On Fri 17-01-20 12:31:47, Pali Rohár wrote: > 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. Ah, OK, thanks for clarification. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
