Weirdness of "mount -o remount,rw" with write-protected floppy

[Evgeny Stambulchik <Evgeny.Stambulchik@weizmann.ac.il>]

Hello,

here is what I found the hard way (having lost the configuration of a 
firewall...):

# mount /dev/fd0 /mnt/floppy/
mount: block device /dev/fd0 is write-protected, mounting read-only
# mount -o remount,rw /mnt/floppy
# echo $?
0
# touch /mnt/floppy/blabla
# ls -l  /mnt/floppy/blabla
-rwxr-xr-x  1 root root 0 Oct 27 13:18 /mnt/floppy/blabla*
# umount /mnt/floppy
# echo $?
0
# mount /dev/fd0 /mnt/floppy/
mount: block device /dev/fd0 is write-protected, mounting read-only
# ls -l  /mnt/floppy/blabla
ls: /mnt/floppy/blabla: No such file or directory

The kernel actually tries to write to the floppy - the light is blinking 
(so everything looks nice from outside). Of course, there are kernel 
errors, seen in dmesg like:

 > end_request: I/O error, dev fd0, sector 21
 > Buffer I/O error on device fd0, logical block 21
 > lost page write due to I/O error on fd0

but these don't propagate to the user space in any way.

The bug is present in both 2.4 and 2.6, and is specific to floppy 
devices. Other RO media I tried (CDROM, RO-exported NFS) are partially 
OK, in the sense that a write attempt returns an error; however, "mount 
-o remount,rw" always returns success (this might be a bug in mount).

Regards,

Evgeny