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

["Pali Rohár" <pali.rohar@gmail.com>]

On Friday 17 January 2020 12:22:54 Jan Kara wrote:
> On Thu 16-01-20 16:46:43, Pali Rohár wrote:
> > 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.
> 
> No. But CD-RW means that the media needs "erasing" before overwriting so
> using 'Rewritable' partitions there is fine and in the kernel we do want to
> force such mounts read-only because we don't support "erasing", do we?

I guess that this formulation as you wrote is the reason why all
formatting tools decided to use Overwritable type for CD-RW.

But it is not completely truth. You need erase CD-RW before formatting,
not before rewriting blocks on it. And kernel already supports rewriting
one random block on CD-RW media via pktcdvd.ko layer (part of mainline
kernel).

So to mount CD-RW media with UDF fs on it in R/W mode, you need:

1) erase & format CD-RW media (e.g. via cdrwtool)
2) setup pktcdvd layer for CD-RW media (e.g. via pktsetup or via /sys)
3) mount pktcdvd block device with udf fs

So, kernel supports UDF on CD-RW media also in R/W mode, just it is not
straightforward as for other hard disk block devices.

> > > > 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?
> 
> Sure, attached.
> 
> > Also question is, what with those 2.50 CD-RW Writonce partitions and
> > with Spartable which creates Nero Linux?
> 
> Well, we force them read-only and that's what we should do because we don't
> support CD-RW in the kernel... But I may be missing something.

We support CD-RW in kernel, but now I realized that we do not support
UDF 2.50 in R/W mode. So this should be OK for now (until UDF 2.50 R/W
mode is implemented).

-- 
Pali Rohár
pali.rohar@gmail.com