linux-kernel - Re: udf: Prevent write-unsupported filesystem to be remounted read-write

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Message-ID: <20190122132228.GF13149@quack2.suse.cz>
Date:   Tue, 22 Jan 2019 14:22:28 +0100
From:   Jan Kara <jack@...e.cz>
To:     Kevin Weidemann <kwe-lnx@...tn.eu>
Cc:     Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Pali Rohár <pali.rohar@...il.com>
Subject: Re: udf: Prevent write-unsupported filesystem to be remounted
 read-write

Hello,

On Mon 14-01-19 11:30:11, Jan Kara wrote:
> On Mon 14-01-19 01:33:30, Kevin Weidemann wrote:
> > this is RE the patch 8515b9edf7a0c9dcf1ad218ccc783700db217336 (Upstream
> > commit a9ad01bc759df79b0012f43ee52164391e31cd96) in 4.18.20.
> > 
> > I have an issue with UDF. I used to be able to create a UDF fs in a way
> > that is very similar to this:
> > 
> > `# mkudffs --label=.... -m dvd -b 512 /dev/mapper/cryptdev1`
> > 
> > I used to proceed mounting it, noticing it got mounted as readonly
> > (because the accesstype of the udf fs is readonly (as decided per -m
> > dvd)), so I remounted it as rw and then was able to fill it with data.[1]
> > 
> > Now, this doesn't work anymore since the last time I did that. I figured
> > out it must have to do with the commit mentioned above.  All I get now,
> > during a remount-as-rw attempt, is:
> > `cannot remount /dev/mapper/cryptdev1 read-write, is write-protected`.
> > 
> > I am not sure if this counts as a regression, because I see the point in
> > not only auto-mounting such filesystems as readonly, but also preventing
> > remounting as rw, however it breaks my use case.
> > 
> > However, I noticed the following, too:
> > case A) when having a "readonly udf" on a readwrite device, the
> > filesystem mounts as readonly (old behavior) and also prevents remounting
> > as rw (new behavior)
> 
> This works as intended.
> 
> > case B) when having a "readwrite udf" on a readonly device, the
> > filesystem mounts as readwrite (!), but the writes end up being invisibly
> > discarded
> 
> That's a bug. I'll fix it. Thanks for reporting this!

I have checked this and it works for me as designed:
(none):~# mount -t udf /dev/ubdb /mnt
mount: block device /dev/ubdb is write-protected, mounting read-only
UDF-fs: warning (device ubdb): udf_load_vrs: No anchor found
UDF-fs: Scanning with blocksize 512 failed
UDF-fs: warning (device ubdb): udf_load_vrs: No anchor found
UDF-fs: Scanning with blocksize 1024 failed
UDF-fs: INFO Mounting volume 'LinuxUDF', timestamp 2017/01/20 10:17 (1000)

So if the block device is read-only, you will get only read only mount.
Also remount read-write gets prevented:

(none):~# mount -o remount,rw /mnt/
mount: block device /dev/ubdb is write-protected, mounting read-only

Or with more recent util-linux (2.31.1):
(none):/ # mount -t udf /dev/ubdb /mnt
UDF-fs: warning (device ubdb): udf_load_vrs: No anchor found
UDF-fs: Scanning with blocksize 512 failed
UDF-fs: warning (device ubdb): udf_load_vrs: No anchor found
UDF-fs: Scanning with blocksize 1024 failed
UDF-fs: INFO Mounting volume 'LinuxUDF', timestamp 2017/01/20 10:17 (1000)
mount: /mnt: WARNING: device write-protected, mounted read-only.

and remount:
(none):/ # mount -o remount,rw /mnt
mount: /mnt: cannot remount /dev/ubdb read-write, is write-protected.

The kernel is 5.0-rc3. What kernel were you testing with?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR