#121 ioctl functions 08/48 41/61 address the wrong sector

[H. Peter Anvin]

On FreeDOS kernel 2040, ioctl (INT 21, AX=440D) subfunctions CH=08/48 CL=41/61 address the sector relative to the beginning of the filesystem/partition.

On MS-DOS 6.22 the same functions address the sector relative to the beginning of the device, so that head=0 cylinder=0 sector=0 refers to the MBR.

[Anthony Williams]

What exposed this, SysLinux (Memdisk)?

The latest stable kernel is 2041. While I doubt it will help, you could also consider trying to reproduce this bug with that.

http://sourceforge.net/projects/freedos/files/Kernel/2041/

If you want to try latest unreleased kernel, check here (and also consider emailing him directly):

https://github.com/PerditionC/fdkernel

If you want a recent binary snapshot (and don't want to build your own with OpenWatcom), try here:

http://www.fdos.org/kernel/testing/git/

[Kenneth Davis]

Bug is confirmed in FD kernel, working on a fix.

[Kenneth Davis]

I still need to do further testing and potential simplification/cleanup but a fixed version is now available for further testing.

http://www.fdos.org/kernel/testing/git/KERNEL.SYS
with changes from source on Github in http://www.fdos.org/kernel/testing/git/absread.diff

[H. Peter Anvin]

This was, indeed, exposed by the Syslinux installer, in particular the -m option (write MBR).

[H. Peter Anvin]

Once this is fixed, it would be great to know if there is a way to detect the faulty FreeDOS versions so we can print an error message rather than overwriting someone's filesystem, or apply a workaround if one exists.

[Kenneth Davis]

Int 0x21 function 0x30 get DOS version, if BH=0xFD and BL < 42 then will use partition relative reads/writes (faulty versions).