Recently I came across an instance where my debugger did not do what I wanted. Rarely do computers disobey me, but this one was unusually stubborn. There was no other option; I had to bend the debugger to my will.
There are many ways to make a computer program do what you want. If you have the source code you can rewrite and recompile the program. If you have a hex editor you can edit the code of the binary. A shim can be used to modify a program at runtime. In this instance I was in a hurry and I was ok with a temporary solution, so I used a debugger to change the execution of the debugger while it ran.
Debug a debugger? Can you do such a thing? Of course you can.
In this example a memory dump was captured of a system and I was asked to determine if the system had run out of desktop heap. Usually the !dskheap command can be used to determine how much heap has been used by each desktop. Unfortunately, this command failed me.
23: kd> !dskheap
Error Reading TotalFreeSize from nt!_HEAP @ fffffa8019c65c00
Failed to GetHeapInfo for desktop @fffffa8019c65c00
EnumDsktps failed on Winsta: 19c4f090FillWinstaArray failed
The error indicates that the command couldn’t read from the _HEAP structure at fffffa8019c65c00 for desktop fffffa8019c65c00. Further investigation found that reason I got this error is that the heap for the desktop in question is not valid memory. Because the memory is described by a prototype PTE I assume that the heap has not been initialized (Note: See Windows Internals’ Memory Management chapter for more information about proto PTEs).
23: kd> dt win32k!tagDESKTOP fffffa8019c65c00
+0x000 dwSessionId : 0
+0x008 pDeskInfo : 0xfffff900`c05e0a70 tagDESKTOPINFO
+0x010 pDispInfo : 0xfffff900`c0581e50 tagDISPLAYINFO
+0x018 rpdeskNext : 0xfffffa80`19c6ef20 tagDESKTOP
+0x020 rpwinstaParent : 0xfffffa80`19c4f090 tagWINDOWSTATION
+0x028 dwDTFlags : 0x110
+0x030 dwDesktopId : 0x19c65c00`00000003
+0x038 spmenuSys : (null)
+0x040 spmenuDialogSys : (null)
+0x048 spmenuHScroll : (null)
+0x050 spmenuVScroll : (null)
+0x058 spwndForeground : (null)
+0x060 spwndTray : (null)
+0x068 spwndMessage : 0xfffff900`c05e0d90 tagWND
+0x070 spwndTooltip : 0xfffff900`c05e0fa0 tagWND
+0x078 hsectionDesktop : 0xfffff8a0`00ef08e0 Void
+0x080 pheapDesktop : 0xfffff900`c05e0000 tagWIN32HEAP
+0x088 ulHeapSize : 0x18000
+0x090 cciConsole : _CONSOLE_CARET_INFO
+0x0a8 PtiList : _LIST_ENTRY [ 0xfffffa80`19c65ca8 - 0xfffffa80`19c65ca8 ]
+0x0b8 spwndTrack : (null)
+0x0c0 htEx : 0n0
+0x0c4 rcMouseHover : tagRECT
+0x0d4 dwMouseHoverTime : 0
+0x0d8 pMagInputTransform : (null)
23: kd> dd 0xfffff900`c05e0000
fffff900`c05e0000 ???????? ???????? ???????? ????????
fffff900`c05e0010 ???????? ???????? ???????? ????????
fffff900`c05e0020 ???????? ???????? ???????? ????????
fffff900`c05e0030 ???????? ???????? ???????? ????????
fffff900`c05e0040 ???????? ???????? ???????? ????????
fffff900`c05e0050 ???????? ???????? ???????? ????????
fffff900`c05e0060 ???????? ???????? ???????? ????????
fffff900`c05e0070 ???????? ???????? ???????? ????????
23: kd> !pte fffff900`c05e0000
PXE at FFFFF6FB7DBEDF90 PPE at FFFFF6FB7DBF2018 PDE at FFFFF6FB7E403010 PTE at FFFFF6FC80602F00
contains 000000076245C863 contains 0000000762569863 contains 000000045FA17863 contains F8A000F4F9780400
pfn 76245c ---DA--KWEV pfn 762569 ---DA--KWEV pfn45fa17 ---DA--KWEV not valid
There are many desktops in this session and I wanted to know about the usage of the other desktops, but the !dskheap command stopped after just one error. I needed to force it to continue after this error, so I launched a debugger to debug my debugger. There is a command to do this, just run .dbgdbg.
23: kd> .dbgdbg
Debugger spawned, connect with
For clarity I will call the original debugger where I ran !dskheap debugger1, and the new debugger spawned by .dbgdbg debugger2 .
Before switching to debugger2 I need to know what I am going to debug. The error message gives a hint about where to set a breakpoint, I am looking for a failure from GetHeapInfo.
I need to know which module GetHeapInfo is in, the .extmatch match command indicates which module contains the !dskheap command.
23: kd> .extmatch dskheap
Switching to debugger2 I set a breakpoint on kdexts!GetHeapInfo. Use Ctrl+C to trigger a debug break in cdb (this is the same as a Ctrl+Break in windbg).
0:004> bp kdexts!GetHeapInfo
Switch back to debugger1 and run the !dskheap command.
In debugger2 I have hit the breakpoint.
Breakpoint 0 hit
000007f9`4237b9b0 4055 push rbp
The error says GetHeapInfo failed, so I am interested in what this function returns. To see what GetHeapInfo returns I go up one level in the stack and set a new breakpoint on the code just after it returns. This new breakpoint will also dump the return value of GetHeapInfo (return values are always in the rax register).
000007f9`4237b483 4885c0 test rax,rax
0:000> r rax
0:000> bc *
0:000> bp 000007f9`4237b483 "r rax"
The next time the breakpoint hit the return value was 1, which in this instance means GetHeapInfo failed. This is where I exerted my control over the computer: I forced the return value to 0.
0:000> r rax=0
I ran through the other breakpoints and changed rax as necessary.
Everything was going well, until the computer defied me again. The !dskheap output computes the percentage of heap usage by dividing the bytes used by the size of the heap. Unfortunately, the size of the heap was left at 0 for the two heaps where I changed the return value. It is well known that only Chuck Norris can divide by zero; to prevent a roundhouse kick to your computer the processor generates an exception.
(2d0.928): Integer divide-by-zero - code c0000094 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
000007f9`4237b90e 49f7f3 div rax,r11
0:000> r r11
Fortunately debugger1 handles the divide by zero exception and it is easy to run !dskheap again.
Back in debugger2 I set a new breakpoint on the div instruction that outputs the divisor. When the divisor (r11) is 0 I changed it to a non-zero value to avoid the wrath of Mr. Norris.
0:000> bp 000007f9`4237b90e
0:000> bp 000007f9`4237b90e "r r11"
breakpoint 1 redefined
0:000> r r11=1
Finally, back in debugger1 I now have complete output for !dskheap. After a few strategic modifications of the program’s execution I got it to output the data I wanted. As it turns out we aren’t out of desktop heap after all.
Error Reading TotalFreeSize from nt!_HEAP @ fffffa801a53ea30
Winstation\Desktop Heap Size(KB) Used Rate(%)
WinSta0\Default 20480 0%
WinSta0\Disconnect 0 0%
WinSta0\Winlogon 192 2%
Service-0x0-3e7$\Default 7680 1%
Service-0x0-3e4$\Default 7680 0%
Service-0x0-3e5$\Default 7680 0%
Service-0x0-26f46a$\Default 0 0%
Service-0x0-2706f2$\Default 7680 0%
Total Desktop: ( 51392 KB - 8 desktops)
Session ID: 0
This is Ron Stock from the Global Escalation Services team and I recently had the task of live debugging a customer’s remote server. In debug circles we use what is known as a crash cart to live debug production servers. The phrase conjures up visions of a wheeled cabinet containing an emergency defibrillator, a heart monitor and latex gloves. Luckily for our purposes, the term merely denotes a machine setup with the Debugging Tools for Windows. This life saving machine is attached to the ailing production server for debugging and no medical degree is required.
The ailing production server is referred to as the Target Computer and the Debugging Tools for Windows are installed on the Host computer. The machines are attached with either a null-modem cable, 1394 cable, a special USB cable, or an ethernet cable (network debugging was added in Windows 8). Below I outline serial debugging because this is the most common technique. In future articles I plan to discuss configuring the other methods.
Serial Connection Setup
A null-modem cable is a serial cable used to send data between two serial ports and it can be cheaply purchased at most electronics stores. Be aware these are different from standard serial cables because the transmit and receive lines are cross linked.
