Michael. Joanne.
I do think this discussion is slightly getting out of hand… so I suggest
to focus on what its up to the kernel to do and what is not. And to
focus only on what is up to the RDB parser, cause the patch is on
changing that. The RDB parser is not responsible for what any file
system may do. Securing AFFS would be a different, important, topic.
With that mail I am probably out as this discussion took already quite a
bit of time.
But for details, read on:
Michael Schmitz - 30.06.18, 07:26:
Am 30.06.18 um 15:56 schrieb jdow:
As far as I can guess from the code, pb_Environment[3] (number
of
heads)
and pb_Environment[5] (number of sectors per cylinder) are
abitrarily
chosen so the partition size can be expressed as a difference
between
pb_Environment[9] and pb_Environment[10] (low and high cylinder
addresses), which places restrictions on both partition size and
alignment that depend on where on the disk a partition is
placed?
If you do not teach the OS to ignore Cylinder Blocks type entries
and
use some math on heads and blocks per track the disk size is
relatively stuck modulo using large blocks.
As long as AmigaOS and Linux agree on how to express start and end
offset for the partitions, that's fine.
But I read your other mail to mean that we're stuck to 2 TB disks
for
now. I don't follow that - we can have partitions of 2 TB each by
maxing
out rdb_CylBlocks as long as we use 512 bytes per block (since the
product of cylinders and blocks per cylinder is limited to 32
bits) and using one cylinder per partition (32 bits available
there as well)?
But the rdb_CylBlocks limit also means we're safe with 64 bit
sector_t in Linux. Best add a check in the parser to warn us if
the product of head count and sectors per cylinder overflows 32
bit though.
Cheers,
Michael
How long did it tale s to get to 10 TB disks from 2 TB disks. And a
new SD Card spec allows for 128 TB disks. Block sizes get sort of
ridiculous as you get past about 8k bytes or about 32 TB or about
two
years from now.
I get that - I just don't get why 32 bits for cylinders plus 32 bits
for blocks per cylinder equals 2 TB (4G of 512 byte blocks). But I
don't know what other limits exist that may restrict the total number
of blocks to 32 bits.
I think for the total device size:
The raw, theoretical limit on the maximum device capacity is about
2^105 bytes:
32 bit rdb_Cylinders * 32 bit rdb_Heads * 32 bit rdb_Sectors * 512
bytes/sector for the HD size in struct RigidDiskBlock
is correct. Do you agree, Joanne?
http://wiki.amigaos.net/wiki/RDB
If so, we can remove that limit from the discussion and focus on what
remains.
As for the partition sizes, how about
Partition size
For the partitions the maximum size is:
32 bit (de_HighCyl + 1 - de_LowCyl) (to get the partition size) * 32
bit de_Surfaces * 32 bit de_SectorsPerTrack * 512 bytes/sector in
struct DosEnvec (=pb_Environment[]) in struct PartitionBlock.
That's from the physical drive part, the actual disk size limit for >
the partitions may be much smaller depending on the partitioning
software, if it's only using the logical sizes instead, which is
likely the case, it's only 8 ZiB with 512 bytes/sector: 32 bit
rdb_HiCylinder * 32 bit rdb_CylBlocks * 512 bytes/sector = 2^73
bytes. For using the logical sizes using simple uint64 calculations
(with some overflow checks) should be enough, for more a math library
with support for larger integers needs to be used which probably no
partitioning software does.
taken from the same wiki page?
I however do not get what it means with "logical sizes".
Joanne, what is your feedback on this?
Do you want to create disks that will fail on AmigaDOS? AmigaDOS, as
far as I know, makes heavy use of Cylinder Blocks values. It
calculating Cylinder Blocks overflows when creating the disk's RDBs
the user MUST be informed it is
I'm not at all planning to create disks for AmigaDOS. I just need to
know what combinations of cylinders, heads and sectors are possible to
encounter on disks that have been created with native tools. Well,
Do we really need to add checks to those possible combinations values,
Joanne? Cause, if we do, how do we find out?
Again, the Linux RDB parser just *reads* the RDB.
And already on the Amiga side it can happen that one partitioning tool
does not understand the other. That Rigid Disk Block is not quite as
rigid as the name would suggest, you already said its very flexible and
that is not always a good thing. I have seen a Phase 5 RDB partitioning
tool *crush* an RDB created by HDToolBox cause one calculated from 0 and
one from zero.
I do think is neither the duty nor the responsibility of the Linux
kernel to check for such crap.
Especially as, as I will point out further down in the mail, it is
difficult to impossible to actually know for sure which combinations any
of the partitioning tools on native os´s allowed.
Even if we´d find out the possible combinations of the official tools,
there are RDB tools like RDBSalv and what not.
I´d say it is important to avoid over complicating things. We cannot
fulfill and it is not our responsibility to save the user from any brain
damage any partitioning tool on either native OS or Linux created
*within* the kernel.
So I´d recommend:
- Make the calculations bail out on overflows and refuse to load the RDB
if any calculation overflows.
- Ideally also secure AFFS this way.
- Have the warning about 64 bit disk support on native OS
and *be done with it*. For the kernel.
assuming sufficient amounts of braindamage in the corresponding Linux
tools, knowing the absolute outer limits of what these tools could do
would be nice as well, but someone using amiga-fdisk to create a RDSK
block for a 10 TB disk fully deserves any punishment that invites.
