kern/56976: bogus wedge auto-discovery for ancient NetBSD installs

[martin%NetBSD.org@localhost]

>Number:         56976
>Category:       kern
>Synopsis:       bogus wedge auto-discovery for ancient NetBSD installs
>Confidential:   no
>Severity:       serious
>Priority:       low
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Aug 22 14:35:00 +0000 2022
>Originator:     Martin Husemann
>Release:        NetBSD 9.99.99
>Organization:
The NetBSD Foundation, Inc.
>Environment:
System: NetBSD seven-days-to-the-wolves.aprisoft.de 9.99.99 NetBSD 9.99.99 (GENERIC) #573: Thu Aug 18 17:03:04 CEST 2022 martin%seven-days-to-the-wolves.aprisoft.de@localhost:/work/src/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:

Booting a 9.3 or -current kernel on a default install of NetBSD 4.0 causes
the kernel to auto-discover bogus wedges, which prevent mounting of /.

The wedges are off by one (e.g. original wd0a starts at sector 63 according
to disklabel, but the wedge is configured starting at block 64).

Might be related to disklabel moving from start-of-disk to inside-the-NetBSD-
MBR-partiton.

>How-To-Repeat:

Set up a new VM, boot the 4.0 amd64 ISO, do a plain default install.
Switch to a 9.3 ISO and reboot. Notice you can't mount wd0a.

check output from fdisk, disklabel and "dkctl wd0 listwedges", notice
different offsets for dk0 and wd0a.

>Fix:
n/a