WinDbg : Parsing Dispatcher Object Queues & the !exqueue Command Extension

WinDbg : Parsing Dispatcher Object Queues & the !exqueue Command Extension

Windows maintains several differ net work queues for different objects. Depending on the priority of the object it is pushed into the respective queue. The !exqueue command can be used to view the different queues and the threads which are part of it.

kd> !exqueue

**** Critical WorkQueue ( Threads: 6/512, Concurrency: 0/1 )

THREAD 83a379e8  Cid 0004.0018  Teb: 00000000 Win32Thread: 00000000 WAIT

THREAD 83a37710  Cid 0004.001c  Teb: 00000000 Win32Thread: 00000000 WAIT

THREAD 83a37438  Cid 0004.0020  Teb: 00000000 Win32Thread: 00000000 WAIT

THREAD 83a37160  Cid 0004.0024  Teb: 00000000 Win32Thread: 00000000 WAIT

THREAD 83a25020  Cid 0004.0028  Teb: 00000000 Win32Thread: 00000000 WAIT

THREAD 84fedd48  Cid 0004.0750  Teb: 00000000 Win32Thread: 00000000 WAIT

**** Delayed WorkQueue ( Threads: 7/512, Concurrency: 0/1 )

THREAD 83a25d48  Cid 0004.002c  Teb: 00000000 Win32Thread: 00000000 WAIT

THREAD 83a25a70  Cid 0004.0030  Teb: 00000000 Win32Thread: 00000000 WAIT

THREAD 83a25798  Cid 0004.0034  Teb: 00000000 Win32Thread: 00000000 WAIT

THREAD 83a254c0  Cid 0004.0038  Teb: 00000000 Win32Thread: 00000000 WAIT

THREAD 83a26020  Cid 0004.003c  Teb: 00000000 Win32Thread: 00000000 WAIT

THREAD 83a26d48  Cid 0004.0040  Teb: 00000000 Win32Thread: 00000000 WAIT

THREAD 83a26a70  Cid 0004.0044  Teb: 00000000 Win32Thread: 00000000 WAIT

**** HyperCritical WorkQueue ( Threads: 1/512, Concurrency: 0/1 )

THREAD 83a26798  Cid 0004.0048  Teb: 00000000 Win32Thread: 00000000 WAIT

To parse the list of waiting threads on an object, first find the queue type of object (it is easier with that, else use any process in your mind, but then those might not have waiters). 

In this example we are using _EPROCESS but the command !exqueue, shown above, will give a list of all executable queues, and also can be used to chose one such queue with multiple waiters.

NOTE : To run this experiment in a better fashion on process type objects, we should create a process and make another process open a handle on it and wait on it (WaitForSingleObject). An example is the Winlogon process which has multiple waiters if multiple sessions are running.

kd> !process 0 0 lsass.exe

PROCESS 848b0720  SessionId: 0  Cid: 0200    Peb: 7ffd8000  ParentCid: 018c

    DirBase: 1ee780e0  ObjectTable: 8781e0e8  HandleCount: 560.

    Image: lsass.exe

kd> dt nt!_EPROCESS 848b0720  Pcb.Header.

   +0x000 Pcb         : 

      +0x000 Header      : 

         +0x000 Type        : 0x3 ''

         +0x001 TimerControlFlags : 0 ''

         +0x001 Absolute    : 0y0

         +0x001 Coalescable : 0y0

         +0x001 KeepShifting : 0y0

         +0x001 EncodedTolerableDelay : 0y00000 (0)

         +0x001 Abandoned   : 0 ''

         +0x001 Signalling  : 0 ''

         +0x002 ThreadControlFlags : 0x26 '&'

         +0x002 CpuThrottled : 0y0

         +0x002 CycleProfiling : 0y1

         +0x002 CounterProfiling : 0y1

         +0x002 Reserved    : 0y00100 (0x4)

         +0x002 Hand        : 0x26 '&'

         +0x002 Size        : 0x26 '&'

         +0x003 TimerMiscFlags : 0 ''

         +0x003 Index       : 0y0

         +0x003 Processor   : 0y00000 (0)

         +0x003 Inserted    : 0y0

         +0x003 Expired     : 0y0

         +0x003 DebugActive : 0 ''

         +0x003 ActiveDR7   : 0y0

         +0x003 Instrumented : 0y0

         +0x003 Reserved2   : 0y0000

         +0x003 UmsScheduled : 0y0

         +0x003 UmsPrimary  : 0y0

         +0x003 DpcActive   : 0 ''

