Hooking Disk Access

The disk-hooking sample is a set of source files (and the built executables) supplied at present with no comment other than what you see here and in the source code. The sources build four test rigs. These are:

  1. A Windows 95 VSD called DISKHOOK.VXD.

    It hooks disk access underneath IOS.VXD on Windows 95 and higher. It loads automatically if placed in the IOSUBSYS subdirectory of the Windows SYSTEM directory.

  2. A Windows for Workgroups 3.11 VSD called DISKHOOK.D32.

    It hooks disk access underneath IOS.386 on Windows for Workgroups 3.11 only. It loads automatically if placed in the SYSTEM subdirectory of the Windows installation.

  3. A VxD called DISKHOOK.386.

    It hooks disk access between BLOCKDEV.386 and the FastDisk driver (typically WDCTRL) or between IOS.386 or IOS.VXD and the port driver. This VxD is to be loaded via a device statement in SYSTEM.INI. It is intended for Windows 3.10 and higher, but a VSD is clearly a superior solution in later versions.

  4. A DOS program called DISKHOOK.EXE.

    It hooks int 13h and then runs a command processor. Running the disk hooking program before running Windows gives you a global int 13h hook. Running the disk hooking program from a DOS Prompt inside Windows gives you a local int 13h hook.

The action of each test rig is to modify the appearance of the first sector on cylinder 0 head 1 of the first physical hard disk, a preset location having been chosen so that the sample can concentrate mostly on the mechanics of intercepting disk access. The particular location chosen for the sample usually stores a DOS boot sector. If this is not true for your machine, do not run these test rigs. The specific change to the appearance is to find the first occurrence of the word “disk” in the sector (this word being very likely to appear in an error message somewhere in the sector) and to toggle the case of some of the letters. The DOS test rig toggles the case of the first two letters. The VxD test rigs toggle the case of the last two letters.

All VxD source code is written in assembly language using include files from the Windows 95 DDK. For detailed expectations read the MAKEFILE in each directory.

The Windows 95 and Windows for Workgroups 3.11 VSDs have the same source code (although they must be compiled with different include files). Two of the source files involve small amounts of conditional assembly. One reason is trivial, namely that the different VSD versions have to check for their respective versions of the IOS. Another reason is that only the Windows 95 VSD needs to support the AEP_BOOT_COMPLETE function.

Although the VSD source code is presented separately from the source code for the DISKHOOK.386 version that works for Windows 3.10, it should be noted that there is a strong correspondence between source files and procedures. Indeed, the two versions of the PLAY.ASM source file can be matched very nearly line by line. Port drivers supersede FastDisk drivers. The IOS supersedes BlockDev. VSDs, which sit between the IOS and a port driver are a very natural development of the sort of VxD that sits between BlockDev and a FastDisk driver.

The trouble with that progression is that Microsoft didn’t document it very much. Indeed, I expect that hooking 32-bit disk access underneath BlockDev on Windows 3.10 and higher was very rare in commercial software. Nothing undocumented is required when hooking VxD services to bend them to your will, but you need a sense of adventure even to see the possibility, before needing a good understanding of what you’re hooking into. The replacement of BlockDev by IOS formalised the notion of passing disk I/O through a sequence of drivers on the way to one that actually controls the hardware. For many VxD programmers, this will have been the first opportunity for hooking disk I/O inside Windows. Yet to my knowledge the existence of this support before Windows 95 never was documented by Microsoft.

For distribution, the binaries and source code are compressed into a zip file: click here to download the Hooking Disk Access sample.

[Note added 29th July 2015] The preceding link breaks from time to time when this site's hosting provider, Fluccs, quarantines the target file as some sort of security threat. Curiously, what their virus scanner, CXS, objects to is not an infected executable in the zip file. It instead dislikes the C and/or C++ source files, citing them as a "Suspicious file type [application/x-c]". Make of that what you will.