spa_prop_get: require caller to supply output nvlist

[robn]

[Sponsors: Klara, Inc., Wasabi Technology, Inc.]

Motivation and Context

@lundman reported a use-after-free. This is the fix.

Description

All callers to spa_prop_get() and spa_prop_get_nvlist() supplied their own preallocated nvlist (except ztest), so we can remove the option to have them allocate one if none is supplied.

This sidesteps a bug in spa_prop_get(), where the error var wasn't initialised, which could lead to the provided nvlist being freed at the end.

How Has This Been Tested?

Completed ZTS run.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[lundman]

$ git revert 9adc94db5c6094bc460f7b8f0c8c17684b1c87c6
[temp cdfe80041d] Revert "Temp fix: add support for prefetching tables into the ARC"
$ git cherry-pick 366d6fecf6bb3c59668b0f3b89f2a610595f3d2f
[temp 235d223322] spa_prop_get: require caller to supply output nvlist
$ zpool create BOOM PHYSICALDRIVE1
$ zpool destroy BOOM

$ git diff
diff --git a/module/zfs/spa.c b/module/zfs/spa.c
-       int err = 0;
+       int err = 4815162342;

Looks good to me, both in success and simulated-error-case.

[glebius]

This is exactly the patch I was writing right now until @amotin pointed me to this pull request. I also observe the use after free panic.