot wait while the IRQL is above dispatch level. So when this anomaly happens Windows will intentionally crash the machine. We trapped in CcMapData because we touched the address c226d800 which was not valid because it’s paged out. If we look at implementation of CcMapData on MSDN http://msdn.microsoft.com/en-us/library/windows/hardware/ff539155(v=vs.85).aspx we see that this function can only be called below Dispatch Level, but the current CPU IRQL is 2.
So how did this happen? The most likely possibility is that some driver on the stack raised the IRQL by calling KeRaiseIrql and then forgot to lower it by calling KeLowerIrql. There are many drivers on this stack and anyone could be the culprit. It is difficult to track when one of these may have done it as it’s long gone.
We could have enabled IRQL Checking using Driver Verifier to find the culprit but we have to enable it on all drivers listed in the stack (and possibly others), which could cause some performance issues. Also this was not the only crash we were seeing; there were various stop codes. So before going the verifier route I decided to dig more into this dump. I noticed there is an address on the stack for which the module name is not getting resolved. This looked odd (suspicious) as most of the time the debugger does a great job of finding the module. I started investigating what is this module. It turned out to be a Trojan “TrojanDropper:Win32/Sirefef.B”. http://www.microsoft.com/security/portal/Threat/Encyclopedia/Entry.aspx?Name=TrojanDropper%3AWin32%2FSirefef.B
0: kd> !analyze -v
* Bugcheck Analysis *
An attempt was made to access a pageable (or completely invalid) address at an
interrupt request level (IRQL) that is too high. This is usually
caused by drivers using improper addresses.
If a kernel debugger is available get the stack backtrace.
Arg1: c226d800, memory referenced
Arg2: d0000002, IRQL
Arg3: 00000000, bitfield :
bit 0 : value 0 = read operation, 1 = write operation
bit 3 : value 0 = not an execute operation, 1 = execute operation (only on chips which support this level of status)
Arg4: 808b64a6, address which referenced memory
0: kd> kv
ChildEBP RetAddr Args to Child
f78ae41c 808b64a6 badb0d00 00000000 00000001 nt!KiTrap0E+0x2a7 (FPO: [0,0] TrapFrame @ f78ae41c)
f78ae4cc f71a6f2d 8b22d520 f78ae4fc 00000400 nt!CcMapData+0x8c (FPO: [Non-Fpo])
f78ae4ec f71a4494 f78ae7ec 8b64c150 01c6d800 Ntfs!NtfsMapStream+0x4b (FPO: [Non-Fpo])
f78ae560 f71a6df0 f78ae7ec 8b3a7100 e7c50ce0 Ntfs!NtfsReadMftRecord+0x86 (FPO: [Non-Fpo])
f78ae598 f71a6fac f78ae7ec 8b3a7100 e7c50ce0 Ntfs!NtfsReadFileRecord+0x7a (FPO: [Non-Fpo])
f78ae5d0 f718e46d f78ae7ec e7c50cd8 e7c50ce0 Ntfs!NtfsLookupInFileRecord+0x37 (FPO: [Non-Fpo])
f78ae67c f718e541 f78ae7ec e7c50cd8 f718e2cb Ntfs!NtfsWalkUpTree+0xbe (FPO: [Non-Fpo])
f78ae6d8 f718e263 f78ae7ec e7c50cd8 00000000 Ntfs!NtfsBuildNormalizedName+0x44 (FPO: [Non-Fpo])
f78ae704 f7196c2e f78ae7ec 8801d600 e7c50da0 Ntfs!NtfsQueryNameInfo+0x4b (FPO: [Non-Fpo])
f78ae774 f71a0ff6 f78ae7ec 88081cb0 8b5f9260 Ntfs!NtfsCommonQueryInformation+0x291 (FPO: [Non-Fpo])
f78ae7d8 f71a102f f78ae7ec 88081cb0 00000001 Ntfs!NtfsFsdDispatchSwitch+0x12a (FPO: [Non-Fpo])
f78ae8f4 8081df85 8b3a7020 88081cb0 88081cb0 Ntfs!NtfsFsdDispatchWait+0x1c (FPO: [Non-Fpo])
f78ae908 f721fd28 8b2fd220 8b60d308 8b54b020 nt!IofCallDriver+0x45 (FPO: [Non-Fpo])
f78ae934 8081df85 8b5f9260 88081cb0 88081cb0 fltMgr!FltpDispatch+0x152 (FPO: [Non-Fpo])
f78ae948 f721fd28 88081ed0 8b60d308 c000000d nt!IofCallDriver+0x45 (FPO: [Non-Fpo])
f78ae974 8081df85 8b54b020 88081cb0 88081cb0 fltMgr!FltpDispatch+0x152 (FPO: [Non-Fpo])
f78ae988 f5c62bec 88081cb0 8b54a980 88081cb0 nt!IofCallDriver+0x45 (FPO: [Non-Fpo])
WARNING: Stack unwind information not available. Following frames may be wrong.
f78ae99c f5c5e3ee 8b03b690 00000000 f78ae9c0 CtxSbx+0x5bec
f78ae9ac 8081df85 8b03b690 88081cb0 8b0528e0 CtxSbx+0x13ee
f78ae9c0 f61074e1 8b0528e0 8801d600 f78aea00nt!IofCallDriver+0x45 (FPO: [Non-Fpo])
f78ae9e4 f61075d0 8b0a9b80 00081cb0 f78aea08 CtxAltStr+0x44e1
f78ae9f4 8081df85 8b0a9b80 88081cb0 88081ed0 CtxAltStr+0x45d0
f78aea08 8b5cfc89 00000000 87ef0000 87ef9002 nt!IofCallDriver+0x45 (FPO: [Non-Fpo])
f78aec90 8081df85 8b5ef610 8b08c110 8b08c2a8 0x8b5cfc89 <------------ This looks odd what is this module?
