INFO: MmMapLockedPages Returns Actual Virtual Address In SP4

MORE INFORMATION

In Windows NT 4.0 Service Pack 3 and earlier versions, if the buffer described by the MDL is not page-aligned then you have to add the offset stored in the MDL with the base virtual address returned by the function to get the actual address as shown here:

VirtualAddress = (PVOID) (((ULONG) MmMapLockedPages(Mdl, Mode)) +  MmGetMdlByteOffset(Mdl));
				

In Service Pack 4 and later versions, this function has been altered to consider the byte offset stored in the MDL and returns the actual virtual address as shown here:

VirtualAddress = (PVOID) MmMapLockedPages(Mdl, Mode);
				

To fix the above mentioned inconsistencies and make the code portable across all versions of Windows NT, use the following:

VirtualAddress = (PVOID) (((ULONG) MmMapLockedPages(Mdl, Mode)) | MmGetMdlByteOffset(Mdl));
				

or

VirtualAddress = (PVOID)(((ULONG)PAGE_ALIGN(MmMapLockedPages(Mdl, Mode))) + MmGetMdlByteOffset(Mdl));
				

Microsoft has confirmed the pre-Service Pack 4 behavior of this function to be a bug and has fixed it in order to resolve memory addressing issues on future releases of Windows NT; Windows NT 64-bit in particular.