IAMGraphBuilderCallback and Intelligent Connect tuning

There was a question asked about how IAMGraphBuilderCallback interface is used to prevent from particular filter insertion during Intelligent Connect.

First of all, there is sample code at The March Hare‘s website:

The IAMGraphBuilderCallback class can be used to remove problematic dshow filters when building a graph with intelligent connect.  This sample does not provide the UI to blacklist filters but will give you a better head start than the information in the documentation.  See the header file for an example of how to use it in your code.  Add your own code to the SelectedFilter or CreatedFilter methods to remove any unwanted filters.

The idea behind the method can be easily illustrated with a code snippet. Each time Filter Graph Manager considers trying a filter, before its instantiation it calls (if provided) IAMGraphBuilderCallback::SelectedFilter method, where there is an option to reject it (before it even was instantiated using CoCreateInstance).

The code snippet below checks selected filter and instructs to reject it if it is an FFDshow Audio Decoder filter.

// IAMGraphBuilderCallback
STDMETHOD(SelectedFilter)(IMoniker* pMoniker) throw()
    ATLTRACE(atlTraceCOM, 4, _T("...\n"));
        const CStringW sMonikerDisplayName = _FilterGraphHelper::GetMonikerDisplayName(pMoniker, NULL);
        ATLTRACE(atlTraceGeneral, 4, _T("sMonikerDisplayName %ls\n"), sMonikerDisplayName);
        static const LPCTSTR g_pszFfdshowAudioDecoderClassIdentifier = _T("{0F40E1E5-4F79-4988-B1A9-CC98794E6B55}");
        if(sMonikerDisplayName.Find(CStringW(g_pszFfdshowAudioDecoderClassIdentifier)) >= 0)
            return E_FAIL; // Die!
        return Exception;
    return S_OK;

DirectShow Filter Graph Spy on Vista

I have been receiving comments that Filter Graph Spy tool does not work with Microsoft Vista operating system. I never had a moment to check until recently, and this time I realized that it really does not work. I am definitely aware of dramatic changes introduced with this operating system, and in particular UAC feature, virtualization and changes in security. No wonder this was the first guess that security was the cause, however the investigation showed there was a trail of issues underneath…

Investigation details deserve a separate post, while this one briefly outlines the issues and also accompany the repository update with a version compatible with Vista OS.

First of all, COM registration of the DLL (which definitely requires privilege elevation) succeeded on Vista. This means the registration procedure did not encounter any errors on the way, or poorly written code ignored the problem. It appeared that the source of the problem was CoTreatAsClass API, which failed to do the requested action, however returned status code indicating successful operation. This definitely looks like a bug and further comments on this particular behavior are expected to appear on Windows Applications Security MSDN forum, where I opened a topic on the matter.

With a COM TreatAs feature activated, the class’s behavior to instantiate instead of DirectShow’s CLSID_FilterGraph is restored, and in particular the DLL generates FilterGraphSpy.log log file on filter graph activity. Note that log file location is OS dependent (due to Vista’s permissions and file system virtualization):

  • pre-Vista OS: root of syste drive, typically C:\
  • starting Vista, administrator with elevated privileges: CSIDL_COMMON_APPDATA, typically C:\ProgramData (note this directory is hidden by deafult)
  • starting Vista, without elevated administrator privileges: CSIDL_LOCAL_APPDATA, typically C:\Users\$(UserName)\AppData\Local (note that AppData directory is hidden by deafult)

Still even with the log file generated and indicating activation of the spy, it was unable to connect to remote graph through the running object table (ROT). It appeared that the ROT entires are there where expected, it was OK to get an object from ROT and the problem came from QueryInterface code:

CComPtr<IUnknown> pFilterGraphUnknown;
ATLENSURE_SUCCEEDED(pRunningObjectTable->GetObject(pMoniker, &pFilterGraphUnknown));
CComQIPtr<IMyFilterGraph> pFilterGraph = pFilterGraphUnknown; // E_NOINTERFACE

The call reached the original object but COM subsystem was unable to marshal the interface through apartments to enable interprocess communication on it. The reason for this is absence of PSFactoryBuffer class (CLSID {92A3A302-DA7C-4A1F-BA7E-1802BB5D2D02}), which provides proxy/stub pairs for marshaling well known DirectShow interfaces in the Vista’s version of quartz.dll. As mentioned by Microsoft’s Mike Wasson, this class was moved from quartz.dll into Vista SDK’s proppage.dll, so in order to obtain connectivity to remote DirectShow graphs starting Vista, one needs to install this DLL with Windows SDK, or otherwise have it registered with the operating system.

Also note that DirectShow-enabled applications that have their filter graphs published on ROT will need an utility application such as GraphEdit started with the same permissions (elevated or not) in order to be able to access ROT entires.