         +0x000 Lock        : 0n2490371

         +0x004 SignalState : 0n0

         +0x008 WaitListHead : _LIST_ENTRY [ 0x84fda4b0 - 0x83b39d20 ]

Now, the WaitListhead is actually of type KWAIT_BLOCK with the first element as LIST_ENTRY, so...

kd> dt nt!_KWAIT_BLOCK 0x84fda4b0 

   +0x000 WaitListEntry    : _LIST_ENTRY [ 0x83b39d20 - 0x848b0728 ]

   +0x008 Thread           : 0x84f9caf8 _KTHREAD

   +0x00c Object           : 0x848b0720 Void

   +0x010 NextWaitBlock    : 0x84fda4c8 _KWAIT_BLOCK

   +0x014 WaitKey          : 0x10

   +0x016 WaitType         : 0x1 ''

   +0x017 BlockState       : 0x2 ''

To parse the list of threads waiting, we use the inbuilt ability of the dt command to parse lists. To read more about this, please refer to this blog post.

kd> dt nt!_KWAIT_BLOCK 0x84fda4b0 -l WaitlistEntry.Flink -y Thread

WaitlistEntry.Flink at 0x84fda4b0

---------------------------------------------

   +0x000 WaitListEntry :  [ 0x83b39d20 - 0x848b0728 ]

   +0x008 Thread : 0x84f9caf8 _KTHREAD

WaitlistEntry.Flink at 0x83b39d20

---------------------------------------------

   +0x000 WaitListEntry :  [ 0x848b0728 - 0x84fda4b0 ]

   +0x008 Thread : 0x848e5430 _KTHREAD

WinDbg : Walking Windows Linked Lists (LIST_ENTRY)

WinDbg : Walking Windows Linked Lists (LIST_ENTRY)

The Windows implementation of the linked list is slightly different from how we are used to seeing it. Most of us start programming data structures with this simple, yet highly used data structure. We usually define a linked list node as follows: 

typedef struct _Node{

    int       ndata;

    _Node *pNext;

} NODE, *PNODE, **PNODE;

Using such a construct tightly couples the data part of the list to the list implementation. If someone later needs to store a character instead of integer, they have to rewrite the node structure. One way to avoid the problem is to use void pointers. redefining the node structure this way would solve the problem partially.

typedef struct _Node{

    void    *pDataBlob;

    _Node *pNext;

} NODE, *PNODE, **PNODE;

The above construct solves most problems, except that the memory for the data blob will have to be elsewhere, and not part of the node itself. This is not a big problem most of the time, unless one wants to serialize the Node and pass it.

Windows has a slightly different way of implementing linked lists.  Windows defines the entry to the list as:

typedef struct _LIST_ENTRY {

  struct _LIST_ENTRY  *Flink;

  struct _LIST_ENTRY  *Blink;

} LIST_ENTRY, *PLIST_ENTRY;

This is exported in the debugger, and we can use the dt command to view it as well.

kd> dt _LIST_ENTRY

ntdll!_LIST_ENTRY

   +0x000 Flink            : Ptr32 _LIST_ENTRY

   +0x004 Blink            : Ptr32 _LIST_ENTRY

Anyone wanting to use linked lists would then use this structure in the following manner:

typedef struct _MY_DATA_STRUCT{

    void             *pDataBlob;

    LIST_ENTRY   List;

} MY_DATA_STRUCT, *PMY_DATA_STRUCT, **PMY_DATA_STRUCT;

Observe, that the LIST_ENTRY struct is embedded inside the data structure we want the list to be of. It is not a pointer. The idea here is that  a separate pointer be maintained which can be used to traverse from one list entry to another, thus we would land up somewhere in the middle of the data structure, then we use the popular CONTAINING_RECORD macro to find the base of the structure.

Cumbersome? Not really. Depending on where you place the LIST_ENTRY inside your structure, you can use this to your advantage as well.

Assume that instead of defining MY_DATA_STRUCT the way we did, we do it this way:

typedef struct _MY_DATA_STRUCT{

    LIST_ENTRY   List;

    void             *pDataBlob;    

} MY_DATA_STRUCT, *PMY_DATA_STRUCT, **PMY_DATA_STRUCT;

In this scenario, since the LIST_ENTRY is the first member, we do not need to find the beginning of the struct at all, a simple C-style cast of one pointer type to another would make the entire structure accessible.

