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Exercise 2: Solving UIPI Issues

Task 1 – Explore the Ping Pong Sample

In this task, you will investigate the Visual Studio solution to understand what the code does.

  1. Double-click the PingPongBroken solution

    Note:
    Help

    Visual Studio 2008 should start and load the solution. The solution contains four projects: two native and two managed. Each pair is the Ping Pong application itself and a convenient loader that starts one process with standard user privileges and the other with administrator privileges.

  2. Open the PingPong.cpp fileThe PingPongDlgProc function is the modal dialog procedure handling the bulk of the work. To coordinate properly, the RegisterWindowsMessage function is used to obtain a global (technically, globally to the process’ Windows Station) message id for inter-process communication. The resulting message, uMsgBall, is the one posted between windows of the two running processes.
  3. The WinMain function searches for another instance of the same executable by looking for the existence of another window with the same caption (using the FindWindow function)

  4. A timer is used to wait 500 msec in between message passing

Task 2 – Fix the Code

The PostMessage function used for message passing fails if its intended message is to a window belonging to a process with a higher privilege level. However, we can use a message filter to let some messages through:

  1. After the message registers in the WM_INITDIALOG handler, use the ChangeWindowFilter function to let the registered message pass through, regardless of privilege levels.

  2. Add a call to that function with the MSGFLT_ADD flag to add the message to the filter. The code should look like the following:uMsgBall = RegisterWindowMessage(sMessageBall);
    if(!uMsgBall) return FALSE;

    // allow our message to come in even if sent by lower privilege
    // process
    ChangeWindowMessageFilter(uMsgBall, MSGFLT_ADD);

  3. Build the project and test it with NativePingPongLoader.Exe. It should work as expected. The complete solution is in the PingPongFixed folder.

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