bin/60644: dump -X/-x: fssN search logic fails if snapshot is mounted

["campbell+netbsd%mumble.net@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60644
>Category:       bin
>Synopsis:       dump -X/-x: fssN search logic fails if snapshot is mounted
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Aug 25 21:25:00 +0000 2026
>Originator:     Taylor R Campbell
>Release:        current, 11, 10, 9, ...
>Organization:
The NetBusyDump Foundation, Inc.
>Environment:
>Description:

	For its -X/-x options to dump with a snapshot, dump(8) has
	logic to find a free /dev/fssN node for the snapshot, by
	opening them one by one and checking whether ioctl(FSSIOCSET)
	fails with EBUSY.

	However, if, before it has found a free node, it finds an
	fss(4) node that is not just configured but also _mounted_,
	then ioctl(FSSIOCTSET) will fail with EPERM instead of EBUSY,
	and so the loop will fail.

>How-To-Repeat:

	fssconfig fss0 / /
	mount /dev/fss0 /mnt
	dump -X -f /dev/null -0a /

>Fix:

	Yes, please!

	Really, the right thing here is probably to just bite the
	bullet and make /dev/fss a proper cloning device.