How to boot from GPT partition without "bootme" attribute?

[Mark Millard]

Lev Serebryakov lev at FreeBSD.org wrote on
Tue Oct 30 18:37:14 UTC 2018 :

>  I have disk with GPT scheme and three partitions:
> 
> p1 - freebsd-boot
> p2 - freebsd-ufs
> p3 - freebsd-ufs
> 
>  pmbr is installed on this disk, and gptboot is installed on p1. Both p2
> and p3 contains valid FreeBSD installation, with /boot/loader, kernel,
> and everything.
> 
>  I have attribute "bootme" set on p3, but not on p2.
> 
>  What should I do to boot from p2?
> 
>  I've tried to interrupt gptboot and override its choice:
> 
>  0:ad(0p3)/boot/loader
> 
>  with
> 
>  0:ad(0p2)/boot/loader
> 
>  After that loader, loaded from p2, loads kernel from p3 and boots
> system from p3!

Are the kernel's on p2 and p3 distinct in an identifiable way?
Can you be sure it was not a mix of the p2 kernel and p3 world
that booted? I ask because . . .

One way to control what world is booted is to adjust the
/etc/fstab where the /boot/kernel/kernel is loaded from,
having that /etc/fstab to point to a different / area. I do
this on small, single board computers to get the kernel
from a microsd card but world from a USB storage media
device. (I tend to use some form of labeling style
reference to avoid device numbering dependencies.) The
/etc/fstab where world is from has / agreeing and directs
swap partition bindings and such that are appropriate to
the specific world.

(I've frequently had a world on the microsd card that the
initial /etc/fstab can be edited to point to. This gives
me a way to boot if there is a problem for the USB media.)

I've done such things in gpt and non-gpt contexts.

Any chance that that /etc/fstab initially used points
to p3's world for / ?

There are also things like /boot/loader.conf having
something like:

vfs.root.mountfrom='ufs:/dev/gpt/MyRoot'

to control where things are booted from.

>  If I have MBR, I could override "active" slice in boot0 MBR loader
> interactively.
> 
>  Is it analogous feature for GPT?



===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)