Plug the null-modem cable into a serial port on each of the computers. The serial port on the target computer must be built into the system, add on components such as PCI cards will not work for serial debugging on the target computer.
Target Computer setup
1. To enable debugging enter the following command from an elevated command prompt.
bcdedit /debug on
2. In most systems the default debug settings are sufficient. The current settings can be verified with the below command.
3. Use the below command if you need to change the debug settings, where x is the number of the COM port connected to the null modem cable on the target machine and rate is the baud rate desired for debugging. The rate is usually 115200.
bcdedit /dbgsettings serial debugport:x baudrate:115200
5. Reboot the target computer.
Host Computer setup
1. First install the Windows Debugging Tools on the host computer. Navigate to the Windows Software Development Kit (SDK) currently located at this link http://msdn.microsoft.com/en-US/windows/hardware/hh852363 and choose the download option.
a. If you are not able to install the SDK on the host computer, the debugging tools for windows can be installed to a different system and the debugger directory can be copied to the host computer.
2. Click Next until you see the Select the features you want to install screen.
3. Select only the option named Debugging Tools for Windows and click the Install button. I typically install the tools to a directory named C:\debugger
4. After the Windows Debugging Tools are installed I set my symbol path by setting the environment variable _NT_SYMBOL_PATH. I recommend setting it to the public symbol server SRV*c:\localsymbols*http://msdl.microsoft.com/download/symbols. If you prefer, you can specify any path in place of ‘c:\localsymbols’.
5. Open the debugger by running windbg.exe from the c:\debugger folder.
6. On the File menu, choose Kernel Debug.
7. In the Kernel Debugging dialog box, open the COM tab.
8. In the Baud rate box, enter the same rate you selected for the Target Machine in the steps above. This is usually 115200.
9. In the Port box, enter COMx where x is the COM port connected to the null modem cable on this computer. In my example I plugged my null modem cable to com port 1 so I typed com1 in the field.
a. It is not necessary to use the same port number on both the target and the host. For example, it is possible to use com1 on the target and com2 on the host.
10. Click OK and you’ll receive a message indicating the Host computer is waiting to connect.
11. From the Debug menu select Break will cause the debugger to break into the target machine and give you the opportunity to debug your ailing production server. Good Luck!!
Hello Debuggers! This is Ron Stock from the Global Escalation Services team and I recently worked an interesting case dispatched to our team because Services.exe was terminating. Nothing good ever happens when Services.exe exits. In this particular case, client RDP sessions were forcibly disconnected from the server and the server machine was shutting down unexpectedly. This is the message encountered at the console on the server.
The customer was able to trigger the crash by changing anything to do with a particular non-Microsoft service in the Services MMC (e.g. changing the Startup Type, changing the dependencies, stopping the service etc...) To protect our vendor friend, I refer to this service as FriendlyService throughout this article. We could have stopped the investigation right here and implicated the FriendlyService vendor, however as you will see, this service was merely a victim.
When investigating a process termination, I routinely gather a process dump to start my investigation. The frustrating thing about this instance was how none of our debugging tools were generating a dump file when Services.exe terminated. I tried the usual toolset including AdPlus, ProcDump and DebugDiag to no avail. Despite the lack of data from a memory dump, I was still able to piece together a stack and attack the problem through a creative approach. Debugging is an art.
First I reviewed the application log which was loaded with entries like this one.
11/29/2012 04:29:05 PM Information HE2NTSP208 1004 Application Error N/A Faulting application services.exe, version 5.2.3790.4455, faulting module msvcr80.dll, version 8.0.50727.6195, fault address 0x000000000001df67.
From the event log entry above I was able to determine the faulting module, msvcr80.dll, and the relative offset, 0x000000000001df67. This address is basically the return address of the faulting call. As you can see from the ln output below, the function name was msvcr80!wcscpy_s. As I noted above, I wasn’t able to gather a dump during process termination so I had the customer use ProcDump to snap a dump of Services.exe during normal process operation (not a crash dump).
0:000> ln 00000000`78130000 + 0x000000000001df67 (00000000`7814ded0) msvcr80!wcscpy_s+0x97 | (00000000`7814df80) msvcr80!wcsncpy_s
Next I needed to determine which DLLs in the Services.exe process were calling msvcr80!wcscpy_s by reviewing the import tables of the binaries loading in the process. I used the !peb command to dump out the Process Environment Block (PEB). This gave me the list of base addresses for each loaded DLL. I focused mainly on non-Microsoft binaries. To protect our vendor friends, I renamed the DLL to ThirdPartyServiceMonitor.Dll in this article.
This is the output from the !peb command with the base addresses in the left column.
PEB at 000007fffffde000
Ldr.InInitializationOrderModuleList: 00000000000d2df0 . 00000000001a3600
Ldr.InLoadOrderModuleList: 00000000000d2d20 . 000000000019cbc0
Ldr.InMemoryOrderModuleList: 00000000000d2d30 . 000000000019cbd0
Base TimeStamp Module
100000000 49882047 Feb 03 04:45:27 2009 C:\WINDOWS\system32\services.exe
77ec0000 4ecbcd57 Nov 22 10:27:03 2011 C:\WINDOWS\system32\ntdll.dll
77d40000 49c51cdd Mar 21 11:59:09 2009 C:\WINDOWS\system32\kernel32.dll
7ff7fc00000 45d6ccae Feb 17 03:36:46 2007 C:\WINDOWS\system32\msvcrt.dll
7ff7fee0000 4a61f064 Jul 18 10:55:16 2009 C:\WINDOWS\system32\ADVAPI32.dll
7ff7fd30000 4c6ba77a Aug 18 04:27:22 2010 C:\WINDOWS\system32\RPCRT4.dll
7ff7e9c0000 4a37438e Jun 16 02:02:38 2009 C:\WINDOWS\system32\Secur32.dll
77c20000 45e7c5c2 Mar 02 00:35:46 2007 C:\WINDOWS\system32\USER32.dll
7ff7fc90000 490062ac Oct 23 06:40:28 2008 C:\WINDOWS\system32\GDI32.dll
7ff7c680000 45d6ccab Feb 17 03:36:43 2007 C:\WINDOWS\system32\USERENV.dll
7ff7c450000 45d6cc90 Feb 17 03:36:16 2007 C:\WINDOWS\system32\SCESRV.dll
7ff7e490000 45d6cc04 Feb 17 03:33:56 2007 C:\WINDOWS\system32\AUTHZ.dll
7ff77370000 4fedd464 Jun 29 11:14:28 2012 C:\WINDOWS\system32\NETAPI32.dll
7ff7c410000 45d6cca5 Feb 17 03:36:37 2007 C:\WINDOWS\system32\umpnpmgr.dll
7ff7d4d0000 45d6ccb8 Feb 17 03:36:56 2007 C:\WINDOWS\system32\WINSTA.dll
7ff65470000 45d6cc5c Feb 17 03:35:24 2007 E:\Program Files\ThirdPartyDirectory\ThirdParty2.dll
400000 424360e9 Mar 24 19:52:57 2005 C:\WINDOWS\system32\msvcp60.dll
7ff7d500000 45d6cc3b Feb 17 03:34:51 2007 C:\WINDOWS\system32\IMM32.DLL
e50000 4df9462e Jun 15 18:54:22 2011 E:\Program Files\ThirdPartyDirectory\ThirdPartyServiceMonitor.dll
Using the base address of ThirdPartyServiceMonitor, I dumped the header to find the Import Address Table Directory.
0:000> !dh 00000000`00e50000
1E000 [ 758] address [size] of Import Address Table Directory
Using the dps command I dumped all of the functions in the import table of ThirdPartyServiceMonitor.dll. I found msvcr80!wcscpy_s in the function list. This indicates ThirdPartyServiceMonitor.dll makes calls to the msvcr80!wcscpy_s.
0:000> dps 00000000`00e50000+ 1E000 l758/@$ptrsize
00000000`00e6e458 00000000`7814d890 msvcr80!_wcsnicmp
00000000`00e6e460 00000000`7814db20 msvcr80!_wcsicmp
00000000`00e6e498 00000000`7814ded0 msvcr80!wcscpy_s
Since this was the only third party DLL with msvcr80!wcscpy_s in its import table, I was able to continue piecing together my stack. ThirdPartyServiceMonitor.dll was calling msvcr80!wcscpy_s and causing Services.exe to crash. At this point in the investigation, the stack looks like this-
In my quest to continue building my “conceptual” stack without a crash dump file, I reviewed the status code from the “System Shutdown” dialog displayed when Services.exe terminated. Notice the status code -1073741811 in the error. What the heck does that mean?
