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From:   Martin Steigerwald <martin@lichtvoll.de>
To:     jdow <jdow@earthlink.net>
Cc:     Michael Schmitz <schmitzmic@gmail.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Matthew Wilcox <willy@infradead.org>,
        David Sterba <dsterba@suse.cz>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>,
        linux-m68k <linux-m68k@lists.linux-m68k.org>
Subject: Re: moving affs + RDB partition support to staging?
Date:   Wed, 27 Jun 2018 10:03:51 +0200
Message-ID: <3093795.YrKfDMMbMq@merkaba>
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Dear Joanne.

jdow - 27.06.18, 08:24:
> You allergic to using a GPT solution? It will get away from some of
> the evils that RDB has inherent in it because they are also features?
> (Loading a filesystem or DriveInit code from RDBs is just asking for
> a nearly impossible to remove malware infection.) Furthermore, any 32
> bit system that sees an RDSK block is going to try to translate it.
> If you add a new RDB format you are going to get bizarre and probably
> quite destructive results from the mistake. Fail safe is a rather
> good notion, methinks.
> 
> Personally I figure this is all rather surreal. 2TG of junk on an
> Amiga system seems utterly outlandish to me. You cited another
> overflow potential. There are at least three we've identified, I
> believe. Are you 100% sure there are no more? The specific one you
> mention of translating RDB to Linux has a proper solution in the RDB
> reader. It should recover such overflow errors in the RDB as it can
> with due care and polish. It should flag any other overflow error it
> detects within the RDBs and return an error such as to leave the disk
> unmounted or mounted read-only if you feel like messing up a poor
> sod's backups. The simple solution is to read each of the variables
> with the nominal RDB size and convert it to uint64_t before
> calculating byte indices.
> 
> However, consider my inputs as advice from an adult who has seen the
> Amiga Elephant so to speak. I am not trying to assert any control. Do
> as you wish; but, I would plead with you to avoid ANY chance you can
> for the user to make a bonehead stupid move and lose all his
> treasured disk archives. Doing otherwise is very poor form.

I am pretty confident that larger than 2 TiB disks are fully supported 
within AmigaOS 4, as I outlined in my other mail.

So with all due respect: I used a larger than 2 TiB disk in AmigaOS 4 in 
2012 already *just* fine. I even found I had the same questions back 
then, and researched it. Which lead to this official article back then:

http://wiki.amigaos.net/wiki/RDB

I am also pretty sure that AmigaOS still uses RDB as partitioning 
format. They support MBR. I don?t think AmigaOS 4.1 supports GPT. 
Whether to implement that of course is the decision of AmigaOS 4 
development team. I am no longer a member of it since some time.

Linux m68k should already be able to use disks in GPT format, but you 
likely won?t be able to read them in AmigaOS, unless there is some third 
party support for it meanwhile.