f78aeca4 f7241607 8b08c2a8 00000000 f78aece8nt!IofCallDriver+0x45 (FPO: [Non-Fpo])
f78aecb4 f72412b2 8b08c2a8 8b392b70 87f11974 CLASSPNP!SubmitTransferPacket+0xbb (FPO: [Non-Fpo])
f78aece8 f7241533 00000000 00000e00 87f11808 CLASSPNP!ServiceTransferRequest+0x1e4 (FPO: [Non-Fpo])
f78aed0c 8081df85 8b392ab8 00000000 8b2a2670 CLASSPNP!ClassReadWrite+0x159 (FPO: [Non-Fpo])
f78aed20 f74c80cf 8b2b7d80 87f11998 f78aed44 nt!IofCallDriver+0x45 (FPO: [Non-Fpo])
f78aed30 8081df85 8b6e4020 87f11808 87f119bc PartMgr!PmReadWrite+0x95 (FPO: [Non-Fpo])
f78aed44 f7317053 87f119d8 8b6e8148 882b9888 nt!IofCallDriver+0x45 (FPO: [Non-Fpo])
f78aed60 8081df85 8b2b7cc8 87f11808 87f119fc ftdisk!FtDiskReadWrite+0x1a9 (FPO: [Non-Fpo])
f78aed74 f72cf4f5 885360cc 88536098 87f26810 nt!IofCallDriver+0x45 (FPO: [Non-Fpo])
f78aed90 f72d1517 87f11808 885360cc 8b60c470 volsnap!VspDecrementIrpRefCount+0x14f (FPO: [Non-Fpo])
f78aeda8 f72c0398 87f26810 87f26810 88536098 volsnap!VspWriteVolumePhase22+0x3d (FPO: [Non-Fpo])
f78aee30 f72d15d3 00f26810 885360cc 00000000 volsnap!VspAcquireNonPagedResource+0xc6 (FPO: [Non-Fpo])
f78aee50 8081e123 00000000 87f215e8 88536098 volsnap!VspWriteVolumePhase2+0x59 (FPO: [Non-Fpo])
f78aee80 f7241829 f78aeeb0 f72413ec 8b392ab8 nt!IopfCompleteRequest+0xcd (FPO: [Non-Fpo])
f78aee88 f72413ec 8b392ab8 87f215e8 00000001 CLASSPNP!ClassCompleteRequest+0x11 (FPO: [Non-Fpo])
f78aeeb0 8081e123 00000000 8b08c428 8b08c5c0 CLASSPNP!TransferPktComplete+0x1fd (FPO: [Non-Fpo])
f78aeee0 f7266545 8b6e30e8 8b08c428 f78aef24 nt!IopfCompleteRequest+0xcd (FPO: [Non-Fpo])
f78aeef0 f7265a8a 87f856a8 00000001 00000000 SCSIPORT!SpCompleteRequest+0x5e (FPO: [Non-Fpo])
f78aef24 f7265130 8b6e30e8 87f856a8 f78aef9b SCSIPORT!SpProcessCompletedRequest+0x6a7 (FPO: [Non-Fpo])
f78aef9c 8083211c 8b6e30a4 8b6e3030 00000000 SCSIPORT!ScsiPortCompletionDpc+0x2bd (FPO: [Non-Fpo])
f78aeff4 8088dba7 f3f4f92c 00000000 00000000 nt!KiRetireDpcList+0xca (FPO: [Non-Fpo])
f3f4f948 80a603d9 ffdffa02 f3f4f980 f3f4f980 nt!KiDispatchInterrupt+0x37 (FPO: [Uses EBP] [0,0,1])
f3f4f964 80a60577 8b6c226c f3f4f980 8088d91d hal!HalpCheckForSoftwareInterrupt+0x81 (FPO: [Non-Fpo])
f3f4f970 8088d91d 8b304c00 000001a3 f3f4fa04 hal!HalEndSystemInterrupt+0x67 (FPO: [Non-Fpo])
f3f4f970 809395e7 8b304c00 000001a3 f3f4fa04 nt!KiInterruptDispatch+0x5d (FPO: [0,2] TrapFrame @ f3f4f980)
f3f4fa04 80939c37 88139178 88028301 00000008 nt!ObpAllocateObject+0x199 (FPO: [Non-Fpo])
f3f4fa38 808f8d28 00000000 8b76aad0 f3f4fa70 nt!ObCreateObject+0x129 (FPO: [Non-Fpo])
f3f4fb44 80937a40 8b6afd10 00000000 88028398 nt!IopParseDevice+0x710 (FPO: [Non-Fpo])
f3f4fbc4 80933b74 00000000 f3f4fc04 00000042 nt!ObpLookupObjectName+0x5b0 (FPO: [Non-Fpo])
f3f4fc18 808eaee7 00000000 00000000 dffa7c01 nt!ObOpenObjectByName+0xea (FPO: [Non-Fpo])
f3f4fc94 808ec181 077ef198 c0140000 077ef14c nt!IopCreateFile+0x447 (FPO: [Non-Fpo])
f3f4fcf0 808eec10 077ef198 c0140000 077ef14c nt!IoCreateFile+0xa3 (FPO: [Non-Fpo])
f3f4fd30 808897ec 077ef198 c0140000 077ef14c nt!NtCreateFile+0x30 (FPO: [Non-Fpo])
f3f4fd30 7c82847c 077ef198 c0140000 077ef14c nt!KiFastCallEntry+0xfc (FPO: [0,0] TrapFrame @ f3f4fd64)
077ef22c 00000000 00000000 00000000 00000000 0x7c82847c
The current CPU IRQL is 2, at which pagefaults cannot be serviced, hence windows crashed the machine.
0: kd> !irql
Debugger saved IRQL for processor 0x0 -- 2 (DISPATCH_LEVEL)
Un-assembling from the return address of this unknown driver, to try to confirm that this is really driver code and that it really does belong in this call stack:
0: kd> ub 8b5cfc89
8b5cfc62 8bd7 mov edx,edi
8b5cfc64 c60605 mov byte ptr [esi],5
8b5cfc67 895e18 mov dword ptr [esi+18h],ebx
8b5cfc6a c7460400020000 mov dword ptr [esi+4],200h
8b5cfc71 c7460809000000 mov dword ptr [esi+8],9
8b5cfc78 c7461c70fb5c8b mov dword ptr [esi+1Ch],8B5CFB70h
8b5cfc7f c64603e0 mov byte ptr [esi+3],0E0h
8b5cfc83 ff158c805d8b call dword ptr ds:[8B5D808Ch]
Sure does call IofCallDriver
0: kd> dps 8B5D808Ch l1
8b5d808c 8081df40 nt!IofCallDriver
So what is this driver? In Windows every image starts with a PE header which starts with letters “MZ”. So I started looking for PE header. I took the offset of the module on the stack and went back to its page boundary, then I started moving 1 page back at a time looking for the PE header.