To sum everything up, to install Alax.Info DirectShow Filter Graph Spy on Vista:

  • get the latest FilterGraphSpy.dll
  • regsvr32 FilterGraphSpy.dll on target system from administrative command prompt, with elevated privileges (note you should have a log file FilterGraphSpy.log generated in CSIDL_COMMON_APPDATA directory, with a few lines indicating registration success)
  • get Windows SDK and make $(WindowsSDK)\Bin\proppage.dll file registered on target system (also administrative regsvr32), note that it is necessary to restart DirectShow-enabled applications and GraphEdit after DLL registration to get graphs visible through ROT
  • mind the log file directories with FilterGraphSpy.log file

A partial Visual C++ .NET 2008 source code is available from SVN, release binary included.

Hint on how to easily find your filter graph on running object table (ROT)

With a lot of DirectShow Filter Graphs published on Running Object Table, especially those automatically published by Filter Graph Spy utility, it might be a bit tricky to locate your graph of interest in the list.

Filter graphs are published with a textual moniker item name of predefined format, which is recognized by GraphEdit or GraphStudio, or similar utilities as an item corresponding to a filter graph. A template for such a string is “FilterGraph %08x pid %08x“, which obviously only contains raw pointer address and process identifier, which only help a bit in looking up for proper graph interactively. However, it is important how exactly applications are recognizing filter graph related names. For example, GraphStudio does it the following way:

while (emon->Next(1, &moniker, &f) == NOERROR) {

    // is this a graph object ?
    LPOLESTR    displayname;
    moniker->GetDisplayName(bindctx, NULL, &displayname);

    CString        name(displayname);
    if (name.Find(_T("!FilterGraph")) == 0) {

The items that match are then listed in GUI with an original string, which means that it is possible to provide an informational suffix to be able to locate the graph in a more convenient way, e.g. with a process image name, not only identifier (“; process: …” was appended to the item name):

GraphStudio's ROT Items

GraphEdit is using another method and is more strict in selecting among available items. AFAIR earlier versions did not allow custom suffixes in item names, however the latest version still picks the items up from the global list. However, GraphEdit does not show graph’s original item name, so suffixes are merely useless with GraphEdit.

GraphEdit ROT Items

Filter Graph Spy utility was updated to automatically append process name suffix, which should be OK for both GrapEdit and GraphStudio. Still, the feature can be disabled through registry DWORD value named “Enable ROT Moniker Item Name Suffix” under HKEY_LOCAL_MACHINE\SOFTWARE\Alax.Info\Utilities. The value of zero, or missing, is the default behavior to enable suffixes. The value of 1 disables the feature, the value of 2 makes sure it is enabled.

static CConstIntegerRegistryValue g_nEnableRotMonikerItemNameSuffix(_T("Enable ROT Moniker Item Name Suffix")); // 0 Default, 1 Disable, 2 Enable
if(g_nEnableRotMonikerItemNameSuffix != 1)
    TCHAR pszPath[MAX_PATH] = { 0 };
    _W(GetModuleFileName(NULL, pszPath, DIM(pszPath)));
    CString sItemName = AtlFormatString(_T("%s; process: %s"), m_RunningFilterGraph.GetDefaultMonikerItemName(GetControllingUnknown()), FindFileName(pszPath));
    m_RunningFilterGraph.SetFilterGraph(GetControllingUnknown(), CStringW(sItemName));
} else

A partial Visual C++ .NET 2008 source code is available from SVN, release binary included.

More on DirectShow Filter Graph Spy

Having DirectShow Filter Graph Spy installed in the system, I noticed a new and weird effect that when Media Player Classic plays a list of files and each MP3 file being currently decoded shows decoder’s system tray icon, a switch to a new file leaves an old icon in place and adds a new icon for a new decoder.  I thought this might be also a decoder’s problem which I also recently updated to a new version, but then I noticed other artifacts, with other software and again with Media Player Classic, that if I drop a new file onto the player and it starts playing it, old file is still being played as if player just releases all referenced to the graph and never calls IMediaControl::Stop to force stop of playback.

It was clear at that point that it is related to filter graph spy and the problem needs a fix. The problem definitely looked like the spy leave outstanding references to the filter graph and when the application releases graph and it is supposed to be automatically destroyed, it is still being alive and possibly, such as in case of Media Player Classic, even running.

The first clue was that being published to running object table (ROT), the object gets more than one additional reference on it and as such it never gets removed from ROT until application terminates as ROT keeps a reference to the graph and there is noone to remove it. By original design spy published filter graph on ROT and removed one reference immediately after that to compensate the ROT’s reference (one only) making it a weak reference. So when all references are released from the filter graph but ROT reference, the graph gets destroyed and in FinalRelease class on the COM object we re-add the reference back and automatically remove the object from ROT as well. The whole scheme works in assumption that ROT adds one reference only, if there are more then the object is referenced by the ROT forever.

Debugging however showed that ROT adds a single reference and there is no problem from this side. However I immediately noticed  another weird behavior in the same method FinalConstruct. An underlying (real) filter graph object, i.e. CFilterGraph from quartz.dll, is adding a reference on the outer COM object when being queried for a private interface from implicit (also known as main, raw) IUnknown when created as aggregated object. And it is not exactly the behavior one would expect from aggregated object because it basically break proper reference counting. I realize that aggregating is a rather rare technique and DirectShow COM base is one of the very first COM bases at all which then stopped being developed, so perhaps this problem was not identified on time and later left unfixed at all, so in present situation there should rather be a workaround.

