PRB: Windows 3.1 SDK HOOKS Sample Causes a GP Fault

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

SYMPTOMS

An application that installs a WH_CBT hook function that is run along with the HOOKS sample application will cause a general protection (GP) fault in the system if the computer-based training (CBT) hook is enabled and the hook procedure receives a CBT notification.

CAUSE

In the HOOKS sample, CBTFunc() is incorrectly casting lParam (a DWORD value) to a WORD when it calls CallNextHookEx() to pass the info on to the next hook in the hook chain. This causes the receiving hook function to receive the CBT notification with a modified lParam (the HIWORD stripped off), thus translating to an invalid pointer and eventually causing a GP fault.

A GP fault occurs similarly when an application that installs a WH_CALLWNDPROC hook is run along with the HOOKS sample and the CallWndProc hook menu-item is selected. In the same manner, the HOOKS sample function CallWndProcFunc() incorrectly casts its lParam (a DWORD value) to a WORD on its call to CallNextHookEx().

In addition, the HOOKs sample will GP Fault when the WH_JOURNALRECORD hook is installed and some other application calls SetSysModalWindow(). As mentioned in the Windows 3.1 SDK help,

   If a WH_JOURNALRECORD hook is in place when SetSysModalWindow is called,
   the hook is called with a hook code of HC_SYSMODALON (for turning on the
   system-modal window) or HC_SYSMODALOFF (for turning off the system-modal
   window).

RESOLUTION

Modify both the CBTFunc() and CallWndProcFunc() calls to CallNextHookEx() to correctly cast lParam to a DWORD.

In the case of the journal record hook, make the following change to the function JournalRecordFunc in hooksdll.c:

   if( nCode >= 0) {
   // do not  record if SysModalOn or SysModalOff and let other filter know
   // about it.
      if(nCode == HC_SYSMODALON || nCode == HC_SYSMODALOFF)
      {
      // let other hook filters know about this
         CallNextHookEx(hhookHooks[JOURNALRECORDINDEX], nCode, wParam,
            lParam);
         return;
      }