0: kd> dc 8b5cf000
8b5cf000 0010b9f2 f02b0000 3b063c8b 83137538 ......+..<.;8u..
8b5cf010 c08304e9 04f98304 b85fee73 00000001 ........s._.....
8b5cf020 9cb8c35e b98b5d86 00000010 ff8bd02b ^....]......+...
8b5cf030 3b02348b 83137530 c08304e9 04f98304 .4.;0u..........
8b5cf040 b85fee73 00000001 335fc35e ccc35ec0 s._.....^._3.^..
8b5cf050 83ec8b55 78a04cec 538b5da4 c0b60f56 U....L.x.].SV...
8b5cf060 827ae857 ff330000 33f46589 f845c7db W.z...3..e.3..E.
8b5cf070 00000400 8b084d8b 52510c55 50b4458d .....M..U.QR.E.P
0: kd> dc 8b5cf000-1000
8b5ce000 01c73024 00000000 c70cc483 00800002 $0..............
8b5ce010 845e5f00 b10874db 0815ff01 8b8b5d80 ._^..t.......]..
8b5ce020 6a0c2444 15ff5000 8b5d8028 14c25b5d D$.j.P..(.].][..
8b5ce030 cccccc00 cccccccc cccccccc cccccccc ................
8b5ce040 6a306a56 2415ff00 8b8b5d80 74f685f0 Vj0j...$.].....t
8b5ce050 6a006a53 68026a01 8b5d0892 5d08ae68 Sj.j.j.h..].h..]
8b5ce060 ff006a8b 5d80ac15 ff56508b 5d80a815 .j.....].PV....]
8b5ce070 000d8b8b ff8b5da2 5d80b015 6a006a8b .....].....].j.j
0: kd> dc 8b5cf000-1000*2
8b5cd000 0689c033 89044689 46890846 1046890c 3....F..F..F..F.
8b5cd010 89144689 ec831846 1c46891c 57204689 .F..F.....F..F W
8b5cd020 85244689 8d0574f6 02eb284e c033c933 .F$..t..N(..3.3.
8b5cd030 41890189 08418904 8b34468b 4e891048 ...A..A..F4.H..N
8b5cd040 14508b10 8b145689 4e891848 1c508b18 ..P..V..H..N..P.
8b5cd050 8b1c5689 4e891848 1c508b20 2424448d .V..H..N .P..D$$
8b5cd060 244c8d50 186a510c 8b245689 6a302454 P.L$.Qj..V$.T$0j
8b5cd070 46c65205 46c70005 00000008 0c46c700 .R.F...F......F.
Finally I found the PE header for this image.
0: kd> dc 8b5cf000-1000*3
8b5cc000 00905a4d 00000003 00000004 0000ffff MZ..............
8b5cc010 000000b8 00000000 00000040 00000000 ........@.......
8b5cc020 00000000 00000000 00000000 00000000 ................
8b5cc030 00000000 00000000 00000000 000000d0 ................
8b5cc040 0eba1f0e cd09b400 4c01b821 685421cd ........!..L.!Th
8b5cc050 70207369 72676f72 63206d61 6f6e6e61 is program canno
8b5cc060 65622074 6e757220 206e6920 20534f44 t be run in DOS
8b5cc070 65646f6d 0a0d0d2e 00000024 00000000 mode....$.......
Using the built-in debugger extension !dh I dumped the header of this image to find the name. Unfortunately there is no name for this image. This address 8b5cc000 is not in the loaded module list, which raised further suspicion. Either it is hiding its load address or this driver was not loaded by standard loading mechanism. This module date shows it’s pretty recent build.
0: kd> !dh 8b5cc000
File Type: DLL
FILE HEADER VALUES
14C machine (i386)
4 number of sections
4EA3461E time date stamp Sun Oct 23 04:09:26 2011
0 file pointer to symbol table
0 number of symbols
E0 size of optional header
32 bit word machine
OPTIONAL HEADER VALUES
10B magic #
9.00 linker version
A400 size of code
2000 size of initialized data
0 size of uninitialized data
46C0 address of entry point
1000 base of code
----- new -----
10000000 image base
1000 section alignment
200 file alignment
1 subsystem (Native)
5.00 operating system version
0.00 image version
5.00 subsystem version
10000 size of image
400 size of headers
00100000 size of stack reserve
00001000 size of stack commit
00100000 size of heap reserve
00001000 size of heap commit
0 DLL characteristics
0 [ 0] address [size] of Export Directory
C91C [ 3C] address [size] of Import Directory
0 [ 0] address [size] of Resource Directory
0 [ 0] address [size] of Exception Directory
0 [ 0] address [size] of Security Directory
F000 [ 3FC] address [size] of Base Relocation Directory
0 [ 0] address [size] of Debug Directory
0 [ 0] address [size] of Description Directory
0 [ 0] address [size] of Special Directory
0 [ 0] address [size] of Thread Storage Directory
0 [ 0] address [size] of Load Configuration Directory
0 [ 0] address [size] of Bound Import Directory
C000 [ 1A8] address [size] of Import Address Table Directory
0 [ 0] address [size] of Delay Import Directory
0 [ 0] address [size] of COR20 Header Directory
0 [ 0] address [size] of Reserved Directory
SECTION HEADER #1
A354 virtual size
1000 virtual address
A400 size of raw data
400 file pointer to raw data
0 file pointer to relocation table
0 file pointer to line numbers
0 number of relocations
0 number of line numbers
(no align specified)
SECTION HEADER #2
13AC virtual size
C000 virtual address
1400 size of raw data
A800 file pointer to raw data
SECTION HEADER #3
4B0 virtual size
E000 virtual address
200 size of raw data
BC00 file pointer to raw data
SECTION HEADER #4
576 virtual size
F000 virtual address
600 size of raw data
BE00 file pointer to raw data
0: kd> !lmi 8b5cc000
Loaded Module Info: [8b5cc000]
ffffffff8b5cc000 is not a valid address
0: kd> lmvm 8b5cc000
start end module name
Checking the import table it does have calls to raise and lower the IRQL. We get to the import table by taking the base address plus the offset to the Import Address Table Directory. The below output is just a snippet of the whole table.
