Issues trying to display a configuration window for my screensaver (from a GUI app running with high mandatory integrity level)

Question

I have an old project that comes with its own screensaver. The GUI application for the project has an option to "Configure screensaver" that should bring up my screensaver configuration window (that you would normally get from a Control Panel -> Display -> Personalization -> Screensaver -> Settings.)

To display the configuration window, one needs to call the screensaver process with the /c parameter as described here. The screensaver itself is placed into the system directory, i.e. "C:\Windows\System32".

So to emulate this from my (32-bit) GUI application I do the following:

//Error checks are omitted for brevity
BOOL bResult = FALSE;

TCHAR buffSysFldr[MAX_PATH];
buffSysFldr[0] = 0;
::GetSystemDirectory(buffSysFldr, SIZEOF(buffSysFldr));

//Make the path, which basically becomes:
//  "C:\Windows\System32\mysvr.scr" /c
TCHAR buff[MAX_PATH];
buff[0] = 0;
::StringCbPrintf(buff, sizeof(buff), L"\"%s\\mysvr.scr\" /c", buffSysFldr);

STARTUPINFO si = {0};
PROCESS_INFORMATION pi = {0};
ZeroMemory(&si,sizeof(si));
ZeroMemory(&pi,sizeof(pi));
si.cb = sizeof(si);

PVOID OldValue;
Wow64DisableWow64FsRedirection(&OldValue);

//And run it
if(CreateProcess(NULL, buff, NULL, NULL, FALSE, 
    NORMAL_PRIORITY_CLASS | CREATE_NEW_CONSOLE, 
    NULL, NULL, &si, &pi))
{
    //Success
    bResult = TRUE;
}

Wow64RevertWow64FsRedirection(OldValue);

CloseHandle(pi.hProcess);
CloseHandle(pi.hThread);

When I deploy it to the Windows 7 machine, the code above returns bResult = TRUE but my screensaver's configuration window is never shown.

I tried it on both 32-bit and 64-bit Windows 7 thinking that Wow64 redirection has something to do with it, but it still didn't change the outcome.

Any idea why this doesn't work?

PS. When I copy and paste that command into a command prompt it runs just fine and displays my configuration window w/o a problem:

EDIT: After further review, it turns out that the issue seems to be connected to the fact that my GUI applications runs with High Mandatory Integrity Level, or S-1-16-12288, which seems to be the reason why the screensaver's configuration window is not shown. If I run my GUI app with its regular Medium integrity level, or S-1-16-8192, the configuration window shows OK.

Otherwise, I can see that my screensaver's RegisterDialogClasses method is called OK, but then when I return TRUE from it, ScreenSaverConfigureDialog is never called.

Since integrity level was introduced to prevent processes with lower levels to send messages to processes with higher levels, evidently the screensaver configuration mechanism tries to send some messages to my GUI app (for whatever undocumented reason) and fails and then silently fails as well ...

So this is the latest. I'll appreciate any input on this from whoever knows how to solve this (apart from lowering the integrity level of my GUI app, that I obviously don't want to do.)

Answer 1

God damn Microsoft documentation (sorry, I just wasted the whole day today trying to fix it.)

For whoever else runs into this -- it turns out one needs to call it as such:

"C:\Windows\System32\mysvr.scr" /c:N

where N is a window handle of the parent window expressed as an integer. Found it out by checking command line of the configuration window displayed via Control Panel (did it using Process Explorer.)

In my case, since my GUI process runs with higher integrity level, doing the following will suffice:

HWND hWndToUse = ::GetDesktopWindow();
::StringCbPrintf(buff, sizeof(buff), L"\"%s\\mysvr.scr\" /c:%d", buffSysFldr, (int)hWndToUse);

And that's it!