libzfs: scrub: only include start and end nv pairs if needed for scrub

[siv0]

Motivation and Context

This patch addresses running zpool scrub <pool> with ZFS 2.4 userspace while the loaded kernel module is still 2.3, failing with:

cannot scrub <pool>: the loaded zfs module does not support an option
for this operation. A reboot may be required to enable this option.

This should help users who are still stuck on an older kernel module, while their distribution ships newer ZFS userspace.

This was observed in the Proxmox community forum:
https://forum.proxmox.com/threads/.180467/

Description

Checking for the source of the message via strace showed the scrub ioctl failing and setting errno to ZFS_ERR_IOC_ARG_UNAVAIL[0]. With that and the comments in module/zfs/zfs_ioctl.c[1] commit: 894edd0 seemed like a likely cause for the backward incompatibility.

The corresponding kernelspace code in module/zfs/zfs_ioctl.c defaults to a setting of 0 if either parameter is not set, so not providing the nvpairs in case both are 0 should not make a semantic difference.

[0]

zfs/include/sys/fs/zfs.h

Line 1762 in d35951b

ZFS_ERR_IOC_ARG_UNAVAIL,

[1]

zfs/module/zfs/zfs_ioctl.c

Line 7799 in d35951b

* ZFS_ERR_IOC_ARG_UNAVAIL An unrecognized nvpair was encountered

Fixes: 894edd0 ("Add TXG timestamp database")

How Has This Been Tested?

Tested by:

• loading zfs.ko in version 2.3.6
• running zpool scrub testpool with zpool from master (error occurs)
• running zpool scrub testpool with this patch applied (scrub is started)

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• 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.

[behlendorf]

Looks good. Thanks for the compatibility fix.