0: kd> dps 8b5cc000+c000
8b5d8000 80a603f4 hal!KfLowerIrql
8b5d8004 80a5ff00 hal!KeGetCurrentIrql
8b5d8008 80a600b4 hal!KfRaiseIrql
Dumping the entire image contents in memory to find more clues about this driver. Again, I am only showing snippets of the whole output.
0: kd> dc 8b5cc000 L?10000/4
8b5cc070 65646f6d 0a0d0d2e 00000024 00000000 mode....$.......
8b5d5e20 76000000 66697265 77252079 00000a5a ...verify %wZ...
8b5d5e30 31000000 35343332 39383736 33323130 ...1234567890123
8b5d5e40 37363534 31303938 35343332 39383736 4567890123456789
8b5d5e50 33323130 37363534 31303938 35343332 0123456789012345
8b5d5e60 39383736 33323130 37363534 31303938 6789012345678901
8b5d5e70 00343332 66000000 646e756f 67697320 234....found sig
8b5d5e80 7574616e 3d206572 0a752520 b4000000 nature = %u.....
Interestingly this image has other images (modules) embedded in it. We can see other PE headers, which again have no name. This looks highly suspicious and resembles behavior used by malicious software.
8b5d0970 f775c085 4848c78b 4dc35f5e 0300905a ..u...HH^_.MZ...
8b5d0980 04000000 ff000000 b80000ff 00000000 ................
8b5d0990 40000000 00000000 00000000 00000000 ...@............
8b5d09a0 00000000 00000000 00000000 00000000 ................
8b5d09b0 00000000 c8000000 0e000000 000eba1f ................
8b5d09c0 21cd09b4 cd4c01b8 69685421 72702073 ...!..L.!This pr
8b5d09d0 6172676f 6163206d 746f6e6e 20656220 ogram cannot be
8b5d09e0 206e7572 44206e69 6d20534f 2e65646f run in DOS mode. 8b5d2200 4d00004e 0300905a 04000000 ff000000 N..MZ...........
8b5d2210 b80000ff 00000000 40000000 00000000 ...........@....
8b5d2220 00000000 00000000 00000000 00000000 ................
8b5d2230 00000000 00000000 00000000 f0000000 ................
8b5d2240 0e000000 000eba1f 21cd09b4 cd4c01b8 ...........!..L.
8b5d2250 69685421 72702073 6172676f 6163206d !This program ca
8b5d2260 746f6e6e 20656220 206e7572 44206e69 nnot be run in D
8b5d2270 6d20534f 2e65646f 240a0d0d 00000000 OS mode....$....
After ensuring my AV definitions were up to date, I decided to dump this memory contents into a file onto my machine. The moment contents were written my AV Microsoft Forefront Endpoint Protection caught a Trojan in this file.
0: kd> .writemem c:\temp\drv.sys 8b5cc000 L?10000 Writing 10000 bytes................................
Every Wednesday (usually) we post a debug tip to our twitter page at https://twitter.com/#!/ntdebugging. This blog is an archive of these tips to allow our readers to find this information easily. Periodically we post an updated blog with the current archive. Follow us on twitter if you want to see the new tips as we post them.
The goal of these tips is to share debug commands, and forms of commands (parameters, flags, etc) that we in Platforms Global Escalation Services find useful. I hope you can add these commands to your toolkit and they will help you debug more efficiently.
!thread/!process [address] e - on x64 will not show you the meaningless Args to Child information.
.frame /c [FrameNumber] - sets context to specificied stack frame. On x64 provides more reliable register information than .trap.
kn - Dumps call stack with frame numbers, easier than counting stacks for .frame.
.frame /r [FrameNumber] - same as .frame /c, but shows registers without changing context.
Note: With .frame /c or /r you can only trust the nonvolatile registers. See http://bit.ly/dik4OR for vol/nonvol regs.
k=rbp rip FrameCount - Dumps call stack starting at rbp/rip on x64. Useful when the stack is corrupt.
.process/.thread /p /r [address] - sets new process context, sets .cache forcedecodeuser, and reloads user symbols.
!process [address] 17 - Sets the context for this command, avoids the need for .process to see user stacks. Try !process 0 17
~~[ThreadID]s - Changes threads in user mode. Use Thread ID number from output such as !locks. Ex: ~~[1bd4]s
runas /netonly /u:<account> windbg.exe - Launch windbg with domain account. Use when dbg computer isn't in domain and symbol server is.
!heap -p -a <address> - Shows information about the heap block containing <address>, even if you aren't using pageheap.
ub - Unassembles starting at a location prior to your address. Accepts l<number> to specify how many instructions to go back. ub . l20
!stacks 2 [FilterString] - Finds kernel mode call stacks that contain the FilterString in a symbol.
!thread [address] 17 (or 1e on x64) - Sets context for this command, avoids the need for .thread/.process for user stacks.
.hh [Text] - Opens the debugger help. [Text] is the topic to lookup in the index. Example: .hh !pte
?? can dump structs using C++ style expressions. Ex: ??((nt!_KTHREAD*)(0xfffffa800ea43bb0))->ApcState
bp /t EThread - Sets a kernel mode breakpoint that only triggers when hit in the context of this thread.
bp /p EProcess - Sets a kernel mode breakpoint that only triggers when hit in the context of this process.
gc - If you run 'p' and hit a breakpoint, gc takes you where p would have gone if you had not hit the bp.
gu - Go until the current function returns. Effectively this unwinds one stack frame. #windbg
pc - Steps through until the next 'call' instruction. Combine with other commands to find who returned your error> pc;p;r eax
pt - Steps through until the next 'ret' instruction. Similar to gu, but pt stops on the ret and gu stops after the ret.
