gpart: More nuance for GPT support
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Authored by imp on Oct 17 2023, 2:28 AM.

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emaste
marcel
zlei

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rG09c999b1557a: gpart: More nuance for GPT support

Summary

A careful reading of the GPT standard shows that one may have fewer than
128 entries in your GPT table. While the standard requires that we
reserve enough space (32 512-byte-LBAs or 4 4096-byte-LBAs), it also
explicitly allows one to specify fewer actual partitions (since that
controls what is in the CRC). It requires that the first LBA to be 32
(512 sectors) or 6 (4k sectors) or larger. That requirement is not
enforced (it's not listed as one of validation criteria for the GPT).
We should likely do so in the future.

To that end, allow a default number of entries to use (defent) on
creation to be different (larger) than the minimum number of legal
entries. For gpt, these numbers work out to 128 and 1 respectively. For
all the others, make minent == defent so this is a nop for those
partitioning schemes.

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rG FreeBSD src repository

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Event Timeline

imp requested review of this revision.Oct 17 2023, 2:28 AM

imp created this revision.

Harbormaster completed remote builds in B54023: Diff 128887.Oct 17 2023, 2:28 AM

imp added a parent revision: D42245: mkimg: Ensure GPT Entry Array is at least 16k.Oct 17 2023, 2:28 AM

imp added a child revision: D42247: gpart: Add warning when the start sector is too low..

emaste added inline comments.Oct 17 2023, 3:56 AM

sys/geom/part/g_part.h
121–124	strange indentation, but I assume is just Phabricator rendering bug
sys/geom/part/g_part_gpt.c
157–158	is it possible for this to introduce any new breakage? e.g. do we start violating the start LBA with something like `gpart create -s gpt -n 1`?

jrtc27 added a subscriber: jrtc27.Oct 17 2023, 6:49 AM

jrtc27 added inline comments.

sys/geom/part/g_part.h
121–124	Show Raw File (Right) confirms the indentation looks sensible in reality

Looks good to me.

sys/geom/part/g_part.h
121–123	One tab before comment should be sufficient.

This revision is now accepted and ready to land.Oct 21 2023, 4:12 PM

guest-patmaddox added a subscriber: guest-patmaddox.Dec 13 2023, 9:57 PM

I'm unsure about this

sys/geom/part/g_part_gpt.c
157–158	I don't think so

I'm unsure about this

sys/geom/part/g_part_gpt.c
157–158	But I'll experiment

I'm going to commit this, but will be inclined to revert on problems rather than powering through them.

sys/geom/part/g_part_gpt.c
157–158	So, you can create non-conforming GPT Tables, but they are well-formed for most other software. You can't create something that's so crazy as to be inconsistent enough for other software to reject. The defaults are good, and but we allow more flexibility, which is consistent with trying not to be overly prescriptive.