Thanks,
Martin

> 
> {o.o}   Joanne "Said enough, she has" Dow
> 
> On 20180626 18:07, Michael Schmitz wrote:
> > Joanne,
> > 
> > As far as I have been able to test, the change is backwards
> > compatible (RDB partitions from an old disk 80 GB disk are still
> > recognized OK). That"s only been done on an emulator though.
> > 
> > Your advice about the dangers of using RDB disks that would have
> > failed the current Linux RDB parser on legacy 32 bit systems is well
> > taken though. Maybe Martin can clarify that for me - was the 2 TB
> > disk in question ever used on a 32 bit Amiga system?
> > 
> > RDB disk format is meant for legacy use only, so I think we can get
> > away with printing a big fat warning during boot, advising the user
> > that the oversize RDB partition(s) scanned are not compatible with
> > legacy 32 bit AmigaOS. With the proposed fix they will work under
> > both AmigaOS 4.1 and Linux instead of truncating the first
> > overflowing partition at disk end and trashing valid partitions
> > that overlap, which is what Martin was after.
> > 
> > If that still seems too risky, we can make the default behaviour to
> > bail out once a potential overflow is detected, and allow the user
> > to
> > override that through a boot parameter. I'd leave that decision up
> > for the code review on linux-block.
> > 
> > Two more comments: Linux uses 512 byte block sizes for the partition
> > start and size calculations, so a change of the RDB blocksize to
> > reduce the block counts stored in the RDB would still result in the
> > same overflow. And amiga-fdisk is indeed utterly broken and needs
> > updating (along with probably most legacy m68k partitioners). Adrian
> > has advertised parted as replacement for the old tools - maybe this
> > would make a nice test case for parted?
> > 
> > Cheers,
> > 
> >    Michael
> > 
> > On Tue, Jun 26, 2018 at 9:45 PM, jdow <jdow@earthlink.net> wrote:
> >> If it is not backwards compatible I for one would refuse to use it.
> >> And if it still mattered that much to me I'd also generate a
> >> reasonable alternative. Modifying RDBs nay not be even an
> >> approximation of a good idea. You'd discover that as soon as an
> >> RDB uint64_t disk is tasted by a uint32_t only system. If it is
> >> for your personal use then you're entirely free to reject my
> >> advice and are probably smart enough to keep it working for
> >> yourself.
> >> 
> >> GPT is probably the right way to go. Preserve the ability to read
> >> RDBs for legacy disks only.
> >> 
> >> {^_^}
> >> 
> >> On 20180626 01:31, Michael Schmitz wrote:
> >>> Joanne,
> >>> 
> >>> I think we all agree that doing 32 bit calculations on 512-byte
> >>> block
> >>> addresses that overflow on disks 2 TB and larger is a bug, causing
> >>> the issues Martin reported. Your patch addresses that by using
> >>> the correct data type for the calculations (as do other partition
> >>> parsers that may have to deal with large disks) and fixes
> >>> Martin's bug, so appears to be the right thing to do.
> >>> 
> >>> Using 64 bit data types for disks smaller than 2 TB where
> >>> calculations don't currently overflow is not expected to cause
> >>> new issues, other than enabling use of disk and partitions larger
> >>> than 2 TB (which may have ramifications with filesystems on these
> >>> partitions). So comptibility is preserved.
> >>> 
> >>> Forcing larger block sizes might be a good strategy to avoid
> >>> overflow
> >>> issues in filesystems as well, but I can't see how the block size
> >>> stored in the RDB would enforce use of the same block size in
> >>> filesystems. We'll have to rely on the filesystem tools to get
> >>> that right, too. Linux AFFS does allow block sizes up to 4k (VFS
> >>> limitation) so this should allow partitions larger than 2 TB to
> >>> work already (but I suspect Al Viro may have found a few issues
> >>> when he looked at the AFFS code so I won't say more). Anyway
> >>> partitioning tools and filesystems are unrelated to the Linux
> >>> partition parser code which is all we aim to fix in this patch.
