Re: udf: Commit b085fbe2ef7fa (udf: Fix crash during mount) broke CD-RW support

["Pali Rohár" <pali.rohar@gmail.com>]

On Monday 13 January 2020 12:48:38 Jan Kara wrote:
> Hello,
> 
> On Sun 12-01-20 15:47:35, Pali Rohár wrote:
> > Commit b085fbe2ef7fa (udf: Fix crash during mount) introduced check that
> > UDF disk with PD_ACCESS_TYPE_REWRITABLE access type would not be able to
> > mount in R/W mode. This commit was added in Linux 4.20.
> > 
> > But most tools which generate UDF filesystem for CD-RW set access type
> > to rewritable, so above change basically disallow usage of CD-RW discs
> > formatted to UDF in R/W mode.
> > 
> > Linux's cdrwtool and mkudffs (in all released versions), Windows Nero 6,
> > NetBSD's newfs_udf -- all these tools uses rewritable access type for
> > CD-RW media.
> > 
> > In UDF 1.50, 2.00 and 2.01 specification there is no information which
> > UDF access type should be used for CD-RW medias.
> > 
> > In UDF 2.60, section 2.2.14.2 is written:
> > 
> >     A partition with Access Type 3 (rewritable) shall define a Freed
> >     Space Bitmap or a Freed Space Table, see 2.3.3. All other partitions
> >     shall not define a Freed Space Bitmap or a Freed Space Table.
> > 
> >     Rewritable partitions are used on media that require some form of
> >     preprocessing before re-writing data (for example legacy MO). Such
> >     partitions shall use Access Type 3.
> > 
> >     Overwritable partitions are used on media that do not require
> >     preprocessing before overwriting data (for example: CD-RW, DVD-RW,
> >     DVD+RW, DVD-RAM, BD-RE, HD DVD-Rewritable). Such partitions shall
> >     use Access Type 4.
> > 
> > And in 6.14.1 (Properties of CD-MRW and DVD+MRW media and drives) is:
> > 
> >     The Media Type is Overwritable (partition Access Type 4,
> >     overwritable)
> > 
> > Similar info is in UDF 2.50.
> 
> Thanks for detailed info. Yes, UDF 2.60 spec is why I've added the check
> you mentioned. I was not aware that the phrasing was not there in earlier
> versions and frankly even the UDF 2.60 spec is already 15 years old... But
> the fact that there are tools creating non-compliant disks certainly
> changes the picture :)

I tested also Nero Linux 4 (Nero provides free trial version which is
fully working even in 2020) and it creates 1.50 CD-RW discs in the same
way with Rewritable partition. Interestingly for 2.50 and 2.60 it does
not use Overwritable, but Writeonce (yes, for CD-RW with Spartable).

And because previous UDF specification do not say anything about it, I
would not sat that those discs are non-compliant.

Moreover, is there any tool (for Linux or other system) which uses
Overwritable partition type for CD-RW discs? All which I tested uses
Rewritable.

> > So I think that UDF 2.60 is clear that for CD-RW medias (formatted in
> > normal or MRW mode) should be used Overwritable access type. But all
> > mentioned tools were probably written prior to existence of UDF 2.60
> > specifications, probably targeting only UDF 1.50 versions at that time.
> > 
> > I checked that they use Unallocated Space Bitmap (and not Freed Space
> > Bitmap), so writing to these filesystems should not be a problem.
> > 
> > How to handle this situation? UDF 2.01 nor 1.50 does not say anything
> > for access type on CD-RW and there are already tools which generates UDF
> > 1.50 images which does not matches UDF 2.60 requirements.
> > 
> > I think that the best would be to relax restrictions added in commit
> > b085fbe2ef7fa to allow mounting mounting udf fs with rewritable access
> > type in R/W mode if Freed Space Bitmap/Table is not used.
> > 
> > I'm really not sure if existing udf implementations take CD-RW media
> > with overwritable media type. E.g. prehistoric wrudf tool refuse to work
> > with optical discs which have overwritable access type. I supports only
> > UDF 1.50.
> 
> Yeah, we should maintain compatibility with older tools where sanely
> possible. So I agree with what you propose. Allow writing to
> PD_ACCESS_TYPE_REWRITABLE disks if they don't use 'Freed Space
> Bitmap/Table'. Will you send a patch or should I do the update?

Could you do it, please?

Also question is, what with those 2.50 CD-RW Writonce partitions and
with Spartable which creates Nero Linux?

-- 
Pali Rohár
pali.rohar@gmail.com