This article is not intended to teach you how to use LIST_ENTRY, but to help us debug structures which already are using this construct. So, after this very brief introduction to lists, I leave it to you to write a few programs using this data type, if you are not already familiar with it, to make yourself comfortable.

The rest of this article will focus on traversing such lists inside the debugger.

Commands used in this article Are:

1. !process
2. dt

Lets try to walk the list of active processes. For this we need to find the head of this list. It is exported by the NT kernel.

kd> x nt!psacti*

8274af18          nt!PsActiveProcessHead = <no type information>

Great, so we now have the pointer to the head of this list.

kd> dt nt!_LIST_ENTRY 8274af18          

 [ 0x839afcb0 - 0x851d1400 ]

   +0x000 Flink            : 0x839afcb0 _LIST_ENTRY [ 0x8490da80 - 0x8274af18 ]

   +0x004 Blink            : 0x851d1400 _LIST_ENTRY [ 0x8274af18 - 0x846e98c8 ]

Okay, so it has a set of valid pointers. However, we need to find out which data structure in Windows is using this list. That is when we can type cast the pointer to the right data structure. In this case, the kernel _EPROCESS data structure is the one using this list. Lets find out which offset of _EPROCESS has this list.

Note: _EPROCESS is a large data structure, so the following output is snipped off after the first few lines.

kd> dt nt!_EPROCESS

   +0x000 Pcb              : _KPROCESS

   +0x098 ProcessLock      : _EX_PUSH_LOCK

   +0x0a0 CreateTime       : _LARGE_INTEGER

   +0x0a8 ExitTime         : _LARGE_INTEGER

   +0x0b0 RundownProtect   : _EX_RUNDOWN_REF

   +0x0b4 UniqueProcessId  : Ptr32 Void

   +0x0b8 ActiveProcessLinks : _LIST_ENTRY

   +0x0c0 ProcessQuotaUsage : [2] Uint4B

   +0x0c8 ProcessQuotaPeak : [2] Uint4B

   +0x0d0 CommitCharge     : Uint4B

   +0x0d4 QuotaBlock       : Ptr32 _EPROCESS_QUOTA_BLOCK

   +0x0d8 CpuQuotaBlock    : Ptr32 _PS_CPU_QUOTA_BLOCK

   +0x0dc PeakVirtualSize  : Uint4B

   +0x0e0 VirtualSize      : Uint4B

   +0x0e4 SessionProcessLinks : _LIST_ENTRY

<Output Snipped>

Okay, so we found the offset of the LIST_ENTRY we want. this means that for any LIST_ENTRY address, belonging to a list of processes, subtracting 0x0b8 from it would lead us to the beginning of the _EPROCESS structure.

Note: The offset of this member could vary between one Windows flavor to another, even between service packs. So it it critical to know the right offset for the particular debugging session.

Okay, lets try to validate our example with a simple experiment. lets see for the process lsass.exe how this goes.

kd> !process 0 0 lsass.exe

PROCESS 84cdc860  SessionId: 0  Cid: 0208    Peb: 7ffda000  ParentCid: 018c

    DirBase: 1eed30e0  ObjectTable: 95f00d18  HandleCount: 513.

    Image: lsass.exe

kd> dt nt!_EPROCESS 84cdc860  

   +0x000 Pcb              : _KPROCESS

   +0x098 ProcessLock      : _EX_PUSH_LOCK

   +0x0a0 CreateTime       : _LARGE_INTEGER 0x01cf0ae6`f5dbfe68

   +0x0a8 ExitTime         : _LARGE_INTEGER 0x0

   +0x0b0 RundownProtect   : _EX_RUNDOWN_REF

   +0x0b4 UniqueProcessId  : 0x00000208 Void

   +0x0b8 ActiveProcessLinks : _LIST_ENTRY [ 0x849390e8 - 0x84ccb4c0 ]

   +0x0c0 ProcessQuotaUsage : [2] 0x2cdc

   +0x0c8 ProcessQuotaPeak : [2] 0x2cdc

   +0x0d0 CommitCharge     : 0x219

   +0x0d4 QuotaBlock       : 0x8273ecc0 _EPROCESS_QUOTA_BLOCK

   +0x0d8 CpuQuotaBlock    : (null) 

<output snipped>

So we see that for this particular debugging instance, for lsass.exe, the base of the _EPROCESS is at 0x84cdc860. Adding 0xb8 to it gives us 0x84CDC918. So if this is a valid offset inside the _EPROCESS, then subtracting 0xb8 from it should get us the _EPROCESS base address back. this is exactly what we will do with the nt!PsActiveProcessHead pointer and how do we do that? Well, the versatile dt command already supports walking windows lists. here is how it is done.

