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

[Jon Masters <jonmasters@gmail.com>]

On 10/27/05, Evgeny Stambulchik <Evgeny.Stambulchik@weizmann.ac.il> wrote:

> # mount /dev/fd0 /mnt/floppy/
> mount: block device /dev/fd0 is write-protected, mounting read-only
> # mount -o remount,rw /mnt/floppy
> # echo $?
> 0

Ok. The problem is in the remounting. Both cases rely on the mount
syscall doing the work and the "fault" is that it is returning
successfully in both cases.

When remounting, Linux /does/ check if the corresponding block device
is read-only and won't remount rw onto that (so if the permissions of
the bdev corresponded to the real state of the floppy then all would
be good) but it will if the block dev is writeable but the device is
not. There isn't a generic VFS way to ask if a backing device is
writeable (or do_remount_sb would be using it) - or is there?

The /only/ way I can see to "fix" this is to do a pointless open on
the block device and see if that returns EROFS before allowing a
remount. But I don't know what other hassle that will cause - I'll
make the hack, but someone (Al?) who knows the code will need to
comment because this might completely fuck up a lock somewhere.

Jon.