Booting a Ubuntu base in qemu

Ubuntu_base.md

How to build a Minimal System

• Create filesystems

  1. Create a raw image file of 1G size

     dd if=/dev/zero of=ubuntu_base.img bs=1G count=1

  2. Installing EXT4 file system using mkfs

     mkfs.ext4 ubuntu_base.img

• Mount

  1. Create a folder for mounting

     sudo mkdir /mnt/loopback

  2. Mount it

     sudo mount ubuntu_base.img /mnt/loopback

• Unpacking system files

  sudo tar -xpf /mnt/e/Downloads/ubuntu-base-22.04.1-base-amd64.tar.gz -C /mnt/loopback

  Ubuntu base can be downloaded from https://cdimage.ubuntu.com/ubuntu-base/releases

• Installation using arch-chroot

  sudo apt install arch-install-scripts
  sudo arch-chroot /mnt/loopback

• A few adjustments to the system

  tee /etc/default/locale <<EOF
  LANGUAGE=en_US.UTF-8
  LANG=en_US.UTF-8
  LC_CTYPE=en_US.UTF-8
  EOF
  
  echo "LANG=en_US.UTF-8" > /etc/local.conf
  
  tee -a /etc/fstab <<EOF
  tmpfs /tmp tmpfs rw,nosuid,nodev,size=99%
  tmpfs /var/log tmpfs rw,nosuid,nodev
  EOF
  
  tee /etc/hosts <<EOF
  127.0.0.1       localhost localhost.localdomain
  127.0.1.1       localhost.localdomain
  EOF

• Update software sources

  apt update && apt upgrade -y

• Install the required packages

  apt install -y linux-image-virtual extlinux setools lzip patchelf e2fsprogs python3-minimal sudo finit-sysv qemu-guest-agent

• Install bootloader

  mkdir -p /boot/syslinux
  
  tee /boot/syslinux/syslinux.cfg <<EOF
  PROMPT 0
  TIMEOUT 0
  DEFAULT Base
  
  LABEL Base
          LINUX ../vmlinuz
          APPEND root=/dev/sda rw
          INITRD ../initrd.img
  EOF
  
  extlinux --install /boot/syslinux

• Cheanup

  apt clean

• Other additional work, such as creating users, etc

  echo \
  '#!/bin/sh
  . /usr/share/initramfs-tools/hook-functions
  copy_exec /sbin/e2fsck /sbin
  copy_exec /sbin/fsck /sbin
  copy_exec /sbin/fsck.ext2 /sbin
  copy_exec /sbin/fsck.ext4 /sbin
  copy_exec /sbin/logsave /sbin' \
  | dd of=/etc/initramfs-tools/hooks/e2fsck.sh
  
  chmod +x /etc/initramfs-tools/hooks/e2fsck.sh
  
  update-initramfs -u
  
  adduser --disabled-password lsposed
  passwd -d lsposed
  adduser lsposed adm
  adduser lsposed sudo
  chmod u+w /etc/sudoers
  echo "lsposed ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
  chmod u-w /etc/sudoers
  
  tee /sbin/autologin <<EOF
  #!/bin/sh
  exec /usr/bin/login -f lsposed
  EOF
  
  chmod +x /sbin/autologin
  
  echo "tty [123456] /sbin/getty -L -n -l /sbin/autologin 0 /dev/tty1 linux nowait noclear" >> /etc/finit.conf
  
  dd if=/dev/zero of=/swapfile  bs=1M count=200
  chmod 0600 /swapfile
  mkswap /swapfile
  tee -a /etc/fstab <<EOF
  /swapfile    none    swap    sw    0   0
  EOF

• Exit chroot

  exit

• Unmounting the image

  sudo umount /mnt/loopback

• Shrinking the image

  sudo e2fsck -pf ubuntu_base.img
  resize2fs -M ubuntu_base.img

• Use qemu-img to shrink the image

  qemu-img convert -f raw -c -O qcow2 ubuntu_base.img ubuntu_base.qcow2

• Boot in qemu

  qemu-system-x86_64 ubuntu_base.qcow2 -m 6g -smp 12 --accel whpx --snapshot

It is not convenient to use bash as init, consider using finit-sysv