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

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Message-ID: <20190115083111.qq2mt2p2kn4opwx7@pali>
Date:   Tue, 15 Jan 2019 09:31:11 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Michael Sabolish <sabolish@...com>
Cc:     Jan Kara <jack@...e.cz>, 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 Monday 14 January 2019 19:07:35 Michael Sabolish wrote:
> I can try and make a pull-request for udftune, and I can just copy the API for tune2fs.  It would work something like:
> 
> udftune -O read-only device            (to set read-only access type)
> 
> or:
> 
> udftune -O ^read-only device          (to clear read-only access type (aka set rw))

This API is ambiguous. What does it mean for ^read-only? In UDF you have
following access types: overwritable, rewritable, writeonce, readonly,
pseudo-overwritable, unknown.

So you would need to know to which R/W access type to switch
(overwritable, rewritable, writeonce or pseudo-overwritable).

With information of media type, you could be able to guess correct
access type. But for UDF images stored in VFS there is no media
information. Also you can have uncommon setup, e.g. usage of CD-R
writeonce setup on CD-R/W disc. So "autodetection" of media type would
not work always correctly.

So I think that it would be better to have following API:

  udftune --access-type=<new_access_type>

or

  udftune --change-access-type=<new_access_type>

I understand that you would like to have similar API as tune2fs, but UDF
settings are too different from ext*.

-- 
Pali Rohár
pali.rohar@...il.com