Dear maintainers,

Commit 7dfd6208 ("don't hide /dev/md* arrays if they are provisioned on
whole-disk devices - in that case the md array itself is partitionable,
e.g. for md-fakeraid like imsm (Intel Matrix raid). (Closes: #699431)")
was intended to enable automatic partitioning on Intel Matrix fakeRAID
and keep disabling it on standard Linux md RAID. But it actually allows
almost any md RAID array.

[1] <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=699431>
<https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=699430>

The main logic is implemented in the following function:

is_wholedisk_mdraid () {
	local device="`echo $1 | sed -e 's!/\([0-9]*\)$!\1!'`"
	local mddisk=${device#/dev/}
	local ret=0
	local d

	[ -d /sys/block/$mddisk/md ] || return 1

	for d in /sys/block/$mddisk/slaves/*; do
		case "$d" in
			dm-*|md*)
				;;
			*p[0-9]|*p[0-9][0-9])
				ret=1
				break
				;;
		esac
	done

	return $ret
}

1) `/sys/block/$mddisk/slaves/*` cannot match `dm-*|md*`. Besidses IIUC
it would allow RAID over RAID, LVM or dm-crypt devices, so what is the
point of this check ?

2) `*p[0-9]|*p[0-9][0-9]` is intended to match partitions, but it
actually matches only partitions on devices whose node names end with a
digit (e.g. nvme or mmcblk) and have 'p' before the partition number. So
it does not match on SCSI-like (ATA, USB) drives.

3) RAID members not being partitions does not even guarantee that the
array is fakeRAID. Standard Linux md RAID can use whole unpartitioned
drives as members too. Automatic partitioning on such layout may result
in an unbootable system for both EFI (UEFI firmware cannot read the EFI
partition on Linux md RAID) and BIOS (GRUB may fail to install).

AFAIK d-i no longer supports fakeRAID with dm-raid and supports only
Intel Matrix RAID (IMSM) with md-raid. Is Intel Matrix RAID still used
and should it be supported by d-i ?

If yes, is_wholedisk_mdraid() should be replaced with a proper detection
of IMSM RAID arrays.

If no, either the above commit should be reverted in order to skip any
md RAID array, or any md RAID array should be allowed under the user's
responsibility (possibly resulting in an unbootable system).

Opinions ?