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
Breakpoint 0 hit
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.