Well I easily resolved the cryptic status code by passing 0n-1073741811 to the !error command in the debugger. The “0n” prefix indicates the value should be interpreted as decimal rather than hex by the debugger. I also included the negative symbol “-“ because this also appears in status code.
0:030> !error 0n-1073741811
Error code: (NTSTATUS) 0xc000000d (3221225485) - An invalid parameter was passed to a service or function.
Armed with the status code information, I reviewed the msvcr80!wcscpy_s assembly code to determine if this status code was returned at any point. In the assembly I found a call to a function named msvcr80!_invalid_parameter. In context of the message, “an invalid parameter was passed to a service or function”, this certainly sounds like the code path taken.
0:000> uf msvcr80!wcscpy_s
18 00000000`7814deea e88184feff call msvcr80!_errno(00000000`78136370)
18 00000000`7814deef 4533c9 xor r9d,r9d
18 00000000`7814def2 4533c0 xor r8d,r8d
18 00000000`7814def5 33d2 xor edx,edx
18 00000000`7814def7 33c9 xor ecx,ecx
18 00000000`7814def9 48c744242000000000 mov qword ptr [rsp+20h],0
18 00000000`7814df02 c70016000000 mov dword ptr[rax],16h
18 00000000`7814df08 e873d1feff call msvcr80!_invalid_parameter (00000000`7813b080)
18 00000000`7814df0d b816000000 mov eax,16h
34 00000000`7814df12 4883c438 add rsp,38h
34 00000000`7814df16 c3 ret
It was reasonable to add this call to my conceptual stack because the call tree makes sense.
Because I’m a curious kind of guy, I unassembled msvcr80!_invalid_parameter to peel back another layer of the onion. To my surprise I found a call to msvcr80!_imp_TerminateProcess. BOOM! This explains why the debugger wasn’t catching the process crash. The process was terminating ‘organically’ through a TerminateProcess call rather than crashing due to an exception, however it was unexpectedly terminating. In other words, all of the services running on the machine were not expecting Services.exe to terminate.
88 00000000`7813b155 ff156d200900 call qword ptr [msvcr80!_imp_GetCurrentProcess (00000000`781cd1c8)]
88 00000000`7813b15b ba0d0000c0 mov edx,0C000000Dh
88 00000000`7813b160 488bc8 mov rcx,rax
88 00000000`7813b163 ff1557200900 call qword ptr [msvcr80!_imp_TerminateProcess(00000000`781cd1c0)]
Now I was able to cobble together a fairly accurate stack without a dump file. At this point I could tell ThirdPartyServiceMonitor.dll was passing bad parameters to msvcr80!wcscpy_s. However, this didn’t explain how FriendlyService (mentioned in the blog introduction) was triggering the issue. I needed to go deeper with a live debug by leveraging the stack information I devised.
On the customer’s server I attached Windbg to the Services.exe process and set a breakpoint on msvcr80!_invalid_parameter.
0:001> bp msvcr80!_invalid_parameter
Then I had the customer reproduce the issue by changing the startup type on FriendlyService in the Services MMC. As I mentioned above, this was one way to trigger the issue. BOOM! My breakpoint hit and looked exactly like the conceptual stack I pieced together. Now I was able to determine what ThirdPartyServiceMonitor was passing to msvcr80!wcscpy_s while broken in with the debugger.
Child-SP RetAddr Call Site
00000000`105deb00 00000000`7814df67 msvcr80!_invalid_parameter+0xe3
00000000`105df0c0 00000000`00e53045 msvcr80!wcscpy_s+0x97
00000000`105df100 00000000`00e5947e ThirdPartyServiceMonitor+0x3045
00000000`105df130 00000000`00e58405 ThirdPartyServiceMonitor+0x947e
00000000`105df180 000007ff`7fd69c75 ThirdPartyServiceMonitor+0x8405
00000000`105df1b0 000007ff`7fe9ccc9 rpcrt4!Invoke+0x65
00000000`105df200 000007ff`7fe9d58d rpcrt4!NdrStubCall2+0x54d
I reviewed the assembly of ThirdPartyServiceMonitor at the point in which it calls msvcr80!wcscpy_s. I discovered that the vendor hardcoded the size of the destination string buffer with 200 hex (512 decimal) while the size of the source string buffer was greater than 512 decimal.
0:035> ub 00000000`00e53045
00000000`00e53020 898324040000 mov dword ptr [rbx+424h],eax
00000000`00e53026 488b4738 mov rax,qword ptr [rdi+38h]
00000000`00e5302a 4c8b4010 mov r8,qword ptr [rax+10h] <<<< Source Buffer
00000000`00e5302e 4d85c0 test r8,r8
00000000`00e53031 7412 je ThirdPartyServiceMonitor+0x3045 (00000000`00e53045)
00000000`00e53033 488d8b28040000 lea rcx,[rbx+428h] <<<< Destination Buffer
00000000`00e5303a ba00020000 mov edx,200h <<<<< Buffer Size hardcoded to 200 hex
00000000`00e5303f ff1553b40100 call qword ptr [ThirdPartyServiceMonitor!PlugControl+0xff38 (00000000`00e6e498)] <<<< call to msvcr80!wcscpy_s
00000000`00e53045 eb16 jmp ThirdPartyServiceMonitor +0x305d (00000000`00e5305d) <<<< return from msvcr80!wcscpy_s
0:035> .formats 200
Here is the definition of wcscpy_s from MSDN
wchar_t *strDestination, // Location of destination string buffer
size_t numberOfElements, // Size of the destination string buffer.
const wchar_t *strSource // Null-terminated source string buffer.
Here are the parameters passed to msvcr80!wcscpy_s. As a reminder, the x64 compiler uses rcx to pass the first parameter, rdx for the second, and r8 for the third. In this case the buffer size was the second parameter in rdx and the source buffer was in r8, the third parameter.
0:035> dq 00000000`00e6e498 l1
0:035> ln 00000000`7814ded0
(00000000`7814ded0) msvcr80!wcscpy_s | (00000000`7814df80) msvcr80!wcsncpy_s
Dumping the source string in r8 showed the string was clearly longer than 512 characters.
0:035> dc 00000000`0103b438
00000000`0103b438 003a0000 0049005c 0074006e 00670065 ..:.\.F.r.e.i.n.
00000000`0103b448 00610072 00690074 006e006f 00670041 d.l.y.S.e.r.v.i.
00000000`0103b458 006e0065 005c0074 0069006c 005c0062 c.e.\.l.i.b.\..
00000000`0103b738 00650070 002e0072 00700061 002e0070 p.e.r...a.p.p...
00000000`0103b748 00610070 00610072 0065006d 00650074 p.a.r.a.m.e.t.e.
00000000`0103b758 002e0072 003d0031 00770020 00610072 r...1.=. .w.r.a.
00000000`0103b768 00700070 00720065 0061002e 00700070 p.p.e.r...a.p.p.
00000000`0103b778 0070002e 00720061 006d0061 00740065 ..p.a.r.a.m.e.t.
00000000`0103b788 00720065 0032002e 0020003d 00720077 e.r...2.=. .w.r.
00000000`0103b798 00700061 00650070 002e0072 00700061 a.p.p.e.r...a.p.
00000000`0103b7a8 002e0070 00610070 00610072 0065006d p...p.a.r.a.m.e.
00000000`0103b7b8 00650074 002e0072 003d0033 00770020 t.e.r...3.=. .w.
00000000`0103b7c8 00610072 00700070 00720065 0061002e r.a.p.p.e.r...a.
00000000`0103b7d8 00700070 0070002e 00720061 006d0061 p.p...p.a.r.a.m.
00000000`0103b7e8 00740065 00720065 0034002e 0020003d e.t.e.r...4.=. .
00000000`0103b7f8 00720077 00700061 00650070 002e0072 w.r.a.p.p.e.r...
00000000`0103b808 00700061 002e0070 00610070 00610072 a.p.p...p.a.r.a.
00000000`0103b818 0065006d 00650074 002e0072 003d0035 m.e.t.e.r...5.=.
00000000`0103b828 00770020 00610072 00700070 00720065 .w.r.a.p.p.e.r.
It was not hard to figure out this string was the imagefile path located at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\FriendlyService. You can see from the string that service takes parameters which increased the size of the image path beyond 512 characters.
