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

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Message-ID: <20190115084832.fypbuhyvgfizh3am@pali>
Date:   Tue, 15 Jan 2019 09:48:32 +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

On Tuesday 15 January 2019 09:41:19 Jan Kara wrote:
> On Tue 15-01-19 09:31:11, Pali Rohár wrote:
> > 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*.
> 
> If you wanted to follow tune2fs interface, you can have e.g.:

Question is if it is a good idea to follow this interface.

> udftune -E access-type=<foo>
> 
> Another question about the feature is - the access type is actually per
> partition and there can be multiple partitions on UDF media. So I think we
> need to specify the partition number in the command and <foo> has to
> actually be something like <partition number>,<access_type>.

Access type is stored in partition descriptor and in UDF (as opposite of
ECMA-167) you can have only one partition descriptor. IIRC there is some
exception when you have two partition descriptors, but then one have to be
readonly and second virtual.

-- 
Pali Rohár
pali.rohar@...il.com