.ignore_missing_pages 1 - supresses the error: "Page 2a49 not present in the dump file. Type ".hh dbgerr004" for details"
.exr -1 shows the most recent exception. Useful in user dumps of crashes, especially for no execute crashes (NX/DEP).
wt - Trace calls until they return to the current address. More useful with -or to get return values. Use -l for depth.
.thread /w - Changes to the WOW64 32-bit context from 64-bit kernel mode. Wow64exts doesn't work in kernel mode.
??sizeof(structure) - Gets the size of a structure, it's easier than counting.
sxe ld:module.dll - Enables an exception which will break into the debugger when module.dll is loaded.
vertarget - Shows OS version of the debug target. Also shows machine name, uptime, and session time (when the dump was taken).
!vm 1 - In a kernel debugger, shows basic information about memory usage. Available, committed, pagefile, pool, sysptes, etc.
.time - Shows session time (when dump was taken) and system uptime. In user mode shows process uptime, kernel/user time.
ba w size [address] - Break on write access only. Replace size with the num bytes you want to watch. Ex: ba w 4 005d5f10
.process -i <address> - Make the process active and break into the debugger. Use in live kernel debugs to get into process context.
.reload /f /o - Overwrites cached files in your downstream symbol store. Useful when your store has corrupt pdbs.
->* - Use with dt to dump pointers. Example: dt _EPROCESS [Address] ObjectTable->*
!for_each_module s -a @#Base @#End "PTag" - Find the drivers using pool tag "PTag".
.unload [DllName] - Unloads the debug extension you didn't intend to load. Omit DllName to unload the last dll loaded.
!exqueue dumps the executive worker queues. Use flags 7f to dump the worker threads and the queues.
lmvm <module> - Dumps information about the module. Remember to use <module> and not <module.dll>.
!thread -t TID - Dump a thread using thread ID rather than thread address. Useful when working with a critical section.
!list - Walks a linked list and displays informatino for each element in a list. See blog later today for an example.
.time -h # - Shows the debug session time using the timezone offset of #. Ex: .time -h 0 shows when a dump was taken in UTC.
!session - Lists all of the user session IDs. A quick way to list the active sessions from a dump of a terminal server.
!session -s SessionID - Changes the current session context to SessionID. Useful when looking at GDI, or other per session data.
| ProcNum s - Switches to process number ProcNum. Use when debugging multiple dumps, or processes, in one windbg.
!! - Launches a shell process and redirects its output to the debugger. The same as .shell, but "bang bang" sounds cooler.
uf Function - Dumps the assembly for Function (name or address). Useful for optimized code that is not contiguous in memory.
uf /c Function - Shows all of the calls made by Function (can be function name or address).
!wow64exts.sw - switches between x64 and x86 contexts. Often used to reverse .thread /w context switch.
Hello debuggers, the debug ninja is back again. Sometimes we have a scenario where a process is using a lot of memory, and the only data we are able to get at the moment is a user dump. Ordinarily data from tools such as umdh or xperf would be preferable because they provide memory usage data over a period of time and can include call stack information. However, umdh requires restarting the process (which loses the state of high memory usage), and xperf requires the installation of the Windows Performance Toolkit which may not always be an immediate option.
When we have such a dump we may not be able to specifically identify what piece of code is generating the high memory usage, but we may be able to narrow the scope of troubleshooting to a specific dll.
The first thing we need to do is identify what type of memory is using most of the address space. The debugger command !address –summary allows us to do this:
0:000> !address -summary
--- Usage Summary ---------------- RgnCount ----------- Total Size -------- %ofBusy %ofTotal
Free 489 7fe`6ff5a000 ( 7.994 Tb) 99.92%
Heap 9094 1`75ed1000 ( 5.843 Gb) 93.47% 0.07%
<unknown> 275 0`12e41000 ( 302.254 Mb) 4.72% 0.00%
Image 937 0`05a6a000 ( 90.414 Mb) 1.41% 0.00%
Stack 138 0`01700000 ( 23.000 Mb) 0.36% 0.00%
Other 14 0`001bd000 ( 1.738 Mb) 0.03% 0.00%
TEB 46 0`0005c000 ( 368.000 kb) 0.01% 0.00%
PEB 1 0`00001000 ( 4.000 kb) 0.00% 0.00%
From this example we can see that most of the memory is used by heap. A process will usually have multiple heaps, each created by a call to HeapCreate. We can examine the size of each of these heaps with !heap –s:
0:000> !heap -s
LFH Key : 0x0000006c1104d280
Termination on corruption : ENABLED
Heap Flags Reserv Commit Virt Free List UCR Virt Lock Fast
(k) (k) (k) (k) length blocks cont. heap
0000000000100000 00000002 16384 12824 16384 1180 254 5 0 3 LFH
0000000000010000 00008000 64 4 64 1 1 1 0 0
00000000003d0000 00001002 1088 708 1088 121 20 2 0 0 LFH
0000000003080000 00001002 1536 700 1536 4 4 2 0 0 LFH
00000000033a0000 00001002 5229696 1377584 5229696 414244 4039 3059 0 2c LFH
External fragmentation 30 % (4039 free blocks)
Virtual address fragmentation 73 % (3059 uncommited ranges)
0000000003380000 00001002 64 8 64 3 1 1 0 0
0000000003600000 00001002 512 56 512 3 1 1 0 0
0000000003c20000 00001002 512 8 512 3 1 1 0 0
0000000003220000 00001002 512 8 512 3 1 1 0 0
0000000003e50000 00001002 512 8 512 3 1 1 0 0
0000000003d00000 00001002 512 148 512 5 3 1 0 0 LFH
From the above output we can see that most of the memory is being used by heap 00000000033a0000.
At this point we need to try to identify what this heap is used for. A brute force method to do this is to search memory with the ‘s’ command.
0:000> s -q 0 l?7fffffffffffffff 00000000033a0000
000007fe`f21810a0 00000000`033a0000 00000000`00000001
The output of the ‘s’ command may be verbose. You will need to manually examine the addresses where ‘s’ finds hits. Most of these addresses will probably be in heap memory, we are looking for an address that matches a module. I snipped the above output to just show the relevant hit, an address inside of a loaded module.