FriendlyService\lib\friendly.3\sbin\friendlywindows-x86-32.exe-s E:\ FriendlyService\conf\wrapper.conf set.DATA_APP=apia "set.DATA_APP_LONG= FriendlyServiceVendor" set.DATA_EXE=E:\ FriendlyService\bin\..\lib\friendly.3\bin set.DATA_HOME=E:\ FriendlyService\bin\..\lib\friendly.3 set.INSTALL_DIR=E:\ FriendlyService\bin\.. wrapper.working.dir=E:\ FriendlyService\bin\.. wrapper.app.parameter.1= wrapper.app.parameter.2= wrapper.app.parameter.3= wrapper.app.parameter.4= wrapper.app.parameter.5= wrapper.app.parameter.6= wrapper.app.parameter.7= wrapper.app.parameter.8= wrapper.app.parameter.9=
The ThirdPartyServiceMonitor hooks any changes made to a service (e.g. changing the Startup Type, changing the dependencies, stopping the service etc...). After hooking the change, ThirdPartyServiceMonitor.dll performs a string copy of the service’s image file path. In most cases this works like a champ, however in this instance, the FriendlyService image path from vendor 1 is really long and the ThirdPartyServiceMonitor from vendor 2 doesn’t account for service image paths exceeding 512 characters. This is the perfect storm! ThirdPartyServiceMonitor needs to remove the hardcoded buffer size.
This example occurred on Windows Server 2003. Starting in Windows 7 and Windows Server 2008 R2, Windows has added functionality for catching processes that exit ‘organically’ through a TerminateProcess call. You can find information on this added functionality on MSDN.
Recently I was working with a developer who had created performance counters that work in Performance Monitor but are never collected in a user defined data collector set. The customer explained that their counters update named shared memory inside the application which should be read by perfmon or the data collector set.
Putting counter data in shared memory is a common technique for performance counter developers. A programmer can update performance data in a block of shared memory in their application and then use a performance extension dll (aka an “extensible counter”) to read from the shared memory.
Shared memory is created by calling CreateFileMapping and MapViewOfFile. This memory is then accessed by another application by calling OpenFileMapping. All applications which use this shared memory must pass the same lpName to CreateFileMapping or OpenFileMapping. An example of using these APIs to implement shared memory is available on MSDN.
Based on the customer’s explanation that they are populating shared memory in their application, and their counters work in Performance Monitor but do not work in a user defined data collector set, I suspected that OpenFileMapping was failing for the data collector set.
User defined data collector sets run in a rundll32.exe process. If you have multiple rundll32.exe processes you may need to identify which one is related to your data collector set. The relevant process has a command line similar to “rundll32.exe C:\Windows\system32\pla.dll,PlaHost”. There are several tools that can be used to identify the command line of the process such as tlist.exe, which is included with the Debugging Tools for Windows.
After attaching a debugger to rundll32.exe, I wanted to break on the ret instruction at the end of the OpenFileMappingW function. This would allow me to determine if the function succeeds or fails. According to MSDN “If the function fails, the return value is NULL. To get extended error information, call GetLastError.”
The uf command is an easy way to unassemble a function and find the ret instruction to break on.
0:001> uf kernelbase!OpenFileMappingW
75b88e0d 8bff mov edi,edi
75b88e79 c9 leave
75b88e7a c20c00 ret 0Ch
0:001> bp 75b88e7a
eax=00000000 ebx=00008022 ecx=7ffd8000 edx=00000002 esi=05abf03c edi=00000000
eip=75b88e7a esp=05abeb20 ebp=05abeb3c iopl=0 nv up ei pl zr na pe nc
cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00000246
In the above output we can see that the eax register is NULL, indicating that the call to OpenFileMapping failed. The !gle command will show the last error and last status.
LastErrorValue: (Win32) 0x2 (2) - The system cannot find the file specified.
LastStatusValue: (NTSTATUS) 0xc0000034 - Object Name not found.
The failure is that OpenFileMapping cannot find the file. The file name is the third parameter to OpenFileMapping. We can get the first three parameters from the kb command.
ChildEBP RetAddr Args to Child
05abf0d0 6abae355 0002001f 00000000 05abeb7c kernelbase!OpenFileMappingW+0x90
WARNING: Stack unwind information not available. Following frames may be wrong.
05abf0f8 7784fe67 02a7ae90 05abf224 05abf254 ninjaprf+0x10edb
05abf110 7784fc97 00472158 02a7ae90 05abf224 advapi32!CallExtObj+0x17
05abf270 7784efaf 05abf2bc 60fcfa02 05abf778 advapi32!QueryExtensibleData+0x735
05abf654 75ff0468 80000004 05abf778 00000000 advapi32!PerfRegQueryValue+0x5da
05abf748 75ffd505 80000004 05abf778 05abf790 kernel32!LocalBaseRegQueryValue+0x366
05abf7b4 61247dc5 80000004 02a7ae90 00000000 kernel32!RegQueryValueExW+0xb7
05abf830 61250595 80000004 02a7ae58 02a7ae90 pdh!GetSystemPerfData+0x92
05abf89c 6124c753 02a407d0 05abf8e8 61241928 pdh!GetQueryPerfData+0xa4
05abf8b8 61254463 02a407d0 05abf8e8 60fcf32f pdh!PdhiCollectQueryData+0x32
05abf90c 611c6d04 02a58f08 00000000 75ffc3e0 pdh!PdhUpdateLogW+0xa2
05abf9bc 611be128 0045c968 00000000 00000000 pla!HPerformanceCounterDataCollector::Cycle+0x48
05abf9bc 00000000 0045c968 00000000 00000000 pla!PlaiCollectorControl+0x3b7
0:008> da 05abeb7c
The user defined data collector set is failing to open the file "Local\NINJAPERF_S-1-5-18". This is the name that the performance extension dll ninjaprf.dll has given to its shared memory.
Based on the customer’s description this operation works in Performance Monitor. Next, I attached a debugger to perfmon and set the same breakpoint.
eax=000009f8 ebx=00008022 ecx=a7330000 edx=080ee678 esi=06798070 edi=00000000
eip=760be9bb esp=0a84e564 ebp=0a84e580 iopl=0 nv up ei pl zr na pe nc
cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00000246
760be9bb c20c00 ret 0Ch
In the above output we can see that eax is a handle number, indicating that the function succeeded.
Looking at the file being opened we can see why this works in one scenario and not in another.
0:016> kb 1
0a84e580 698e4ab9 0002001f 00000000 0a84e5c0 kernelbase!OpenFileMappingW+0x90
0:016> da 0a84e5c0
The file name used is unique for each user. The ninjaprf dll has chosen a file name which includes the SID of the current user. This works for Performance Monitor because the user who starts the application is the same as the user who runs Performance Monitor. However, a user defined data collector set runs in rundll32.exe as the Local System account.
It is possible to run the user defined data collector set as a different user, however the file name being used will not work in that scenario either. By using the “Local\” prefix, the file is created in the local session namespace. The application runs in the user’s session, while rundll32.exe is started by the Task Scheduler service and runs in session 0. This prevents the user defined data collector set from seeing the file created by the application.
If your performance counter uses shared memory to communicate, be aware that your performance extension dll may be run in the context of a different user account and a different user session. Your inter process communication techniques must account for this, or your customers will ask you why your counters do not work outside of Performance Monitor.
What is a bug check 0x133?
Starting in Windows Server 2012, a DPC watchdog timer is enabled which will bug check a system if too much time is spent in DPC routines. This bug check was added to help identify drivers that are deadlocked or misbehaving. The bug check is of type "DPC_WATCHDOG_VIOLATION" and has a code of 0x133. (Windows 7 also included a DPC watchdog but by default, it only took action when a kernel debugger was attached to the system.) A description of DPC routines can be found at http://msdn.microsoft.com/en-us/library/windows/hardware/ff544084(v=vs.85).aspx.
The DPC_WATCHDOG_VIOLATION bug check can be triggered in two ways. First, if a single DPC exceeds a specified number of ticks, the system will stop with 0x133 with parameter 1 of the bug check set to 0. In this case, the system's time limit for single DPC will be in parameter 3, with the number of ticks taken by this DPC in parameter 2. Alternatively, if the system exceeds a larger timeout of time spent cumulatively in all DPCs since the IRQL was raised to DPC level, the system will stop with a 0x133 with parameter 1 set to 1. Microsoft recommends that DPCs should not run longer than 100 microseconds and ISRs should not run longer than 25 microseconds, however the actual timeout values on the system are set much higher.
