Transition to support for 52 partitions

In -current, Theo de Raadt (deraadt@) has started the transition to support for 52 disk partitions (on a subset of hardware architectures):

CVSROOT:	/cvs
Module name:	src
Changes by:	deraadt@cvs.openbsd.org	2025/11/13 13:59:14

Modified files:
	sys/dev/ata    : wd.c 
	sys/kern       : kern_pledge.c 
	sys/sys        : disklabel.h dkio.h 
	sys/scsi       : sd.c 
	sys/dev/isa    : fdreg.h 
	sys/arch/sparc64/dev: fd.c 

Log message:
Begin transition to 52-partition support.  The partition encoding used
to be lowest 4 bits of dev_t, and now becomes 6.  This supplies 64
partitions in struct disklabel.d_partitions[MAXPARTITIONSUNIT], but we
only use 52 of these slots (an architecture can be either 16 partition
or 52 partition, depending on MD define MAXPARTITIONS).  The
52-partition limit is due to single-character representation limit of
a-zA-Z.  We supply a backwards-compat ioctl for a while which can read
an disklabel structure.

This change does not yet store 52-partition information on-disk, and
does not transition any architecture to allowing use of >16 partitions.
Those changes come soon, after this compatibility breaking change settles.

Immediate result is dev_t numbers for /dev/{sd,wd,rd,fd,...}[12345...][a-p]
become incorrect, and need to be repaired.  A sysupgrade will do this for
automatically.  For a hand-build, or a manual kernel replacement, systems
which have root *NOT ON SD0 or WD0*, probably fail into single-user and you
must perform these steps explained in /etc/rc:

#   fsck /dev/rrootdisk
#   mount -uw /dev/rootdisk /
#   cd /dev && ./MAKEDEV redodisks

Please do not try to manual-build through this on a system earlier than 7.8.

with and ok krw

Note that for most using full-disk-encryption, it is likely that root is not on sd0/wd0.

[The first tendrils of this work were committed well before the 7.8 release.]

Update:
Warning - Those with full-disk-encryption should avoid this bump until it is known to be safe. At the time of writing, there is still a problem -
https://marc.info/?l=openbsd-misc&m=176319377915316&w=2

Update:
We have a confirmed report of successful sysupgrade(8) across the bump on a full-disk-encryption (FDE) amd64 system using the (new) snapshot with BUILDINFO:

Build date: 1763270116 - Sun Nov 16 05:15:16 UTC 2025

Relevant commit appears to be:

CVSROOT:	/cvs
Module name:	src
Changes by:	deraadt@cvs.openbsd.org	2025/11/15 12:00:19

Modified files:
	sys/arch/amd64/stand/libsa: softraid_amd64.c 
	sys/arch/arm64/stand/efiboot: softraid_arm64.c 
	sys/arch/i386/stand/libsa: softraid_i386.c 
	sys/arch/riscv64/stand/efiboot: softraid_riscv64.c 

Log message:
bootblock softraid code was assuming struct disklabel fits in a 512 byte
buffer.  That is no longer true, since the on-disk is being seperated
from the in-memory.
ok krw florian hshoexer