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

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Message-ID: <20190114120023.wkftfz6pwatehpfe@pali>
Date:   Mon, 14 Jan 2019 13:00:23 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     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

Hello!

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.

-- 
Pali Rohár
pali.rohar@...il.com