The search of memory revealed that the heap 00000000033a0000 is used by the module useheap.dll, specifically it is part of the global ‘Blob’ class.
0:000> ln 000007fe`f21810a0
At this point we don’t know specifically what code in useheap.dll has allocated a lot of heap, however we have significantly narrowed the scope of the problem. We can now determine if there is a known issue with heap usage in useheap.dll that is addressed in a later version. We may also know from experience that this module uses a lot of memory under specific circumstances, such as a high volume of work sent to this service.
I hope this example helps the next time you have high memory usage and only have a user dump to troubleshoot with. Good luck!
Last time, we discussed how applications place data on the clipboard, and how to access that data using the debugger. Today, we'll take a look at how an application can monitor the clipboard for changes. Understanding this is important because it is a place where Windows allows 3rd-party code to "hook" into the system. If you experience unexpected behavior with copying and pasting, a program using these hooks may be misbehaving. We’ll start by covering the hooking mechanisms for clipboard, and then review how to identify what applications, if any, are using these hooks in the debugger.
There are three ways to monitor the clipboard for changes - clipboard viewers, clipboard format listeners, and querying the clipboard sequence number. We will focus on the first two as these allow an application to register for notifications whenever the clipboard is updated. The third method simply allows an application to check and see if a change has occurred, and should not be used in a polling loop.
The Clipboard Viewer functionality has been around since Windows 2000, if not earlier. The way it works is pretty simple - an application interested in receiving clipboard change notifications calls SetClipboardViewer and passes a handle to its window. Windows then stores that handle in a per-session win32k global, and anytime the clipboard is changed Windows sends a WM_DRAWCLIPBOARD message to the registered window.
Of course, multiple applications are allowed to register their windows as clipboard viewers - so how does Windows handle that? Well, if an application calls SetClipboardViewer and another clipboard viewer was already registered, Windows returns the handle value of the previous viewer's window to the new viewer. It is then the responsibility of the new viewer to call SendMessage every time it receives a WM_DRAWCLIPBOARD to notify the next viewer in the chain. Each clipboard viewer also needs to handle the WM_CHANGECBCHAIN message, which notifies all viewers when one of the viewers in the chain is removed, and specifies what the next viewer in the chain is. This allows the chain to be maintained.
An obvious problem with this design is it relies on each clipboard viewer application to behave correctly, not to terminate unexpectedly, and to generally be a good citizen. If any viewer decided not to be friendly, it could simply skip notifying the next viewer in line about an update, rendering the next viewer and all subsequent viewers impotent.
To address these problems, the Clipboard Format Listener mechanism was added in Windows Vista. This works in much the same way as the clipboard viewer functionality except in this case Windows maintains the list of listeners, instead of depending on each application to preserve a chain.
If an application wishes to become a clipboard format listener,it calls the AddClipboardFormatListener function and passes in a handle to its window. After that, its window message handler will receive WM_CLIPBOARDUPDATE messages. When the application is ready to exit or no longer wishes to receive notifications, it can call RemoveClipboardFormatListener.
Now that we've covered the ways to register a viewer/listener, let's take a look at how to identify them using the debugger. First, you'll need to identify a process in the session you are interested in checking for clipboard monitors. It can be any win32 process in that session -we just need to use it to locate a pointer to the Window Station. In this case, I'll use the Notepad window I used in part 1:
kd> !process 0 0 notepad.exe
SessionId: 1 Cid: 0374 Peb: 7fffffd8000 ParentCid: 0814
DirBase: 1867e000 ObjectTable: fffff9803d28ef90 HandleCount: 52.
If you are doing this in a live kernel debug, you'll need to change context into the process interactively (using .process /I< address> then hit g and wait for the debugger to break back in). Now DT the process address as an _EPROCESS and look for the Win32Process field:
kd> dt _EPROCESS fffff980366ecb30 Win32Process
+0x258 Win32Process : 0xfffff900`c18c0ce0 Void
Now DT the Win32Process address as a win32k!tagPROCESSINFO and identify the rpwinsta value:
kd> dt win32k!tagPROCESSINFO 0xfffff900`c18c0ce0 rpwinsta
+0x258 rpwinsta : 0xfffff980`0be2af60 tagWINDOWSTATION
This is our Window Station. Dump it using dt:
kd> dt 0xfffff980`0be2af60 tagWINDOWSTATION
+0x000 dwSessionId : 1
+0x008 rpwinstaNext : (null)
+0x010 rpdeskList : 0xfffff980`0c5e2f20 tagDESKTOP
+0x018 pTerm : 0xfffff960`002f5560 tagTERMINAL
+0x020 dwWSF_Flags : 0
+0x028 spklList : 0xfffff900`c192cf80 tagKL
+0x030 ptiClipLock : (null)
+0x038 ptiDrawingClipboard: (null)
+0x040 spwndClipOpen : (null)
+0x048 spwndClipViewer : 0xfffff900`c1a4ca70 tagWND
+0x050 spwndClipOwner : 0xfffff900`c1a3ef70 tagWND
+0x058 pClipBase : 0xfffff900`c5512fa0 tagCLIP
+0x060 cNumClipFormats : 4
+0x064 iClipSerialNumber : 0x16
+0x068 iClipSequenceNumber : 0xc1
+0x070 spwndClipboardListener : 0xfffff900`c1a53440 tagWND
+0x078 pGlobalAtomTable: 0xfffff980`0bd56c70 Void
+0x080 luidEndSession : _LUID
+0x088 luidUser : _LUID
+0x090 psidUser : 0xfffff900`c402afe0 Void
Note the spwndClipViewer, spwndClipboardListener, and spwndClipOwnerfields. spwndClipViewer is the most-recently-registered window in the clipboard viewer chain. Similarly, spwndClipboardListener is the most recent listener in our Clipboard Format Listener list. spwndClipOwner is the window that set the content in the clipboard.