How to debug a 0x133 (0, …
In the case of a stop 0x133 with the first parameter set to 0, the call stack should contain the offending driver. For example, here is a debug of a 0x133 (0,…) kernel dump:
0: kd> .bugcheck
Bugcheck code 00000133
Arguments 00000000`00000000 00000000`00000283 00000000`00000282 00000000`00000000
Per MSDN, we know that this DPC has run for 0x283 ticks, when the limit was 0x282.
0: kd> k
fffff803`08c18428 fffff803`098525df nt!KeBugCheckEx
fffff803`08c18430 fffff803`09723f11 nt! ??::FNODOBFM::`string'+0x13ba4
fffff803`08c184b0 fffff803`09724d98 nt!KeUpdateRunTime+0x51
fffff803`08c184e0 fffff803`09634eba nt!KeUpdateTime+0x3f9
fffff803`08c186d0 fffff803`096f24ae hal!HalpTimerClockInterrupt+0x86
fffff803`08c18700 fffff803`0963dba2 nt!KiInterruptDispatchLBControl+0x1ce
fffff803`08c18898 fffff803`096300d0 hal!HalpTscQueryCounter+0x2
fffff803`08c188a0 fffff880`04be3409 hal!HalpTimerStallExecutionProcessor+0x131
fffff803`08c18930 fffff880`011202ee ECHO!EchoEvtTimerFunc+0x7d //Here is our driver, and we can see it calls into StallExecutionProcessor
fffff803`08c18960 fffff803`097258b4 Wdf01000!FxTimer::TimerHandler+0x92
fffff803`08c189a0 fffff803`09725ed5 nt!KiProcessExpiredTimerList+0x214
fffff803`08c18ae0 fffff803`09725d88 nt!KiExpireTimerTable+0xa9
fffff803`08c18b80 fffff803`0971fe76 nt!KiTimerExpiration+0xc8
fffff803`08c18c30 fffff803`0972457a nt!KiRetireDpcList+0x1f6
fffff803`08c18da0 00000000`00000000 nt!KiIdleLoop+0x5a
Let’s view the driver’s unassembled DPC routine and see what it is doing
0: kd> ub fffff880`04be3409
fffff880`04be33e0 448b4320 mov r8d,dword ptr[rbx+20h]
fffff880`04be33e4 488b0d6d2a0000 mov rcx,qword ptr [ECHO!WdfDriverGlobals (fffff880`04be5e58)]
fffff880`04be33eb 4883631800 and qword ptr [rbx+18h],0
fffff880`04be33f0 488bd7 mov rdx,rdi
fffff880`04be33f3 ff150f260000 call qword ptr [ECHO!WdfFunctions+0x838(fffff880`04be5a08)]
fffff880`04be33f9 bbc0d40100 mov ebx,1D4C0h
fffff880`04be33fe b964000000 mov ecx,64h
fffff880`04be3403 ff15f70b0000 call qword ptr[ECHO!_imp_KeStallExecutionProcessor (fffff880`04be4000)] //Its Calling KeStallExecutionProcessor with 0x64 (decimal 100) as a parameter
0: kd> u fffff880`04be3409
fffff880`04be3409 4883eb01 sub rbx,1
fffff880`04be340d 75ef jne ECHO!EchoEvtTimerFunc+0x72 (fffff880`04be33fe) //Here we can see it is jumping back to call KeStallExecutionProcessor in a loop
fffff880`04be340f 488b5c2430 mov rbx,qword ptr[rsp+30h]
fffff880`04be3414 4883c420 add rsp,20h
fffff880`04be3418 5f pop rdi
fffff880`04be3419 c3 ret
fffff880`04be341a cc int 3
fffff880`04be341b cc int 3
0: kd> !pcr
KPCR for Processor 0 at fffff80309974000:
Major 1 Minor 1
DpcQueue: 0xfffffa80020ce790 0xfffff880012e4e9c [Normal] NDIS!NdisReturnNetBufferLists
0xfffffa800185f118 0xfffff88000c0ca00 [Normal] ataport!AtaPortInitialize
0xfffff8030994fda0 0xfffff8030972bc30 [Normal] nt!KiBalanceSetManagerDeferredRoutine
0xfffffa8001dbc118 0xfffff88000c0ca00 [Normal] ataport!AtaPortInitialize
0xfffffa8002082300 0xfffff88001701df0 [Normal] USBPORT
The !pcr output shows us queued DPCs for this processor. If you want to see more information about DPCs and the DPC Watchdog, you could dump the PRCB listed in the !pcr output like this:
dt nt!_KPRCB fffff80309974180 Dpc*
Often the driver will be calling into a function like KeStallExecutionProcessor in a loop, as in our example debug. To resolve this problem, contact the driver vendor to request an updated driver version that spends less time in its DPC Routine.
How to troubleshoot a 0x133 (1, …
Determining the cause of a stop 0x133 with a first parameter of 1 is a bit more difficult because the problem is a result of DPCs running from multiple drivers, so the call stack is insufficient to determine the culprit. To troubleshoot this stop, first make sure that the NT Kernel Logger or Circular Kernel Context Logger ETW traces are enabled on the system. (For directions on setting this up, see http://blogs.msdn.com/b/ntdebugging/archive/2009/12/11/test.aspx.)
Once the logging is enabled and the system bug checks, dump out the list of ETW loggers using !wmitrace.strdump. Find the ID of the NT Kernel logger or the Circular logger. You can then use !wmitrace.logsave (ID) (path to ETL) to write out the ETL log to a file. Load it up with Windows Performance Analyzer and add the DPC or DPC/ISR Duration by Module, Function view (located in the Computation group) to your current analysis window:
Next, make sure the table is also shown by clicking the box in the upper right of the view:
Ensure that the Address column is added on the left of the gold bar, then expand each address entry to see individual DPC enters/exits for each function. Using this data, you can determine which DPC routines took the longest by looking at the inclusive duration column, which should be added to the right of the gold bar:
In this case, these DPCs took 1 second, which is well over the recommended maximum of 100 us. The module column (and possible the function column, if you have symbols) will show which driver is responsible for that DPC routine. Since our ECHO driver was based on WDF, that is the module named here.
For an example of doing this type of analysis in xperf, see http://blogs.msdn.com/b/ntdebugging/archive/2008/04/03/windows-performance-toolkit-xperf.aspx.
For additional information about Stop 0x133 errors, see this page on MSDN: http://msdn.microsoft.com/en-us/library/windows/hardware/jj154556(v=vs.85).aspx.
For DPC timing recommendations and for advice on capturing DPC timing information using tracelog, see http://msdn.microsoft.com/en-us/library/windows/hardware/ff545764(v=vs.85).aspx.
Guidelines for writing DPC routines can be found at http://msdn.microsoft.com/en-us/library/windows/hardware/ff546551(v=vs.85).aspx.
In Part 1 of this series we identified a pool leak in non paged pool. In Part 2 and Part 3 of this series we identified what pool tag was leaking. In Part 5 and Part 6 we got call stacks showing the memory being allocated. In this article we are going to discuss a tool that combines this information into one piece of data.
Starting with Windows 7 and Windows Server 2008 R2, Windows has new functionality to track pool allocations and frees using the Windows Performance Toolkit, commonly referred to as xperf. For this example we will be using the WPT from the Windows 8 ADK. When installing the ADK select only the Windows Performance Toolkit option to minimize download time.
Before collecting pool usage data on a 64-bit system, you must disable the paging of data such as drivers and call stacks. The first time you run the Windows Performance Recorder UI and click the Start button you will be prompted that Disable Paging Executive is not set. If you click OK to this dialog WPR will set DisablePagingExecutive and ask that you reboot. To set this ahead of time run the following command from an elevated command prompt, and reboot afterwards:
wpr –disablepagingexecutive on
There are three methods to collect data from a pool leak using the Windows Performance Toolkit. The WPR UI, WPR command line, and xperf command line each provide different methods to collect this data.
Windows Performance Recorder UI:
The easiest way to record a trace is with the Windows Performance Recorder, which presents a GUI to make it easy for a user to record a trace. To collect data regarding a pool leak simply check the “Pool usage” checkbox and click the Start button. Reproduce the leak and after a few minutes click the Save button. Use Cancel to stop the trace after your log is saved. Note that by default the WPR will use a circular log in memory, if you record for a long period of time the log will wrap and data will be lost. A sequential file log will be captured by selecting File as the “Logging mode”, however these logs can become very large in a short period of time on a busy system so it is not recommended to leave the log running for longer than a few minutes.
