Hide devices in chrooted environment

Question

On Linux, I'm looking for a way to run grub-install <device> and update-grub in a chrooted environment without some devices being taken into account by GRUB (I don't want them to appear at boot).

From a system on a disk A, I want to chroot to a disk B with its own system, to install grub there. So grub installs itself on B with entries A and B. It works but when I boot disk B (without A) I see all the entries (including A), which I don't want. Hence my question.

What I tested...

When I booted a fresh Debian Buster install on a disk /dev/sda with a second disk /dev/sdb with too a fresh Debian Buster install, I had :

root@buster:~# blkid -s UUID
/dev/sda1: UUID="0505963d-a522-415c-ba85-57bac4b7e0ae"
/dev/sda5: UUID="40e956ea-05c7-4099-b9c8-3b0c97780db0"
/dev/sdb1: UUID="afc267fa-5f9b-464a-b9c0-02437f83b28f"
/dev/sdb5: UUID="ae03e23b-cdbf-4b46-96d1-0f1b0b5ac13b"

=> 2 disks are listed

The GRUB menu boot was (I wrote a command to display UUID too and the result is in french) :

Debian GNU/Linux [0505963d-a522-415c-ba85-57bac4b7e0ae]
Options avancées pour Debian GNU/Linux [0505963d-a522-415c-ba85-57bac4b7e0ae]

=> only /dev/sda1 is concerned

So, I prepared the chroot environment :

root@buster:~# DEST=/dev/sdb
root@buster:~# mkdir -p /mnt${DEST}1
root@buster:~# mount -t ext4 ${DEST}1 /mnt${DEST}1
root@buster:~# for i in /dev /proc /sys /run /sys ; \
               do mount -B $i /mnt${DEST}1$i ; done
root@buster:~# DEST=$DEST chroot /mnt${DEST}1

From the chrooted environment :

root@buster:/# cat /etc/fstab 
...
UUID=afc267fa-5f9b-464a-b9c0-02437f83b28f /               ext4    ...
UUID=ae03e23b-cdbf-4b46-96d1-0f1b0b5ac13b none            swap    ...

root@buster:/# blkid -s UUID
/dev/sda1: UUID="0505963d-a522-415c-ba85-57bac4b7e0ae"
/dev/sda5: UUID="40e956ea-05c7-4099-b9c8-3b0c97780db0"
/dev/sdb1: UUID="afc267fa-5f9b-464a-b9c0-02437f83b28f"
/dev/sdb5: UUID="ae03e23b-cdbf-4b46-96d1-0f1b0b5ac13b"

=> the chrooted environment saw the same things as the NON-chrooted one

I installed grub on it :

root@buster:/# grub-install ${DEST}
root@buster:/# update-grub

=> success, no errors !

But when the GRUB menu is displayed (the same command as these used above) :

Debian GNU/Linux [afc267fa-5f9b-464a-b9c0-02437f83b28f]
Options avancées pour Debian GNU/Linux [afc267fa-5f9b-464a-b9c0-02437f83b28f]
Debian GNU/Linux 10 (buster) (sur /dev/sda1) [0505963d-a522-415c-ba85-57bac4b7e0ae]
Options avancées pour Debian GNU/Linux 10 (buster) (sur /dev/sda1) [0505963d-a522-415c-ba85-57bac4b7e0ae]

=> the partitions /dev/sdb1 and /dev/sda1 are listed, but I do NOT want /dev/sda1

So I am looking for a way to install GRUB in a chrooted environment without some devices being taken into account by GRUB.

I searched in web but I did not find a way to achieve this and if it is possible. How can I do it?

---

OK so I found a solution that more or less satisfies me which is pretty system agnostic.

The idea is to make os-prober NOT executable before installing GRUB and reverting back after (source of idea) :

os_prober_path=$( which os-prober ) && perms=$( getfacl -e $os_prober_path ) \ 
  && chmod a-x $os_prober_path
grub-install ${DEST}
update-grub
[[ $os_prober_path ]] && echo "$perms" |setfacl -M- $os_prober_path

Finally we have :

# needs : gawk acl
DEST=/dev/sdb
mkdir -p /mnt${DEST}1
mount -t ext4 ${DEST}1 /mnt${DEST}1
for i in /dev /proc /sys /run /sys ; do mount -B $i /mnt${DEST}1$i; done
DEST=$DEST chroot /mnt${DEST}1
os_prober_path=$( which os-prober ) && perms=$( getfacl -e $os_prober_path ) \
  && chmod a-x $os_prober_path
grub-install ${DEST}
update-grub
[[ $os_prober_path ]] && echo "$perms" |setfacl -M- $os_prober_path
exit
for i in /dev /proc /sys /run /sys ; do umount -l /mnt${DEST}1$i; done
umount -l /mnt${DEST}1
rmdir /mnt${DEST}1

But I always search a way to disable some devices. So if you have another idea...

Answer 1

It looks like you are trying are trying to control the way grub menus are generated. These are generally configured from scripts in /etc/grub.d. I believe the one you are specifically interested in is "os-prober" which finds other installed operating systems.

Of course if you wanted to remove all other operating systems you could just disable os-prober all togeather with chmod ugo-x /etc/grub.d/30_os-prober to stop the script being executable.

The normal case is that other operating systems are not mounted, so the mount points are unlikely to affect the way this script behaves. From memory it attempts to mount drives to investigate what's installed on them.

I see from your scripts that you are bind mounting /dev into your chroot environment. If it's possible to control os-prober then it will be by removing drives from /dev.
You could try copying the device files instead of bind mounting /dev. Then you are free to delete whichever you wish before running update-grub.

Failing that I'm afraid you might be forced to completely disable os-prober and then create your own script that mimics it minus the unwanted drives.