PRB: INT 2Fh 1684h May Return ES == 0 and DI != 0

• Microsoft Windows Device Driver Kit (DDK) for Windows version 3.1

SYMPTOMS

A DLL that calls INT 2Fh function 1684h (Get Device API Entry Point) to communicate with a VxD may receive ES == 0 and DI != 0. This is contrary to the documentation, which indicates that ES and DI will be zero on failure -- meaning if either is non-zero, the function succeeded. However, zero is not a valid code selector, so attempting to call the returned address results in a general protection (GP) fault.

CAUSE

This function uses the VMM service Allocate_PM_Call_Back to obtain an address for the protected-mode DLL to call. Under low memory conditions, it is possible for this service to be unable to create a valid code selector. Internally in VMM, the resulting code selector value is not checked, resulting in it blindly returning a code selector value of zero to the caller of Allocate_PM_Call_Back. In turn, an invalid callback address is returned to the DLL with ES containing zero.

RESOLUTION

Code calling INT 1Fh function 1684h should check to see if ES contains zero on return. If ES is zero and DI is zero, the VxD is not loaded or does not export a protected mode entry point. If ES is zero and DI is non-zero, the VxD is loaded and exports a protected mode entry point, but the entry point address cannot be obtained due to low memory conditions.