Re: moving affs + RDB partition support to staging?

From: jdow
Date: Fri Apr 27 2018 - 04:50:23 EST

OK guys, I am working from th ememory of a 74 year old with a severe head cold. The filesystems were always limited to 2 TB since the seek command was signed 32 bit integer number of BYTEs. So block size has nothing to do with this 2TB issue. That's something else in the descriptions.

The RDBs grew a block size and sector size concept in describing partitions. One had to be a power of two multiple of the other, as I recall. The relationship worked two ways. If you had a Fujitsu Magneto Optical drive with 2k physical sectors you could still allocate space in virtual sectors, block sizes, of 512 bytes. Or you could use much larger blocks than sector size to cut down on the unallocated blocks list. Of course, all OSs do that these days. With a single unsigned 16 bit integer your address space would fill up a 32 GB with 512 byte blocks. We could see this would blow out so we decribed block size and disk size in blocks with 32 bit integers giving 4 TB of 1 BYTE blocks. Supposing the file sizes stored are such as to support an 8k block size that means describing a very large disk should be very easy, even if block size was a 16 bit unsigned integer.

To carry this forward more accurately I'll have to dig up some old archival data and see what it actually says. This isn't going to happen when I sit here sniffling. Maybe in a few days I can dredge up the actual numbers. I seem to remember at the time that without a 64 bit OS the disk addressing limit was 2 TB, since seeks were signed integers. The disk descriptions could be considerably larger.

And before I forget there are two features of the RDBs that I heartily recommend never implementing on Linux. They were good ideas at the time; but, times changed. The RDBs are capable of storing a filesystem driver and some drive init code for the plugin disk driver card. That is giving malware authors entirely goo easy a shot at owning a machine. Martin S., I would strongly suggest that going forward those two capabilities be removed from the RDB readers in AmigaOS as well as Linux OS.

I also note that at one time the RDB parsing facility in Linux benefitted from my hands on the code and handled the block size/sector size issues very nicely. I had an 18 GB disk I wanted to be able to read on Linux. I seem to remember somebody at one time had a "this doesn't make any real world sense" event and had to be reminded that the real world was wider than he envisioned. I don't know what has happened since then.

{^_^} Joanne Dow

On 20180426 04:08, Geert Uytterhoeven wrote:
Hi Martin,

CC jdow

On Thu, Apr 26, 2018 at 12:28 PM, Martin Steigerwald
<martin@xxxxxxxxxxxx> wrote:
You probably put your stick into a cave with ancient sleeping dragons :)

Added in linux-m68k mailing list, as they likely have an opinion on how
to treat affs + RDB partition support. Also added in Jens Axboe about
patching that RDB support broken with 2 TB or larger harddisks issue
which had been in Linux kernel for 6 years while a patch exists that to
my testing back then solves the issue.

While non-native Linux filesystem support (e.g. affs/isofs/...) could be
handled by FUSE, moving RDB partition support to staging is not an option,
as it is the only partitioning scheme that Amigas can boot from.

If there are bugs in the RDB parser that people run into, they should
be fixed.
If there are limitations in the RDB format on large disks, that's still not
a reason to move it to staging (hi msdos partitioning!).

Matthew Wilcox - 26.04.18, 04:57:
On Wed, Apr 25, 2018 at 10:30:29PM +0200, David Sterba wrote:
I had similar toughts some time ago while browsing the fs/
Access to the filesystem images can be reimplemented in FUSE, but
other than that, I don't think the in-kernel code would be missed.

It's hard to know how many users are there. I was curious eg. about
bfs, befs, coda or feevxfs, looked at the last commits and searched
around web if there are any mentions or user community. But as long
as there's somebody listed in MAINTAINERS, the above are not
candidates for moving to staging or deletion.

Yeah, it's pretty sad how few commits some of these filesystems have
had in recent years. One can argue that they're stable and don't need
to be fixed because they aren't broken, but I find it hard to believe
that any of them were better-implemented than ext2 which still sees
regular bugfixes.

Regarding affs there is a severe issue which is not in affs itself but
in the handling of Rigid Disk Block (RDB) partitions, the Amiga
partitioning standard, which is far more advanced than MBR: It overruns
for 2 TB or larger drives and then wraps over to the beginning of the
drive â I bet you can imagine what happens if you write to an area
larger than 2 TB. I learned this with an external 2TB RDB partitioned
harddisk back then, which I used for Sam440ep (a kind of successor for
old, classic Amiga hardware) backup + some Linux related stuff in
another partition.

Joanne Dow, a developer who developed hdwrench.library which HDToolBox
uses for partitioning in AmigaOS 3.5/3.9, provided a patch back then,
but never officially put it officially through upstreaming as I offered
to make a good description and upstream it through Jens Axboe.

I may take this as a reason toâ actually follow through this time,
hopefully remembering all the details in order to provide a meaningful
patch description â but I think mostly I can do just careful copy and
paste. Even tough I believe Joanne DowÂs fix only fixed my bug report
43511, but not 43511 which is more about a safeguarding issue in case of
future overflows, I still think it would be good to go in in case affs +
RDB stays in their current places.