Because these traces can quickly become large, it can be helpful to trace just one pool tag. This can be done in WPR using a custom WPR recording profile. Below is a sample profile that collects pool usage information only for the tag “Leak”, which we identified as the leaking tag in previous articles (Note that you should change the pool tag Leak to the tag you are investigating). Save this text in a file ending in .wprp (ie PoolTagLeak.wprp) and load it in WPR using the Add Profiles button. Check the “Pool usage Tag ‘Leak’” option under Custom measurements. Use the Start button to begin collecting data, reproduce the leak, and use the Save button to save the log. After you have collected the log click Cancel to stop collecting data.
<?xml version="1.0" encoding="utf-8"?>
<WindowsPerformanceRecorder Version="1.0" Author="NTDebugging Blog" Copyright="Microsoft Corporation" Company="Microsoft Corporation">
<SystemCollector Id="SystemCollector" Name="NT Kernel Logger">
<BufferSize Value="1024" />
<Buffers Value="1024" />
<Keyword Value="Pool" />
<Stack Value="PoolAllocation" />
<Stack Value="PoolAllocationSession" />
<Profile Id="PoolUsage.Verbose.File" Name="PoolUsage" Description="Pool usage Tag 'Leak' " LoggingMode="File" DetailLevel="Verbose">
<SystemProviderId Value="SystemProvider" />
<Profile Id="PoolUsage.Verbose.Memory" Name="PoolUsage" Description="Pool usage Tag 'Leak'" Base="PoolUsage.Verbose.File" LoggingMode="Memory" DetailLevel="Verbose" />
Windows Performance Recorder command line:
WPR can also be run from a command line if you need to script its operation, or if you prefer typing text over clicking buttons. By default WPR will be installed at C:\Program Files (x86)\Windows Kits\8.0\Windows Performance Toolkit\wpr.exe.
To start a trace, run the following from a command line:
wpr -start GeneralProfile -start Pool
To save the trace run:
wpr –stop pool.etl “pool leak”
Cancel the trace with:
Optionally, you can use the custom profile defined earlier in this article to trace just one pool tag. This command assumes PoolTagLeak.wprp is in the same folder as wpr.exe, use the full path if your custom profile is located elsewhere. Save and cancel the trace using the above steps.
wpr -start PoolTagLeak.wprp
Xperf command line:
The third way to enable this tracing is with xperf. This is an older tool which has been replaced by WPR, however xperf provides a circular logging functionality that isn’t available in WPR. Circular logging can be useful if you need to run xperf over a longer period of time. The below command enables tracing with xperf for the pool tag Leak using a circular buffer. Note that you should change the pool tag Leak to the tag you are investigating.
xperf -on Base+CSwitch+POOL -stackwalk PoolAlloc+PoolAllocSession –PoolTag Leak -BufferSize 1024 -MaxBuffers 1024 -MaxFile 1024 –FileMode Circular
To save and cancel the xperf trace in one command:
xperf -d pool.etl
Analyzing data with Windows Performance Analyzer:
After you have collected a trace using the method that works best for your scenario, open the etl file in the Windows Performance Analyzer. The below output is from a trace collected with WPR.
Pool analysis will require symbols. Configure the symbol path using the option in the Trace menu. Often the symbol path will be pre-populated, if it is not use srv*c:\symbols*http://msdl.microsoft.com/download/symbols. Click the Load Symbols option from the Trace menu, and be patient while WPA downloads symbols from the symbol server.
In the Graph Explorer click the + next to Memory to drop down the available memory graphs. Right click the Pool Total Allocation Size graph and choose Add graph to New Analysis View. If you are working with a small resolution screen you may want to click the X at the top of the Graph Explorer to close it, the Graph Explorer can be restored from the Window menu.
The key to effective xperf analysis is to sort the data by the appropriate columns. Columns can be added to the chart at the bottom of the view by right clicking the header and choosing the appropriate fields. To perform pool analysis the Type, Paged, Pool Tag, and Stack columns are necessary. Drag each of these columns to the left of the yellow line and sort them in the order shown below. Click the Size column to sort it as the primary.
The Type column indicates when the pool memory was allocated and when it was freed. The term “AIFO” means the pool was Allocated Inside the timeframe of the trace and it was Freed Outside the timeframe of the trace (or perhaps it was never freed at all). The term “AIFI” means the pool was Allocated Inside the timeframe of the trace, and it was also Freed Inside the timeframe of the trace (this memory was not leaked). Because we are interested in memory that was not freed, start by clicking the + next to AIFO.
The Paged column indicates if the pool allocations recorded are Paged or NonPaged. From the perfmon analysis in Part 1 we know that the leak we are troubleshooting in this example is in NonPaged pool. If a perfmon log is not available, the Size column is an indicator of what type of pool was leaked. Click the + next to whichever type of pool is largest in your trace.
The Pool Tag column displays the pool tag associated with each pool allocation. Again, the Size column is an indicator of which tag is leaking. Click the + next to the largest pool tag in your trace.
The Stack column displays the call stack leading up to the allocation. This is the information we are most interested in, it will indicate what driver is allocating the pool and it may indicate why. Click the + next to the largest Stack in your trace. Depending on how many times a particular code path is repeated, your Stack may only partially display and there may be more + options, you can use the right arrow key as a shortcut to open each of these until you see the call to ExAllocatePool. When the complete stack has been displayed the right arrow key will stop expanding stacks.
In the above output we can see that there was a NonPaged pool leak in the tag Leak. The call stack shows that the allocations were made by myfault.sys. A driver developer would have a great use for this information. If this was data from an actual leak the developer would use this output to determine that the leak is occurring due to an IOCTL sent from NotMyfault.exe!WinMain which leads to an allocation made in myfault.sys!MyfaultDeviceControl. A developer can use this information to perform a code review and identify under what conditions MyfaultDeviceControl allocates this pool, under what conditions it should be expected to free it, and why it may not free the memory.
This article concludes our series on troubleshooting pool leaks. We have demonstrated various techniques which each have their own strengths and weaknesses. Each of these techniques has a place in your debugging toolkit and are applicable to different circumstances depending on what your scenario is and what data you have available.
In part 5 we used poolhittag to get call stacks of pool being allocated and freed. This information is often essential to identifying the cause of a memory leak; however it is not always feasible to configure a live kernel debug to obtain this information. Fortunately there are alternative methods to get such call stacks.
Driver verifier has an option to enable pool tracking for a specific driver, or for multiple drivers. This functionality was first introduced in Windows Vista and Windows Server 2008. This information is also captured when driver verifier is used to enable special pool, however for the purposes of this article we will focus on using pool tracking.
The data stored by driver verifier requires a debugger to view. Any method of debugging can be used for this. You can use a live kernel debug as we described in part 4, you can get a memory dump (kernel or complete, a small dump is insufficient), or you can use livekd.
If you have used the steps from Part 1, Part 2, or Part 3, you likely have an idea which drivers are likely involved in creating the pool leak. In this example we are generating the leak using notmyfault, the same tool we have been using in prior examples. As seen in Part 2, the relevant driver is myfault.sys.
Although driver verifier has GUI, the easiest way to enable this functionality is with the below command from an elevated command prompt:
Verifier /flags 8 /driver myfault.sys
The above command will provide the following output, allowing you to confirm that the expected settings are enabled:
New verifier settings:
Special pool: Disabled
Pool tracking: Enabled
Force IRQL checking: Disabled
I/O verification: Disabled
Deadlock detection: Disabled
DMA checking: Disabled
Security checks: Disabled
Force pending I/O requests: Disabled
Low resources simulation: Disabled
IRP Logging: Disabled
Miscellaneous checks: Disabled
You must restart this computer for the changes to take effect.
After rebooting the system, reproduce the memory leak and attach a debugger or generate a memory dump after the memory has been leaked.
Break in with the debugger (Ctrl+Break or Ctrl+C) or load the dump in windbg (File – Open Crash Dump).
Set the symbol path and reload symbols.