Given the window, it is just a few steps to determine the process. This would work forspwndClipViewer, spwndClipboardListener, and spwndClipOwner. First, dt the value as a tagWND. We'll use the spwndClipViewer for this demonstration:
kd> dt 0xfffff900`c1a4ca70 tagWND
+0x000 head : _THRDESKHEAD
+0x028 state : 0x40020008
+0x028 bHasMeun : 0y0
+0x028 bHasVerticalScrollbar : 0y0
We only care about the head - so since it is at offset 0, dt the same address as a _THRDESKHEAD:
kd> dt 0xfffff900`c1a4ca70 _THRDESKHEAD
+0x000 h : 0x00000000`000102ae Void
+0x008 cLockObj : 6
+0x010 pti : 0xfffff900`c4f26c20tagTHREADINFO
+0x018 rpdesk : 0xfffff980`0c5e2f20 tagDESKTOP
+0x020 pSelf : 0xfffff900`c1a4ca70 "???"
Now, dt the address in pti as a tagTHREADINFO:
kd> dt 0xfffff900`c4f26c20 tagTHREADINFO
+0x000 pEThread : 0xfffff980`0ef6cb10 _ETHREAD
+0x008 RefCount : 1
+0x010 ptlW32 : (null)
+0x018 pgdiDcattr : 0x00000000`000f0d00 Void
Here, we only care about the value of pEThread, which we can pass to !thread:
kd> !thread 0xfffff980`0ef6cb10 e
THREAD fffff9800ef6cb10 Cid 087c.07ec Teb: 000007fffffde000 Win32Thread: fffff900c4f26c20 WAIT: (WrUserRequest) UserModeNon-Alertable
Owning Process fffff98032e18b30 Image: viewer02.exe
Attached Process N/A Image: N/A
Wait Start TickCount 5435847 Ticks: 33 (0:00:00:00.515)
Context Switch Count 809 IdealProcessor: 0 LargeStack
Win32 Start Address 0x000000013f203044
Stack Init fffff880050acdb0 Current fffff880050ac6f0
Base fffff880050ad000 Limit fffff880050a3000 Call 0
Priority 11 BasePriority 8 UnusualBoost 0 ForegroundBoost 2IoPriority 2 PagePriority 5
Child-SP RetAddr Call Site
fffff880`050ac730 fffff800`01488f32 nt!KiSwapContext+0x7a
fffff880`050ac870 fffff800`0148b74f nt!KiCommitThreadWait+0x1d2
fffff880`050ac900 fffff960`000dc8e7 nt!KeWaitForSingleObject+0x19f
fffff880`050ac9a0 fffff960`000dc989 win32k!xxxRealSleepThread+0x257
fffff880`050aca40 fffff960`000dafc0 win32k!xxxSleepThread+0x59
fffff880`050aca70 fffff960`000db0c5 win32k!xxxRealInternalGetMessage+0x7dc
fffff880`050acb50 fffff960`000dcab5 win32k!xxxInternalGetMessage+0x35
fffff880`050acb90 fffff800`01482ed3 win32k!NtUserGetMessage+0x75
fffff880`050acc20 00000000`77929e6a nt!KiSystemServiceCopyEnd+0x13 (TrapFrame @ fffff880`050acc20)
00000000`002ffb18 00000000`00000000 0x77929e6a
As you can see, we have a clipboard viewer registered from process viewer02.exe. Because of viewer's process-maintained chain architecture, it isn't easy to see the next process in the chain. However, we can do this for clipboard listeners. Let's look back at our window station:
+0x038 ptiDrawingClipboard : (null)
+0x048 spwndClipViewer : 0xfffff900`c1a4ca70tagWND
+0x050 spwndClipOwner : 0xfffff900`c1a3ef70tagWND
+0x070 spwndClipboardListener: 0xfffff900`c1a53440 tagWND
If we dt the spwndClipboardListener, there is a field that shows the next listener named spwndClipboardListenerNext:
kd> dt 0xfffff900`c1a53440 tagWND spwndClipboardListenerNext
+0x118 spwndClipboardListenerNext : 0xfffff900`c1a50080 tagWND
When you reach the last clipboard format listener's tagWND, its spwndClipboardListenerNext value will be null:
kd> dt 0xfffff900`c1a50080 tagWND spwndClipboardListenerNext
+0x118 spwndClipboardListenerNext : (null)
Using this window address, we can go through the same steps as above to identify this listener's process name. As mentioned earlier, since tagWND is a kernel structure, the OS is maintaining these spwndClipboardListener/spwndClipboardListenerNext pointers, so they aren't susceptible to the chain problems of clipboard viewers.
That wraps up our clipboard coverage. I hope you found it informative. Want to learn more about monitoring the clipboard? This MSDN article is a good resource.
Recently I had the opportunity to debug the clipboard in Windows, and I thought I’d share some of the things I learned. The clipboard is one of those parts of Windows that many of us use dozens (hundreds?) of times a day and don’t really think about. Before working on this case, I had never even considered how it works under the hood. It turns out that there’s more to it than you might think. In this first article, I’ll describe how applications store different types of data to the clipboard and how it is retrieved. My next post will describe how applications can hook the clipboard to monitor it for changes. In both, I’ll include debug notes to show you how to access the data from the debugger.
Let’s start by discussing clipboard formats. A clipboard format is used to describe what type of data is placed on the clipboard. There are a number of predefined standard formats that an application can use, such as bitmap, ANSI text, Unicode text, and TIFF. Windows also allows an application to specify its own formats. For example, a word processor may want to register a format that includes text, formatting, and images. Of course, this leads to one problem, what happens if you want to copy from your word processor and paste into Notepad, which doesn’t understand all of the formatting and pictures?
The answer is to allow multiple formats to be stored in the clipboard at one time. When I used to think of the clipboard I thought of it as a single object (“my text” or “my image”), but in reality the clipboard usually has my data in several different forms. The destination program gets a format it can use when I paste.
So how does this data end up on the clipboard? Simple, an application first takes ownership of the clipboard via the OpenClipboard function. Once it has done that, it can empty the clipboard with EmptyClipboard. Finally, it is ready to place data on the clipboard using SetClipboardData. SetClipboardData takes two parameters; the first is the identifier of one of the clipboard formats we discussed above. The second is a handle to the memory containing the data in that format. The application can continue to call SetClipboardData for each of the various formats it wishes to provide going from best to worst (since the destination application will select the first format in the list it recognizes). To make things easier for the developer, Windows will automatically provide converted formats for some clipboard format types. Once the program is done, it calls CloseClipboard.
