Re: moving affs + RDB partition support to staging?

From: jdow
Date: Wed Jun 27 2018 - 23:50:11 EST

Michael, as long as m68k only supports int fseek( int ) 4 GB * block size is your HARD limit. Versions that support __int64 fseek64( __int64 ) can work with larger disks. RDBs could include RDSK and a new set of other blocks that replace the last two characters with "64" and use __int64 where needed in the various values. That way a clever disk partitioner could give allow normal (32 bit) RDB definitions where possible. Then at least SOME of the disk could be supported AND a very clever filesystem that abstracts very large disks properly could give access to the whole disk. (Read the RDBs first 32 bits. Then if a filesystem or driveinit was loaded re-read the RDBs to see if new 64 bit partitions are revealed.

I could be wrong but I do not think RDBs could be safely modified any other way to work. And, trust me as I bet this is still true, you will need a SERIOUSLY good bomb shelter on the Moon if you change RDBs. Suppose Joe Amigoid uses it, and then Joe Amigoid loads Amigados 2.4 because he wants to run a game that crashes on anything newer. Then Joe got far enough something writes to the disk and data is corrupted. Note further that Amigoids do NOT, repeat NOT, listen to facts in such cases. Hell, some of them never listened to facts about an incident at Jerry Pournelle's place when a 1.1 DPaint session with Kelly Freas hung and we lost a delightful drawing. Jerry reported it. Amigoids screamed. I tried to tell them I was there, it was my machine, and 1.1 was, indeed, crap.

{o.o}

On 20180627 02:00, Michael Schmitz wrote:
Joanne,

I'm not at all allergic to avoiding RDB at all cost for new disks. If AmigaOS 4.1 supports more recent partition formats, all the better. This is all about supporting use of legacy RDB disks on Linux (though 2 TB does stretch the definition of 'legacy' a little). My interest in this is to ensure we can continue to use RDB format disks on m68k Amiga computers which have no other way to boot Linux from disk.

Not proposing to change the RDB format at all, either. Just trying to make sure we translate RDB info into Linux 512-byte block offset and size numbers correctly. The kernel won't modify the RDB at all (intentionally, that is - with the correct choice of partition sizes, Martin might well have wiped out his RDB with the current version of the parser).

The choice of refusing to mount a disk (or mounting read-only) rests with the VFS drivers alone - AFFS in that case. Not touching any of that. At partition scan time, we only have the option of making the partition available (with a warning printed), or refusing to make it available to the kernel. Once it's made available, all bets are off.

From what Martin writes, his test case RDB was valid and worked as expected on 32 bit AmigaOS (4.1). Apparently, that version has the necessary extensions to handle the large offsets resulting from 2 TB disks. Not sure what safeguards are in place when connecting such a disk to older versions of AmigaOS, but that is a different matter entirely.

The overflows in partition offset and size are the only ones I can see in the partition parser - there is no other overflow I've identified. I just stated that in order to place a partition towards the end of a 2 TB disk, the offset calculation will overflow regardless of what combination of rdb->rdb_BlockBytes and sector addresses stored in the RDB (in units of 512 byte blocks) we use:

ÂÂÂÂÂÂÂ blksize = be32_to_cpu( rdb->rdb_BlockBytes ) / 512;


ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ nr_sects = (be32_to_cpu(pb->pb_Environment[10]) + 1 -
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ be32_to_cpu(pb->pb_Environment[9])) *
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ be32_to_cpu(pb->pb_Environment[3]) *
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ be32_to_cpu(pb->pb_Environment[5]) *
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ blksize;
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (!nr_sects)
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ continue;
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ start_sect = be32_to_cpu(pb->pb_Environment[9]) *
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ be32_to_cpu(pb->pb_Environment[3]) *
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ be32_to_cpu(pb->pb_Environment[5]) *
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ blksize;

But in the interest of avoiding any accidental use of a RDB partition where calculations currently overflow, I'll make the default behaviour to bail out (instead of using wrong offset or size as we currently do). Given the 'eat_my_RDB_disk=1' boot option, the user may proceed at their own risk (though I still can't see what harm should result from now translating a well formed v4.1 2 TB disk RDB correctly for the first time).

Whether or not Linux correctly handles AFFS filesystems larger than 1 TB is a matter for VFS experts. Bailing out on nr_sects overflowing ought to prevent accidental use of AFFS filesystems on RDB disks which I suppose is the majority of use cases.

Bugs in partitioning tools on Linux are entirely out of scope - the partitioning tools bypass the partition structure discovered by the kernel, and work straight on the raw device. No protecting against that.

If you can point out a way to cause data loss with these precautions, for a disk 2 TB or larger that was partitioned and used on a recent version or AmigaOS supporting such large disks, I'd consider omitting the 'eat_my_RDB_disk' boot option, and just bail out as the only safe option.

Cheers,

ÂÂÂÂMichael


Am 27.06.2018 um 18:24 schrieb jdow:
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.

{o.o}ÂÂ 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,

ÂÂ Michael



On Tue, Jun 26, 2018 at 9:45 PM, jdow <jdow@xxxxxxxxxxxxx> 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,

ÂÂÂÂÂ 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]Â 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,

ÂÂÂÂ Michael


On Sun, Jun 24, 2018 at 11:40 PM, jdow <jdow@xxxxxxxxxxxxx> 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 DowÂs 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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