If there is any warning due to using limits that exceed what HDToolBox
(AmigaOS upto 3.1), HDToolbox using hdwrench.library (AmigaOS 3.5
onwards), Media Toolbox using SP_engine (AmigaOS 4 onwards) as well as
all the tools in Morphos, AROS and on AmiNet woul use, it would be up to
amiga-fdisk and parted to issue such a warning.
As for the RDB parser, I´d go with robustness principle:
Be conservative in what you send, be liberal in what you accept.
https://en.wikipedia.org/wiki/Robustness_principle
So what ever crap or non crap any partitioning tool creates, check for
any overflow in calculations and only refuse to accept the result in
case there have been an overflow in calculations.
If a value is an ULONG, accept the full 32 bit unsigned integer. Cause
if you don´t, it could mean the user is not able to access the disk with
that RDB.
Partitioning tools however can impose any they think make sense.
As for limits with in native OS partitioning tools, for HDToolbox with
and without hdwrench.library you probably remember, Joanne. As for Media
Toolbox we´d have to ask AmigaOS developers. Remember AmigaOS and
Morphos development are still closed source. So there is nothing except
the SDKs and their Documentation which we can use to know for sure.
The latest AmigaOS 4.1 development kit appears to be version 53.30,
found at. It is for AmigaOS 4.1 Final Edition:
https://www.hyperion-entertainment.biz/index.php/downloads?
view=files&parent=30
(you need to click the link to arrive an an extra page and click the
link there)
base.lha in SDK_Install contains the documentation. I was able to unpack
it with lha x on Linux. However I have no idea where that lha command on
my Debian GNU/Sid setup came from:
% LANG=C dpkg -S /usr/bin/lha
dpkg-query: no path found matching pattern /usr/bin/lha
I bet there is some Linux lha downloadable somewhere, if need be I can
dig for it.
However I used grep and find on various variations of Media Toolbox,
SP_Engine and RDB as well as Rigid Disk Block and found nothing of
interest for our topic.
So it appears that the workings of SP_Engine are buried in source code
we don´t have access to. And well I won´t base any limit checking on
assumptions. So I won´t set any additional limits than the check for
overflowing calculations.
The AmigaOS 3.9 NDK still appears to be available on the website of my
former employee HAAGE&PARTNER GmbH:
https://www.haage-partner.de/download/AmigaOS/NDK39.lha
This one included the hdwrench.library autodocs and releasenotes. I do
not know whether they have something about limits. However, it does not
appear so:
[…] NDK_3.9/Documentation% grep limit Autodocs/hdwrench.doc
This function is intentionally limited writing only to the lower
block
This is a limit about writing the data section of a BootBlock, so
unrelated to our topic.
[…] NDK_3.9/Documentation% grep limit
Releasenotes/hdwrench_lib_relnotes
Michael:
(Actually, I lied there. I do plan to create a RDSK block for a 2 TB
disk image where cylinder, head and sector counts all approach the 32
bit limit, just to see that my overflow checks work as intended. But
that's strictly for Linux testing).
I think that is a good test for the patch. I am impressed you are
willing to put the effort for that into this work.
Okay, that is what I was able to dig out on what is officially
available. It appears that the official developer documentation for
AmigaOS is pretty sparse on any of that. For anything else I´d need to
contact current AmigaOS developers. I am no longer a member of the
AmigaOS team.
Maybe the ADF Format FAQ I mentioned has a bit more, but I did not find
anything obvious on a short glance
http://lclevy.free.fr/adflib/adf_info.html
Also it is a third party source.
unsafe to put on a real Amiga. (I'd also suggest teaching Linux to
understand RDSL, which would be RDSK++ sort of. Then use that if
Cylinder Blocks overflows.) The value you will not be able to fill
in
the DosEnvec structure is:
ULONG de_HighCyl; /* max cylinder. drive specific */
OK, so Cylinder Blocks overflowing is a red flag, and requires to
abort parsing the partition table right away? And HighCyl really
means the max. number of logical blocks, not cylinders (which would
have nr_heads*nr_sects many blocks)? That's probably the cause for my
confusion.
When can they actually overflow? How are they calculated?
If there is no calculation to calculate it and it is just a static value
within the RDB, I´d accept what ever the value is in the RDB, unless a
calculation based on it overflows.
Again, keep it simple.
I think it does not make sense for the Linux kernel to try to outsmart
the user or any of the partitioning tools out there.
The only question is:
Can the partitions be calculated in a way that it is safe to access
them, i.e. that their calculated start and end is really at the location
the start and end is on the disk? Then accept it.
For the file systems: Can the file system safely access this? Then
accept. If not, then decline. *In the filesystem*. This is nothing for
the RDB parser to check so out of scope for the discussion of this
patch.
So accessing larger disks once you hit 2 TB means you must increase
the logical block size. And eventually that will waste HUGE amounts
of files when small files are being stored.
Just like small inodes wastes huge amounts of space for metadata. It's
a tradeoff, and AFFS on a RDSK format disk probably isn't the right
choice for huge disks. Never mind that - if someone _does_ go that
way, we need to make sure we can parse the RDSK information
correctly. And if such a disk causes the 64 bit sector_t in Linux to
overflow, I'd like the parser to spot that, too.
Thanks for your immense patience in explaining all these subtleties to
me.
Cheers,
Michael
Any solution will require action on the part of the people
developing
AmigaDOS follow-ons. You might want to get them motivated, somehow,
and proceed from there with a request to be informed of any RDB
changes. I'd suggest to them that removing sensitivity to Cylinder
Blocks sorts of values from the entire system probably would be
painful but the simplest solution.
{^_^}
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