Create a hybrid USB Image

The goal is to create an image file which can be copied onto a USB stick and booted both via legacy BIOS as well as UEFI.

This document assumes

• a local loop device is used (much faster than directly on USB stick)
• the target USB device is /dev/sdb
• locally, the new system will be mounted under /mnt

Environment variables:

export USB_DEV=/dev/sdb
# note the trailing 'p'!
export TARGET=/dev/loop0p
export MNT_TGT=/mnt

Please be extra careful setting those and ensure these are set at every step, as otherwise this may render the host system unusable.

Preparation

1. Create an empty file slightly smaller than the minimal USB stick size supported, e.g. 7GiB:

   apt-get --yes install pv
   pv --stop-at-size --size 7G /dev/zero > /tmp/usb.img
   

2. make image available as block device

   losetup -fP /tmp/usb.img
   

3. Check the correct loop device is enabled:

   losetup -a
   

4. Create an initial partition table with enough room for MBR/!BIOS, (U)EFI and system, e.g.

   parted --script --align optimal ${TARGET%p} -- mktable gpt
   parted --script --align optimal ${TARGET%p} -- mkpart biosgrub fat32 1MiB 4MiB
   parted --script --align optimal ${TARGET%p} -- mkpart efi fat32 4MiB 512MiB
   parted --script --align optimal ${TARGET%p} -- mkpart linux ext4 512MiB 1024MiB
   parted --script --align optimal ${TARGET%p} -- mkpart linux ext4 1024MiB -1
   parted --script --align optimal ${TARGET%p} -- set 1 bios_grub on
   parted --script --align optimal ${TARGET%p} -- set 2 esp on
   

5. Check layout

   parted ${TARGET%p} -- print

   should show

            
   Model: Loopback device (loopback)
   Disk /dev/loop0: 7516MB
   Sector size (logical/physical): 512B/512B
   Partition Table: gpt
   Disk Flags: 
   
   Number  Start   End     Size    File system  Name      Flags
    1      1049kB  4194kB  3146kB               biosgrub  bios_grub
    2      4194kB  537MB   533MB                efi       boot, esp
    3      537MB   1074MB  537MB                linux
    4      1074MB  7515MB  6441MB               linux
   

6. Create file systems

   mkfs -t vfat -n EFIGRUB ${TARGET}2
   mkfs -t ext4 -L BOOT ${TARGET}3
   mkfs -t ext4 -L SYSTEM ${TARGET}4
   

7. Create first backup image in case something goes wrong

   apt install pigz pv
   pv ${TARGET%p} | pigz -c > /tmp/bootimage.rescue.gz

8. Mount file systems

   mkdir -p $MNT_TGT/
   mount ${TARGET}4 $MNT_TGT
   mkdir -p $MNT_TGT/boot
   mount ${TARGET}3 $MNT_TGT/boot
   mkdir -p $MNT_TGT/boot/efi
   mount ${TARGET}2 $MNT_TGT/boot/efi
   

Installing Debian

1. Bootstrap Debian

   debootstrap buster $MNT_TGT
   

2. Create various mounts/bind-mounts

   mount -t sysfs sys $MNT_TGT/sys
   mount -t proc proc $MNT_TGT/proc
   mount --bind /dev $MNT_TGT/dev
   mount --bind /dev/pts $MNT_TGT/dev/pts
   

3. log into chroot

   chroot $MNT_TGT /bin/bash

4. change hostname

   echo atlastender > /etc/hostname
   

5. add password for root, e.g. bench

   echo "root:bench" | chpasswd --crypt-method SHA512

6. setup /etc/fstab

   (
   echo -e "UUID=\"$(blkid -s UUID -o value ${TARGET}4)\"\t/\text4\terrors=remount-ro\t0\t1"
   echo -e "UUID=\"$(blkid -s UUID -o value ${TARGET}3)\"\t/boot\text4\terrors=remount-ro\t0\t2"
   echo -e "UUID=\"$(blkid -s UUID -o value ${TARGET}2)\"\t/boot/efi\tvfat\tdefaults\t0\t2"
   echo -e "tmpfs\t\t/tmp\ttmpfs\tdefaults,nodev,nosuid,size=2500m\t0\t0" ) > /etc/fstab
   

7. install kernel and BIOS grub (when asked where to install do not install it)

   apt-get --yes --no-install-recommends install linux-image-amd64
   DEBIAN_FRONTEND=noninteractive apt-get --yes --no-install-recommends install grub-pc
   

8. install EFI grub resulting in removal of grub-pc

   apt-get --yes install grub-efi
   sed -i 's/main$/main contrib non-free/' /etc/apt/sources.list
   apt-get update
   apt-get --yes install firmware-linux firmware-linux-free firmware-linux-nonfree
   

9. ensure grub is really installed

   grub-install --removable --no-nvram --no-uefi-secure-boot --target=x86_64-efi
   grub-install --removable --target=i386-pc ${TARGET%p}
   update-grub
   

Install software into image

Now install any software wanted into the chroot

Finishing steps

1. leave the chroot with exit
2. umount everything

   umount $MNT_TGT/boot/efi
   umount $MNT_TGT/boot
   umount $MNT_TGT/dev/pts
   umount $MNT_TGT/dev
   umount $MNT_TGT/proc
   umount $MNT_TGT/sys
   umount $MNT_TGT
   

3. remove loop back device, e.g.

   losetup -D

4. copy image to USB stick

   pv /tmp/usb.img > $USB_DEV
   

-- CarstenAulbert - 03 Jun 2019