It is also worth mentioning that there are two scenarios involved. My CSpy object creates DirectShow CFilterGraph as an aggregated object. And CSpy in its turn may be created as aggregated too. For example, Windows Media Player creates (as most applications in this universe do) filter graph object the regular way, as a standalone COM object. However it appears that Media Player Classic creates filter graph as aggregated, for whatever it needs it for. Because of the discovered problem inner CFilterGraph adds an outstanding reference on the top level “controlling IUnknown”, which is either CSpy’s private IUnkown in case of Windows Media Player, or higher level IUnknown in case of Media Player Classic.

So the proper workaround is to release unwanted references from controlling unknown rather than self:

CComPtr<IUnknown> pUnknown;
ATLENSURE_SUCCEEDED(pClassFactory->CreateInstance(pControllingUnknown, __uuidof(IUnknown), (VOID**) &pUnknown));
// NOTE: DirectShow FilterGraph is incorrectly implementing COM aggregation adding outer reference to interfaces queried from private IUnknown
CComQIPtr<IFilterGraph2> pFilterGraph2 = pUnknown;
pFilterGraph2.p->Release(); // <<--------------------------

It looked like the problem is finally worked around but in aggregated creation scenario it brought another problem up. Since instantiation of inner CFilterGraph takes place in FinalConstruct and CSpy is also being instantiated as aggregated, it appears that outer COM object is sensible to manipulation with its reference counter. CFilterGraph added a reference it should not, CSpy released it in compensation but on this early stage the compensating Release might zero reference counter and might initiate unexpected object destruction. It is not necessarily this way as it depends on outer COM object base, but it seems that Media Player Classic does not do DECLARE_PROTECT_FINAL_CONSTRUCT or its equavalent and things go the worst scenario.

CSpy is an inner object and we don’t have any external instance to hold a temporary reference for us while outer COM object is being created, so a workround for this is to temporarily hold a circular reference to an outer object to let it complete its instanatiation and pass another reference higher up to the controlling application. At which point we are going to be safe to release the temporary reference and normalize reference counters.

    const ULONG nReferenceCount = pControllingUnknown.p->Release();
    if(nReferenceCount == 1)
        m_pTemporaryUnknown = pControllingUnknown;

The only thing is left is to release the temporary reference early enough, but we have a filter graph, don’t we? What is typically done with a filter graph, adding a filter right? And it is a controlling application which has its own reference, who adds filters so we are safe to release temporary reference in IFilterGraph::AddFilter or IGraphBuilder::AddSourceFilter (actually it would not hurt to add this in other methods as well):

STDMETHOD(AddFilter)(IBaseFilter* pFilter, LPCWSTR pszName) throw()
    _Z4(atlTraceCOM, 4, _T("pszName \"%s\"\n"), CString(pszName));
    ReleaseTemporaryUnknown(); // <<--------------------------
    return m_pInnerFilterGraph2->AddFilter(pFilter, pszName);

Additionally, the spy also implements now IMediaControl interface and traces calls to log file, it is convenient.

A partial Visual C++ .NET 2008 source code is available from SVN, release binary included.

reg-FilterGraphSpy.dll.bat and unreg-FilterGraphSpy.dll.bat files in SVN are convenient batch files to register and unregister the spy with the operating system.

DirectShow Filter Graph Spy – CLSID_FilterGraphNoThread

At the time of original implementation I intentionally left off a variation of Filter Graph Manager that runs on application thread, CLSID_FilterGraphNoThread. MSDN says:

CLSID_FilterGraphNoThread creates the Filter Graph Manager on the application’s thread. If you use this CLSID, the thread that calls CoCreateInstance must have a message loop that dispatches messages; otherwise, deadlocks can occur. Also, before the application thread exits, it must release the Filter Graph Manager and all graph objects (such as filters, pins, reference clocks, and so forth).

I have never needed to use this CLSID (by the way Windows SDK has one more CLSID – CLSID_FilterGraphPrivateThread – but it does not seem to be documented) and for this reason I left it for the time a need in it comes up. However it appeared that this CLSID was not created just for fun, there was a need in it. Filter graphs of Windows Media Player playing well known Windows’ clock.avi appeared to be not published on Running Object Table. Why?

Process Monitor showed clearly that Windows Media Player created filter graph through CLSID_FilterGraphNoThread and quite obviously it was not intercepted by Filter Graph Spy (still I wonder what made it different previous time when I could see WMP’s graph).

Continue reading →

DirectShow Filter Graph Spy

Inspired by a post on DirectShow Development Forum, it is sometimes useful to find out how certain application is using DirectShow to capture live video and/or audio, or play certain file etc. Filter graph topology, details about media types on the pins, used interfaces and call order. There is quite an easy step for the developer to spy over running filter graphs system wide by connecting Component Object Model (COM) “Treat As” feature with filter graph remoting through Running Object Table and connecting to remote process graphs using GraphEdit utility. Who could imagine Windows Media Player to build such a filter graph for a video and audio Windows Media file? SRS Wow DMO, equalizer, analyzer and stuff? Continue reading →