geom_part: add support for SGI disk labels

[asomers]

Summary

It's read-only for now, because the purpose isn't to run FreeBSD on
systems that use this partition scheme, but rather merely to read data
from SGI-formatted disks.

Some of the code is based on util-linux, which is explicitly public
domain.

https://github.com/util-linux/util-linux/blob/41644c216ee1c81cd346df17b95453ff4004c7a6/include/pt-sgi.h

MFC after: 2 weeks
Relnotes: yes
Sponsored by: ConnectWise

Test Plan

manually tested with 3 SGI disk images, and compared against SGI's output. Regression test added.

[asomers]

Believe it or not, there's a practical reason for this. I wasn't just tinkering around for fun, honestly.

[imp]

I'll ignore the trend we've had towards retiring older-format support generally...

[asomers]

In D57483#1316905, @imp wrote:

I'll ignore the trend we've had towards retiring older-format support generally...

If you have a better idea for how to read such disks, I'm all ears. Out-of-tree kernel modules are tough to maintain. But maybe a cuse device or something? It's a possibility.

[imp]

In D57483#1316912, @asomers wrote:

In D57483#1316905, @imp wrote:

I'll ignore the trend we've had towards retiring older-format support generally...

If you have a better idea for how to read such disks, I'm all ears. Out-of-tree kernel modules are tough to maintain. But maybe a cuse device or something? It's a possibility.

That's why I'm ignoring it... :) I don't either.

[jlduran]

I echo the words of @imp; I don't even have a real way to test it.
Having said that, this is really cool from a retro-computing perspective.
We need a "It's a U**X system, I know this!" GIF.

[asomers]

It occurs to me that this might be a good use case for an out-of-tree kernel driver written in Rust. It's easy to test, it's small, and it's not wanted within the tree.

[imp]

In D57483#1317295, @asomers wrote:

It occurs to me that this might be a good use case for an out-of-tree kernel driver written in Rust. It's easy to test, it's small, and it's not wanted within the tree.

Yea. There's a fair amount of lift to do this in rust. But being out of tree, you'd have a fair amount of rust api freedom for a time... though that might make it overall a much tougher problem.