1: kd> .symfix c:\symbols
1: kd> .reload
Loading Kernel Symbols
The !verifier command has various options to view information about driver verifier. To view the pool allocations which have been tracked by verifier for notmyfault.sys, use the following:
0: kd> !verifier 3 myfault.sys
Verify Level 8 ... enabled options are:
All pool allocations checked on unload
Summary of All Verifier Statistics
Synch Executions 0x0
Pool Allocations Attempted 0xb
Pool Allocations Succeeded 0xb
Pool Allocations Succeeded SpecialPool 0xa
Pool Allocations With NO TAG 0x1
Pool Allocations Failed 0x0
Resource Allocations Failed Deliberately 0x0
Current paged pool allocations 0x0 for 00000000 bytes
Peak paged pool allocations 0x1 for 00000080 bytes
Current nonpaged pool allocations 0xa for 009CE000 bytes
Peak nonpaged pool allocations 0xa for 009CE000 bytes
Driver Verification List
Entry State NonPagedPool PagedPool Module
fffffa80031b5830 Loaded 009ce000 00000000 myfault.sys
Current Pool Allocations 0000000a 00000000
Current Pool Bytes 009ce000 00000000
Peak Pool Allocations 0000000a 00000001
Peak Pool Bytes 009ce000 00000080
PoolAddress SizeInBytes Tag CallersAddress
fffffa8005400000 0x000fb000 Leak fffff8800447d634
fffffa80052fb000 0x000fb000 Leak fffff8800447d634
fffffa8005200000 0x000fb000 Leak fffff8800447d634
fffffa80050fb000 0x000fb000 Leak fffff8800447d634
fffffa8005000000 0x000fb000 Leak fffff8800447d634
fffffa8004efb000 0x000fb000 Leak fffff8800447d634
fffffa8004e00000 0x000fb000 Leak fffff8800447d634
fffffa8004cfb000 0x000fb000 Leak fffff8800447d634
fffffa8004c00000 0x000fb000 Leak fffff8800447d634
fffffa8004a66000 0x000fb000 Leak fffff8800447d634
At the bottom of the above output is the list of allocations made by notmyfault.sys. For our purposes we are going to assume that these allocations have been leaked, as opposed to just being normal allocations that were not yet freed when the debugger was attached.
The !verifier command has an option to view call stacks for one of the tracked allocations. Keep in mind that the size of the database is limited and only more recent allocations will be kept in the database.
0: kd> !verifier 80 fffffa8005400000
Log of recent kernel pool Allocate and Free operations:
There are up to 0x10000 entries in the log.
Parsing 0x0000000000010000 log entries, searching for address 0xfffffa8005400000.
Pool block fffffa8005400000, Size 00000000000fa000, Thread fffffa80044ceb60
Parsed entry 0000000000010000/0000000000010000...
Finished parsing all pool tracking information.
The above output shows the call stack leading to the pool allocation. This is the same information we had seen in Part 5, however we are able to obtain this information using a dump or livekd, whereas the steps from Part 5 required an invasive debug and extended system downtime.
When you have completed troubleshooting, disable driver verifier with the following command and reboot:
Hey there NTDEBUGGERS my name is Randy Monteleone and today we are going to talk about IRPs. In the past we have talked about the IRP structure in passing and showed a field here and there that can be pulled out and used to find answers to stalled IO. I was recently working on a debugger extension and found something interesting in the IRP I was looking at. I had been looking at a !irp output much like the one pictured below. I found that I was asking myself what do the "Success Error Cancel" fields mean?
After doing some digging and working with a few of my co-workers we found the mystery to the meaning behind these words and why we see them in our output. Lets break this IRP down starting with the ">" marker that indicates the current stack frame. In the output below you see this marker is indicating that we are working on something in partmgr.
1: kd> !irp fffffa809a1f3af0
Irp is active with 9 stacks 5 is current (= 0xfffffa809a1f3ce0)
Mdl=fffffa814e9c4f40: No System Buffer: Thread fffffa80d05b67e0: Irp stack trace.
cmd flg cl Device File Completion-Context
[ 0, 0] 0 0 00000000 00000000 00000000-00000000
Args: 00000000 00000000 00000000 00000000
>[ 4,34] 1c e0 fffffa80920c0060 00000000 fffff880010301b0-00000000 Success Error Cancel <-- What are you trying to tell me?
Args: 00001000 00000000 14625000 00000000
[ 4, 0] 1c e0 fffffa80920c0b90 00000000 fffff88001063010-fffffa80b01eec00 Success Error Cancel
Args: 3993d69568d 00000000 14625000 00000000
[ 4, 0] c e0 fffffa80b01eeab0 00000000 fffff88001d59150-fffffa80d05b2180 Success Error Cancel
Args: 00001000 00000000 3993d69568a 00000000
[ 4, 0] c e1 fffffa80d05b2030 00000000 fffff88001845344-fffff8800d52bb38 Success Error Cancel pending
Args: 00001000 00000000 14615000 00000000
[ 4, 0] 0 0 fffffa80c2829030 fffffa80d04f18a0 00000000-00000000
Args: 00001000 00000000 00002000 00000000
In the example below we see the “Success Error Cancel” fields and in this case we can also see that the pending flag has been set. The pending field indicates that STATUS_PENDING was returned to the caller. This is used so that I/O completion can determine whether or not to fully complete the I/O operation requested by the packet. Drivers can do this by calling IoMarkIrpPending.
Now look at the "cl" column as it holds the key to unlocking what "Success Error Cancel pending" really means.
1: kd> !irp 0xfffffa80`920c2340
Irp is active with 3 stacks 3 is current (= 0xfffffa80920c24a0) <--- _IO_STACK_LOCATION
Mdl=fffffa814e9c4f40: No System Buffer: Thread 00000000: Irp stack trace.
[ f, 0] 1c 0 fffffa809209a060 00000000 fffff880010061a0-fffffa80920bfcc0
Args: fffffa80920c22b0 00000000 00000000 fffffa80920bfcc0
>[ f, 0] 1c e1 fffffa809209a060 00000000 fffff88001a01a00-fffffa80920c2190 Success Error Cancel pending
Args: fffffa80920c22b0 00000000 00000000 fffffa80920bfcc0
Focusing on the cl column we see that our active stack frame is working in MPIO, but what is it trying to tell us about its status and intent? Well to figure that out we need to break down the value "e1" we see listed in our cl or Control column. This field is being used as a flag where the high and low parts represent two different values. We get this value in !irp from an _IO_STACK_LOCATION Control member. This can be found by dumping the stack location displayed by !irp.
1: kd> dt_IO_STACK_LOCATION 0xfffffa80920c24a0
+0x000 MajorFunction : 0xf ''
+0x001 MinorFunction : 0 ''
+0x002 Flags : 0x1c ''
+0x003 Control : 0xe1 '' < -- Control flag
+0x008 Parameters : <unnamed-tag>
+0x028 DeviceObject : 0xfffffa80`9209a060 _DEVICE_OBJECT
+0x030 FileObject : (null)
+0x038 CompletionRoutine : 0xfffff880`01a01a00 long CLASSPNP!TransferPktComplete+0
+0x040 Context : 0xfffffa80`920c2190 Void
So now that we know where we get this value we still have to decode what it means and how this value results in what we see in our !IRP output. Let's split our number into two, our high and low parts. In our example above we had the value e1. Taking our value apart gives us an E and a 1. The first bit of our value will indicates the last returned value as our IRP was processed. In our case this is a 1, meaning that our IRP is Pending. Thus we see the word pending at the end of current stack frame in !irp. There are three possible values for this bit, 0, 1 or 2. 0 being nothing, 1 being pending and 2 being Error Returned.
Moving to our high bits we are left with the "e". Lets take this number and convert it to binary. We end up with 1110. This upper number indicates which invoke types were requested for the completion routine for the driver listed on that stack frame. If we look at MSDN we see that IoSetCompletionRoutine takes in three bool values to set this flag. These options specify whether the completion routine is called if the IRP is completed with that corresponding status.
_In_ PIRP Irp,
_In_opt_ PIO_COMPLETION_ROUTINE CompletionRoutine,
_In_opt_ PVOID Context,
_In_ BOOLEAN InvokeOnSuccess,
_In_ BOOLEAN InvokeOnError,
_In_ BOOLEAN InvokeOnCancel
Doing some source review I was able to pin together how these values translate to the !irp output. If we look back at our binary value of "e" we see that we have a set of bits that get set based on the what the driver wanted to do when an IRP was completed with one of our defined status values.
Cancel = 2
Success = 4
Error = 8
Add each of these values up and the sum is 14 or e in hex. Going back to our binary "1110" we see that our first bit is set to zero or off. This would be the pending and Error Returned values that I mentioned above. The next three bits represent the corresponding Cancel, Success and Error bool values passed to the driver at the time IoSetCompletionRoutine was called for this stack location.
Example : IoSetCompletionRoutine(pirp, pCompletionRoutine,pContex, True,False,True); would yield a value of 6.
