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

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Date:   Mon, 14 Jan 2019 13:30:42 +0100
From:   Jan Kara <jack@...e.cz>
To:     Pali Rohár <pali.rohar@...il.com>
Cc:     Jan Kara <jack@...e.cz>, Michael Sabolish <sabolish@...com>,
        Kevin Weidemann <kwe-lnx@...tn.eu>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: udf: Prevent write-unsupported filesystem to be remounted
 read-write

On Mon 14-01-19 13:00:23, Pali Rohár wrote:
> On Monday 14 January 2019 11:30:11 Jan Kara wrote:
> > > As for the case of remounting as rw if the UDF is ro but the device is
> > > rw, I am not sure what the best idea is to deal with this.  If this new
> > > behavior doesn't count as a regression, is there any way to end up with a
> > > UDF filesystem as specced by the command above (-m dvd -b 512, so with it
> > > being read-only), but still allow for mounting it as rw if the device
> > > supports it? Does udftools offer a way to manipulate the UDF partition
> > > descriptor flag in a pre-existing filesystem after it had already been
> > > created that I am missing?
> > 
> > So I would really prefer to keep the behavior of disallowing remounting
> > read-only partition read write. After all ECMA-167 standard is pretty clear
> > on this saying that for read-only partitions no sectors can be recorded. I
> > understand it is inconvenient if you try to create e.g. a DVD image. So you
> > want partition to be read-only in the end but initially you need it to be
> > writeable so that you can fill-in the contents.
> > 
> > Generally I think a clean solution for this is to provide a way in udftools
> > to switch partition read-only / read-write. Also this is how similar things
> > are achieved for other filesystems. Pali, is there a way to switch
> > accessType of a partition on existing media with udftools? If not, can you
> > look into implementing that please? It should be rather straightforward,
> > the biggest question really is which tool should do this...
> 
> You are not the first who asked for such functionality in udftools.
> Michael (CCed) already experimented with such thing and "hacked"
> udflabel to switch access type from overwritable to readonly.
> 
> I'm not against adding a new tool into udftools project which can
> manipulate UDF access type. Question is design / API of such tool.
> 
> Currently udftools has udfinfo tool which prints lot of information
> about UDF filesystem (including access type). But the only tool which
> modifies UDF filesystem in udftools is udflabel. And udflabel is not
> really the proper place for changing access type.
> 
> So some new tool for modifying UDF filesystem would be better. Which
> other settings of UDF filesystem could be useful to be modifiable? I'm
> thinking about "udftune" where new features for modification could be
> implemented later too.

Yes, that sounds good to me and is in line with what tools for other
filesystems have.

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