However, in case you move affs to staging, I may be less motivated to do
so, but then I suggest you also move RDB partitioning support to
staging, cause this is the one that is known to be dangerously badly for
2 TB or larger disks. And yeah, I admit I did not follow through with
having that patch upstreamed. Probably I did not want to be responsible
in case my description would not have been absolutely accurate or the
patch breaks something else. I do not have that 2 TB drive anymore and
donÂt feel like setting one up in a suitable way in order to go about
this patch, but my testing back then was quite elaborate and I still
feel pretty confident about it.

I totally get your motivation, but I would find it somewhat sad to see
the filesystems you mentioned go into staging. However, as I just shown
clearly, for the user it may be better, cause there may be unfixed
dangerous bugs. FUSE might be an interesting approach, but I bet it will
not solve the maintenance issue. If there is no one maintaining it in
the kernel, I think its unlikely to find someone adapting it to be a
FUSE filesystem and maintaining it. And then I am not aware of FUSE
based partitioning support. (And I think think ideally weÂd had a
microkernel and run all filesystems in userspace processes with a
defined set of privileges, but that is simply not Linux as it is.)

Partitions: Amiga RDB partition on 2 TB disk way too big, while OK in
AmigaOS 4.1

Bug 43521 - Amiga RDB partitions: truncates miscalculated partition size
instead of refusing to use it

Bug 43511 - Partitions: Amiga RDB partition on 2 TB disk way too big,
while OK in AmigaOS 4.1

I forward the relevant mail of Joanne, in

I even have the patch in diff format. And I just checked, the issue is
still unpatched as of 4.16.3.

---------- Weitergeleitete Nachricht ----------

Betreff: Re: Partitions: Amiga RDB partition on 2 TB disk way too big,
while OK in AmigaOS 4.1
Datum: Montag, 18. Juni 2012, 03:28:48 CEST
Von: jdow <jdow@xxxxxxxxxxxxx>
An: Martin Steigerwald <Martin@xxxxxxxxxxxx>
Kopie: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>, linux-
kernel@xxxxxxxxxxxxxxx, Jens Axboe <axboe@xxxxxxxxx>, linux-

On 2012/06/17 14:20, Martin Steigerwald wrote:
Am Sonntag, 17. Juni 2012 schrieb jdow:
On 2012/06/17 09:36, Geert Uytterhoeven wrote:
On Sun, Jun 17, 2012 at 2:58 PM, Martin Steigerwald
<Martin@xxxxxxxxxxxx> wrote:
Am Sonntag, 17. Juni 2012 schrieb jdow:
| JXFS 64 bit file system
| With AmigaOS 4.x a new file system has been introduced called
| JXFS. It is a totally new 64 bit file system that supports
| partitions up to 16 TB in size. It is a modern journalling file
| system, which means that it reduces data loss if data writes to
| the disk are interrupted. It is the fastest and most reliable
| file system ever created for AmigaOS.

Well I asked AmigaOS 4 developers about this issue as well. Lets
what they say about 2 TB limits.

16 TB = 2 TB * 8. Perhaps they increased the block size from 512 to

block/partitions/amiga.c reads the block size from
but after conversion to 512-byte blocks, all further calculations
done on "int", so it will overflow for disks larger than 2 TiB.

Note that in your profile-binary.img, the field is 0x200, i.e. 512
bytes per block,
so I'll have to get a deeper look into your RDB first...
Note that the work I did on the Linux RDB code eons ago took full
cognizance of the physical and virtual block sizes.
I've asked Martin for a digital copy of his RDBs and what he thinks
partition(s) should look like. I should also be told whether the disk
is supposed to be solely Amiga OSs or not. I gather it's not.

Its all in the bug report. profile-binary.img should be it.

Its small so I attach it.

Meanwhile I try to get the currently supported maximum disk size out
the OS 4 developers. Maybe JXFS is playing tricks that other
filesystems do
not play or simply has a different implementation.


I've sent Jens a proposed fix changing these lines in amiga.c.
===8<--- was
struct PartitionBlock *pb;
int start_sect, nr_sects, blk, part, res = 0;
int blksize = 1; /* Multiplier for disk block size */
===8<--- becomes changing line 338 into lines 338-339
struct PartitionBlock *pb;
sector_t start_sect, nr_sects;
int blk, part, res = 0;
int blksize = 1; /* Multiplier for disk block size */

(I'm working without proper tools at the moment or I'd make a real diff.

This fix may not be complete. The RDBs contain provisions for describing
the physical disk block size and how many physical blocks are used in a
file system's logical blocks. This MAY not work quite right large
block sizes.

{^_^} Joanne Dow
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As long as there's someone who can be pestered with bugs, I don't see
the need to push their baby out of the tree. I'm a bit more nervous
about hfsplus; if it has users and no maintainer, those users are at

Perhaps we could advertise on Craigslist or something ... maintainer
wanted for LTR.