When a user hits paste, the destination application will call OpenClipboard and one of these functions to determine what data format(s) are available: IsClipboardFormatAvailable, GetPriorityClipboardFormat, or EnumClipboardFormats. Assuming the application finds a format it can use, it will then call GetClipboardData with the desired format identifier as a parameter to get a handle to the data. Finally, it will call CloseClipboard.
Now let’s take a look at how you can find what data being written to the clipboard using the debugger. (Note that all of my notes are taken from a Win7/2008 R2 system – so things might vary slightly in different versions of the OS.) Since the clipboard is part of Win32k.sys, you’ll need to use a kernel debugger. I like to use win32k!InternalSetClipboardData+0xe4 as a breakpoint. The nice thing about this offset is that it is right after we’ve populated the RDI register with data from SetClipboardData in a structure known as tagCLIP.
kd> u win32k!InternalSetClipboardData+0xe4-c L5
fffff960`0011e278 894360 mov dword ptr [rbx+60h],eax
fffff960`0011e27b 8937 mov dword ptr [rdi],esi
fffff960`0011e27d 4c896708 mov qword ptr [rdi+8],r12
fffff960`0011e281 896f10 mov dword ptr [rdi+10h],ebp
fffff960`0011e284 ff15667e1900 call qword ptr[win32k!_imp_PsGetCurrentProcessWin32Process (fffff960`002b60f0)]
kd> dt win32k!tagCLIP
+0x000 fmt : Uint4B
+0x008 hData : Ptr64 Void
+0x010fGlobalHandle : Int4B
Here’s what a call to SetClipboardData from Notepad looks like:
Child-SP RetAddr Call Site
fffff880`0513a940 fffff960`0011e14f win32k!InternalSetClipboardData+0xe4
fffff880`0513ab90 fffff960`000e9312 win32k!SetClipboardData+0x57
fffff880`0513abd0 fffff800`01482ed3 win32k!NtUserSetClipboardData+0x9e
00000000`001dfad8 00000000`7792e494 USER32!ZwUserSetClipboardData+0xa
00000000`001dfae0 000007fe`fc5b892b USER32!SetClipboardData+0xdf
00000000`001dfb20 000007fe`fc5ba625 COMCTL32!Edit_Copy+0xdf
00000000`001dfb60 00000000`77929bd1 COMCTL32!Edit_WndProc+0xec9
00000000`001dfc00 00000000`779298da USER32!UserCallWinProcCheckWow+0x1ad
00000000`001dfcc0 00000000`ff5110bc USER32!DispatchMessageWorker+0x3b5
00000000`001dfd40 00000000`ff51133c notepad!WinMain+0x16f
00000000`001dfdc0 00000000`77a2652d notepad!DisplayNonGenuineDlgWorker+0x2da
00000000`001dfe80 00000000`77b5c521 kernel32!BaseThreadInitThunk+0xd
So here, we can dt RDI as a tagCLIP to see what was written:
kd> dt win32k!tagCLIP @rdi
+0x000 fmt : 0xd
+0x008 hData : 0x00000000`00270235 Void
+0x010fGlobalHandle : 0n1
Fmt is the clipboard format. 0xd is 13, which indicates this data is Unicode text. We can’t just ‘du’ the value in hData, however, because this is a handle, not a direct pointer to the data. So now we need to look up the handle. To do that, we need to look at a win32k global structure – gSharedInfo:
Evaluate expression: -7284261440224 = fffff960`002f3520
kd> dt win32k!tagSHAREDINFO fffff960`002f3520
+0x000 psi : 0xfffff900`c0980a70 tagSERVERINFO
+0x008 aheList : 0xfffff900`c0800000 _HANDLEENTRY
+0x010 HeEntrySize : 0x18
+0x018 pDispInfo : 0xfffff900`c0981e50 tagDISPLAYINFO
+0x020ulSharedDelta : 0
+0x028 awmControl :  _WNDMSG
+0x218DefWindowMsgs : _WNDMSG
+0x228DefWindowSpecMsgs : _WNDMSG
aheList in gSharedInfo contains an array of handle entries, and the last 2 bytes of hData multiplied by the size of a handle entry the address of our handle entry:
kd> ?0x00000000`00270235 & FFFF
Evaluate expression: 565 = 00000000`00000235
unsigned int64 0x18
kd> ? 0xfffff900`c0800000 + (0x235*0x18)
Evaluate expression: -7693351766792 = fffff900`c08034f8
kd> dt win32k!_HANDLEENTRY fffff900`c08034f8
+0x000 phead : 0xfffff900`c0de0fb0 _HEAD
+0x008 pOwner : (null)
+0x010 bType : 0x6 ''
+0x011 bFlags : 0 ''
+0x012 wUniq : 0x27
If we look in phead at offset 14, we’ll get our data (this offset may vary on different platforms):
kd> du fffff900`c0de0fb0 + 0x14
fffff900`c0de0fc4 "Hi NTDebugging readers!"
Let’s consider one other scenario. I copied some text out of Wordpad, and a number of SetClipboardData calls were made to accommodate different formats. The Unicode format entry looks like this:
Breakpoint 0 hit
fffff960`0011e284 ff15667e1900 call qword ptr[win32k!_imp_PsGetCurrentProcessWin32Process (fffff960`002b60f0)]
+0x008 hData : (null)
+0x010fGlobalHandle : 0n0
hData is null! Why is that? It turns out that the clipboard allows an application to pass in null to SetClipboardData for a given format. This indicates that the application can provide the data in that format, but is deferring doing so until it is actually needed. Sure enough, if I paste the text into Notepad, which needs the text in Unicode, Windows sends a WM_RENDERFORMAT message to the WordPad window, and WordPad provides the data in the new format. Of course, if the application exits before populating all of its formats, Windows needs all of the formats rendered. In this case, Windows will send the WM_RENDERALLFORMATS message so other applications can use the clipboard data after the source application has exited.
That’s all for now. Next time we’ll look at how applications can monitor the clipboard for changes using two hooks. If you want to know more about using the clipboard in your code, this is a great reference.
Part 2 of this article can be found here: