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:
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.