Re: kern/59924: NetBSD improperly does not update FAT0 on FAT12 MSDOS filesystem

[Michael Cheponis <michael.cheponis%gmail.com@localhost>]

I have figured out how to succeed with my use case, which is, indeed, to
use the raw floppy (via e.g. /dev/rsd3a).

And, if one does 'mount_msdos /dev/rsd3a /a[' and then 'disklabel -W sd3a'
  then... standard reads / writes to files, deletion, creation, all work
properly.

The PROBLEM, as I see it, is: mount_mdsos needs to, effectively, ALSO do
"disklabel -W <disk>" because otherwise, other parts of the kernel protect
some sectors on cylinder 0 (prevent writes to it) because on other
filesystem types, that's the superblock etc.  In the case of MS-DOS,
however, you must keep the two FATs synchronized (they are both in cylinder
0 on an actual 1.44 floppy).  FAT 0 is not updated unless 'disklabel -W
<disk>' is performed first.

I created two small shell scripts

msmount:

echo example: msmount sd3a a
# this takes -x bit off msdos files
mount_msdos -m 0644 -M 0755 /dev/$1 /$2
disklabel -W $1

and the corresponding 'msumount' :

echo 'example: msumount  sd3a a'
umount /$2
disklabel -N $1

This fixes the problem.

Thanks everybody,
Mike



On Sun, Jan 18, 2026 at 1:45 AM Michael van Elst via gnats <
gnats-admin%netbsd.org@localhost> wrote:

> The following reply was made to PR kern/59924; it has been noted by GNATS