Remember the important thing here is not that the !irp output is trying to tell us that one of these things happened. It's telling us that this driver would like to be notified if one of those things does happen. This area also provides us with information if pending or or an error is returned.
Well that's all I have for now, for more documentation for the _IRP and _IO_STACK_LOCATION structures please see the following links to MSDN.
More on IRP: http://msdn.microsoft.com/en-us/library/windows/hardware/ff550694(v=vs.85).aspx
More on _IO_STACK_LOCATION: http://msdn.microsoft.com/en-us/library/windows/hardware/ff550659(v=vs.85).aspx
In Part 4 we narrowed the source of the leaked pool memory to the specific driver which is allocating it, and we identified where in the driver this allocation was taking place. However, we did not capture contextual information such as the call stack leading up to this code. Also, we didn’t capture information about when this allocated pool is freed. In this article we will use the PoolHitTag feature to break into the debugger when a specific tag is used.
As in Part 4, a live debug must be configured to use this feature. The debugging tools have instructions in the debugger.chm help file, under Debugging Tools for Windows\Debuggers\Installation and Setup\Kernel-Mode Setup (see screenshot in part 4).
These steps are typically only effective if you are able to perform them while the leak is happening. There may be a scenario in which a developer wants to know what “normal” looks like, but most often the steps in this article are used to investigate “broken”.
The PoolHitTag is a global in the kernel binary. When this global is set to a pool tag, the system will break into the debugger whenever pool with this tag is allocated or freed. By default the PoolHitTag is set to ffffff0f.
1: kd> dc nt!PoolHitTag l1
fffff800`016530fc ffffff0f ....
To turn on this feature, edit the PoolHitTag to the tag that is known to leak. The value 3261654c is little endian ASCII for the string ‘Lea2’. I found this value in the “Confirm your edits” step in Part 4.
1: kd> ed nt!PoolHitTag 3261654c
fffff800`016530fc 3261654c Lea2
With PoolHitTag now set to the leaking tag, issue the ‘g’ command to release debugger and it will automatically break in whenever the Lea2 tag is used.
1: kd> g
Break instruction exception - code 80000003 (first chance)
nt! ?? ::FNODOBFM::`string'+0x24a2a:
fffff800`014798f6 cc int 3
In the above example the debugger broke in because the ‘int 3’ instruction triggered a breakpoint. The symbols seem to indicate that we are in a function named “?? ::FNODOBFM::`string'”, but this is simply a lack of symbolic information for this optimized code. Unassembling the surrounding code shows that this code is a piece of ExpAllocateBigPool, one of the functions used in the kernel to allocate pool allocations larger than 4096 bytes.
1: kd> u fffff800`014798f6
fffff800`014798f7 e9ee8e0800 jmp nt!ExpAllocateBigPool+0x13a (fffff800`015027ea)
At this point we can dump the call stack and see the full context of what is happening when this memory is allocated.
1: kd> k
fffff880`04ec1680 fffff800`0161090e nt! ??::FNODOBFM::`string'+0x24a2a
fffff880`04ec1770 fffff880`0496e634 nt!ExAllocatePoolWithTag+0x82e
fffff880`04ec1860 fffff880`0496e727 myfault+0x1634
fffff880`04ec19b0 fffff800`017fca97 myfault+0x1727
fffff880`04ec1a10 fffff800`017fd2f6 nt!IopXxxControlFile+0x607
fffff880`04ec1b40 fffff800`014e0ed3 nt!NtDeviceIoControlFile+0x56
fffff880`04ec1bb0 00000000`7756138a nt!KiSystemServiceCopyEnd+0x13
00000000`000df4c8 000007fe`fd5fa249 ntdll!ZwDeviceIoControlFile+0xa
00000000`000df4d0 00000000`7740683f KERNELBASE!DeviceIoControl+0x75
00000000`000df540 00000000`ff222384 kernel32!DeviceIoControlImplementation+0x7f
00000000`000df590 00000000`00000000 NotMyfault+0x2384
Repeating the ‘g’ and ‘k’ commands multiple times will begin to give you an understanding of the various ways this code may be used. This can be automated by modifying the ‘int 3’ instruction and using a breakpoint. Note that system performance may suffer because output to the debug port is serialized.
The commands shown below use addresses specific to big pool allocations (larger than 4KB). The ‘int 3’ instruction may be located elsewhere depending on the scenario you are debugging.
To modify the operation from a debug break to a breakpoint, change the ‘int 3’ to ‘nop’. In x86 and x64 the opcode for ‘nop’ is 90. Coincidentally these instructions are the same length.
1: kd> eb fffff800`014798f6 90
Confirm that the instruction was reset properly.
1: kd> u fffff800`014798f6 l1
fffff800`014798f6 90 nop
Set a breakpoint on the ‘nop’ instruction and configure the breakpoint to automatically dump the stack and go the debugger.
1: kd> bp fffff800`014798f6 "k;g"
If you find that the pool is sometimes allocated and occasionally freed, you may need to edit the ‘int 3’ used when ExFreePool is called, and set a similar breakpoint on that address.
fffff800`0160f5b7 cc int 3
1: kd> eb fffff800`0160f5b7 90
1: kd> bp fffff800`0160f5b7 "k;g"
Once you have sufficient data to understand the scenario where the memory is allocated and freed use Ctrl+Break to break into the debugger, clear the breakpoints and reset the PoolHitTag. Then go the debugger to allow the system to continue normal operation.
1: kd> bc *
1: kd> ed nt!PoolHitTag ffffff0f
The data collected with these steps should provide an indication to a developer of what memory is being leaked and when.
PoolHitTag isn’t the only option for collecting call stack information. Our final articles will cover alternative techniques for obtaining this information.
In our previous articles we discussed various techniques for identifying a pool memory leak and narrowing the scope of the leak to an individual pool tag. Knowing the leaking pool tag is often sufficient to identify the cause of the problem and find a solution. However, there may be a scenario where multiple drivers use the same pool tag (such as DDK) or when one driver uses the same tag in multiple places. In this scenario you will need more information to identify the source of the leak. In our next several articles we will present techniques to get this information.
This article will present a basic technique where we modify each pool tag to identify what code in which driver is allocating the memory that gets leaked.
This technique requires a live debug of the problematic system. There are many resources with steps for how to configure a system for a live debug. The debugging tools have instructions in the debugger.chm help file, under Debugging Tools for Windows\Debuggers\Installation and Setup\Kernel-Mode Setup.
Using the same technique as in Part 3, identify where the tag in question is used.
0: kd> !for_each_module s -a @#Base @#End "Leak"
fffff880`0496e3aa 4c 65 61 6b 3b c1 0f 42-c1 41 8d 49 fd 8b d0 ff Leak;..B.A.I....
fffff880`0496e621 4c 65 61 6b 3b c1 0f 42-c1 33 c9 8b d0 ff 15 cc Leak;..B.3......
Next, edit each instance so that they are unique. The ASCII code for numeral 1 is 0x31, and the codes for each numeral increase sequentially. Using this information edit each tag to be Lea1, Lea2, etc.
0: kd> eb fffff880`0496e3aa+3 31
0: kd> eb fffff880`0496e621+3 32
Confirm your edits resulted in the expected tags using the dc command.
0: kd> dc fffff880`0496e3aa l1
fffff880`0496e3aa 3161654c Lea1
0: kd> dc fffff880`0496e621 l1
fffff880`0496e621 3261654c Lea2
Now wait for the leak to happen and repeat the steps from Part 3 to identify which of the tags is leaked. This tells you what code allocates the memory that gets leaked. Below we can see that Lea2 is the tag being leaked.
0: kd> !poolused /t5 2
Sorting by NonPaged Pool Consumed
Tag Allocs Used Allocs Used
Lea2 257 263168000 0 0 UNKNOWN pooltag 'Lea2', please update pooltag.txt
nVsC 664 1531552 0 0 UNKNOWN pooltag 'nVsC', please update pooltag.txt
netv 4369 1172224 1 144 UNKNOWN pooltag 'netv', please update pooltag.txt
Leak 1 1024000 0 0 UNKNOWN pooltag 'Leak', please update pooltag.txt
EtwB 94 945136 4 163840 Etw Buffer, Binary: nt!etw
TOTAL 41296 281814544 44077 68102368
Knowing what code allocates the leaked pool may be very valuable to a driver developer who needs to narrow the scope of the problem. Often this information is sufficient for a developer to code review the use of this memory and identify why it would be leaked.
There are times when more information is needed to determine the cause of the leak. A developer may need the call stacks of memory being allocated and freed. We will capture this information in Part 5.