Load any kernel , mount any filesystem as root , with a small ufs boot partition

[Alain De Vos]

Using a small ufs partition you have the flexibility to boot almost anything.
How.
Create a small ufs partition.

gpart  add  -t freebsd-ufs  -a 4k  -s SIZE  -l LABEL   GEOM

This creates a new device.
Format to ufs:

newfs  -O 2  -U  -j  -L VOLNAME  DEVICE

mount the DEVICE,

mkdir /mnt/bootpartition
mount DEVICE  /mnt/bootpartition

Copy over the /boot folder

cp -axfvR /boot /mnt/bootpartition

Now edit the /mnt/bootpartition/boot/loader.conf file.
Three settings are important.

The partition where to boot from e.g., (the loader is intelligent so it can mostly be omited)

loaddev="disk2p9:"

The default device to load the kernel from e.g.,

currdev="zfs:MYZPOOL/ROOT/default:"

The root partition to mount, e.g. for booting on zpool MYZPOOL

vfs.root.mountfrom="zfs:MYZPOOL/ROOT/default"

When you load a kernel not located on the bootpartition the bootpartion itself does not need to have kernel.
So you can

rm -vfR /mnt/bootpartition/kernel
rm -vfR /mnt/bootpartition/kernel.old

Done

umount /mnt/bootpartition

The freebsd bootloader resided on this bootpartition can now very easily be loaded via linux-grub using chainloading. For instance if the freebsd bootpartition is the first partition of the first disk with gpt,
Edit on the linux system /etc/grub.d/40_custom and add,

menuentry "FreeBSD" {
    insmod part_bsd
    insmod part_gpt
    insmod part_msdos
    insmod ufs2
    insmod chain
    root=(hd0,gpt1)
    kfreebsd /boot/loader
}

update-grub