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@xxxxxxxxx