Find the pointer to the list head for Active Processes.

kd> x nt!psact*

8274af18          nt!PsActiveProcessHead = <no type information>

Then find the Flink entry address for this List_ENTRY

kd> dt nt!_LIST_ENTRY 8274af18

 [ 0x839afcb0 - 0x851d1400 ]

   +0x000 Flink            : 0x839afcb0 _LIST_ENTRY [ 0x8490da80 - 0x8274af18 ]

   +0x004 Blink            : 0x851d1400 _LIST_ENTRY [ 0x8274af18 - 0x846e98c8 ]

Now we typecast the pointer we got to _EPROCESS, but we subtract the correct offset, 0x0b8 first. Then we use the -l switch of the dt command to tell it that we want it to parse a list, via the Flink member of _LIST_ENTRY, where the member variable inside _EPROCESS which is this _LIST_ENTRY type is called ActiveProcessLinks. We also ask it to print out the ImageFilename member inside _EPROCESS so that we can see what all processes are currently active.

 

kd> dt nt!_EPROCESS 0x839afcb0-0x0b8 -l ActiveProcessLinks.Flink -y ImageFileName

ActiveProcessLinks.Flink at 0x839afbf8

---------------------------------------------

   +0x0b8 ActiveProcessLinks :  [ 0x8490da80 - 0x8274af18 ]

   +0x16c ImageFileName : [15]  "System"

ActiveProcessLinks.Flink at 0x8490d9c8

---------------------------------------------

   +0x0b8 ActiveProcessLinks :  [ 0x84c3b0e8 - 0x839afcb0 ]

   +0x16c ImageFileName : [15]  "smss.exe"

ActiveProcessLinks.Flink at 0x84c3b030

---------------------------------------------

   +0x0b8 ActiveProcessLinks :  [ 0x84bf2df8 - 0x8490da80 ]

   +0x16c ImageFileName : [15]  "csrss.exe"

ActiveProcessLinks.Flink at 0x84bf2d40

---------------------------------------------

   +0x0b8 ActiveProcessLinks :  [ 0x848a7df8 - 0x84c3b0e8 ]

   +0x16c ImageFileName : [15]  "wininit.exe"

ActiveProcessLinks.Flink at 0x848a7d40

---------------------------------------------

   +0x0b8 ActiveProcessLinks :  [ 0x84c54df8 - 0x84bf2df8 ]

   +0x16c ImageFileName : [15]  "csrss.exe"

ActiveProcessLinks.Flink at 0x84c54d40

---------------------------------------------

   +0x0b8 ActiveProcessLinks :  [ 0x84ccb4c0 - 0x848a7df8 ]

   +0x16c ImageFileName : [15]  "winlogon.exe"

ActiveProcessLinks.Flink at 0x84ccb408

---------------------------------------------

   +0x0b8 ActiveProcessLinks :  [ 0x84cdc918 - 0x84c54df8 ]

   +0x16c ImageFileName : [15]  "services.exe"

ActiveProcessLinks.Flink at 0x84cdc860

---------------------------------------------

   +0x0b8 ActiveProcessLinks :  [ 0x849390e8 - 0x84ccb4c0 ]

   +0x16c ImageFileName : [15]  "lsass.exe"

<Output trimmed>

the list is long, so i trimmed the output. As we see, that one of the process names printed is lsass.exe

ActiveProcessLinks.Flink at 0x84cdc860

---------------------------------------------

   +0x0b8 ActiveProcessLinks :  [ 0x849390e8 - 0x84ccb4c0 ]

   +0x16c ImageFileName : [15]  "lsass.exe"

The address of the ActiveProcessLinks.Flink member is 0x84cdc860. So the address of it's corresponding _EPROCESS would be 0x84cdc860 - 0x0b8 = 0x84CDC860. This is exactly the address we get when we did a !process 0 0 lsass.exe.

kd> !process 0 0 lsass.exe

PROCESS 84cdc860  SessionId: 0  Cid: 0208    Peb: 7ffda000  ParentCid: 018c

    DirBase: 1eed30e0  ObjectTable: 95f00d18  HandleCount: 513.

    Image: lsass.exe

Lists are used extensively inside Windows, and we will use these commands in future blog posts. So I would suggest that you execute these yourselves to familiarize with the concept.