> >>> 
> >>> If you feel strongly about unknown ramifications of any
> >>> filesystems on partitions larger than 2 TB, say so and I'll have
> >>> the kernel print a warning about these partitions.
> >>> 
> >>> I'll get this patch tested on Martin's test case image as well as
> >>> on a RDB image from a disk known to currently work under Linux
> >>> (thanks Geert for the losetup hint). Can't do much more without
> >>> procuring a working Amiga disk image to use with an emulator,
> >>> sorry. The Amiga I plan to use for tests is a long way away from
> >>> my home indeed.
> >>> 
> >>> Cheers,
> >>> 
> >>>       Michael
> >>> 
> >>> Am 26.06.18 um 17:17 schrieb jdow:
> >>>> As long as it preserves compatibility it should be OK, I suppose.
> >>>> Personally I'd make any partitioning tool front end gently force
> >>>> the
> >>>> block size towards 8k as the disk size gets larger. The file
> >>>> systems
> >>>> may also run into 2TB issues that are not obvious. An unused
> >>>> blocks
> >>>> list will have to go beyond a uint32_t size, for example. But a
> >>>> block
> >>>> list (OFS for sure, don't remember for the newer AFS) uses a tad
> >>>> under 1% of the disk all by itself. A block bitmap is not quite
> >>>> so bad. {^_-}
> >>>> 
> >>>> Just be sure you are aware of all the ramifications when you make
> >>>> a
> >>>> change. I remember thinking about this for awhile and then
> >>>> determining I REALLY did not want to think about it as my brain
> >>>> was getting tied into a gordian knot.
> >>>> 
> >>>> {^_^}
> >>>> 
> >>>> On 20180625 19:23, Michael Schmitz wrote:
> >>>>> Joanne,
> >>>>> 
> >>>>> Martin's boot log (including your patch) says:
> >>>>> 
> >>>>> Jun 19 21:19:09 merkaba kernel: [ 7891.843284]  sdb: RDSK (512)
> >>>>> sdb1
> >>>>> (LNX^@)(res 2 spb 1) sdb2 (JXF^D)(res 2 spb 1) sdb3 (DOS^C)(res
> >>>>> 2 spb
> >>>>> 4)
> >>>>> Jun 19 21:19:09 merkaba kernel: [ 7891.844055] sd 7:0:0:0: [sdb]
> >>>>> Attached SCSI disk
> >>>>> 
> >>>>> so it's indeed a case of self inflicted damage (RDSK (512) means
> >>>>> 512
> >>>>> byte blocks) and can be worked around by using a different block
> >>>>> size.
> >>>>> 
> >>>>> Your memory serves right indeed - blocksize is in 512 bytes
> >>>>> units.
> >>>>> I'll still submit a patch to Jens anyway as this may bite others
> >>>>> yet.
> >>>>> 
> >>>>> Cheers,
> >>>>> 
> >>>>>      Michael
> >>>>> 
> >>>>> On Sun, Jun 24, 2018 at 11:40 PM, jdow <jdow@earthlink.net> 
wrote:
> >>>>>> BTW - anybody who uses 512 byte blocks with an Amiga file
> >>>>>> system is
> >>>>>> a famn
> >>>>>> dool.
> >>>>>> 
> >>>>>> If memory serves the RDBs think in blocks rather than bytes so
> >>>>>> it
> >>>>>> should
> >>>>>> work up to 2 gigablocks whatever your block size is. 512 blocks
> >>>>>> is
> >>>>>> 2199023255552 bytes. But that wastes just a WHOLE LOT of disk
> >>>>>> in
> >>>>>> block maps.
> >>>>>> Go up to 4096 or 8192. The latter is 35 TB.
> >>>>>> 
> >>>>>> {^_^}
> >>>>>> 
> >>>>>> On 20180624 02:06, Martin Steigerwald wrote:
> >>>>>>> Hi.
> >>>>>>> 
> >>>>>>> Michael Schmitz - 27.04.18, 04:11:
> >>>>>>>> test results at
> >>>>>>>> https://bugzilla.kernel.org/show_bug.cgi?id=43511
> >>>>>>>> indicate the RDB parser bug is fixed by the patch given
> >>>>>>>> there, so if
> >>>>>>>> Martin now submits the patch, all should be well?
> >>>>>>> 
> >>>>>>> Ok, better be honest than having anyone waiting for it:
> >>>>>>> 
> >>>>>>> I do not care enough about this, in order to motivate myself
> >>>>>>> preparing the a patch from Joanne Dow?s fix.
> >>>>>>> 
> >>>>>>> I am not even using my Amiga boxes anymore, not even the
> >>>>>>> Sam440ep
> >>>>>>> which
> >>>>>>> I still have in my apartment.
> >>>>>>> 
> >>>>>>> So RDB support in Linux it remains broken for disks larger 2
> >>>>>>> TB,
> >>>>>>> unless
> >>>>>>> someone else does.
> >>>>>>> 
> >>>>>>> Thanks.
> >>>>>> 
> >>>>>> --
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> >> 
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-- 
Martin