How to do a chroot

The easy way

• Install Atoms from Flatpak, and you get an easy GUI

The easy way in Void Linux

Using xvoidstrap and xchroot

Bootstrap chroot:

# mkdir <chroot_dir>
# XBPS_ARCH=<chroot_arch> xvoidstrap <chroot_dir> base-container

Chroot:

xchroot <chroot_dir>

Manually

Copy XBPS RSA keys:

# mkdir -p "<chroot_dir>/var/db/xbps/keys"
# cp -a /var/db/xbps/keys/* "<chroot_dir>/var/db/xbps/keys"

Bootstrap chroot:

# XBPS_ARCH=<chroot_arch> xbps-install -S -r <chroot_dir> -R <repository> base-container

For Networking:

# cp /etc/resolv.conf <chroot_dir>/etc
# cp /etc/hosts <chroot_dir>/etc

Chroot:

# mount -t proc none <chroot_dir>/proc
# mount -t sysfs none <chroot_dir>/sys
# mount --rbind /dev <chroot_dir>/dev
# mount --rbind /run <chroot_dir>/run

The manual way

• Optional: Run the commands as root instead of elevating at each command by sudo or doas
• Make directory and enter it
• Unpack a stage-3 Gentoo tarball / Bootstrap a distro
• Copy /etc/resolv.conf to chroot-dir/etc
  • Set permissions: chmod a+r etc/resolv.conf
• Make any other mounts
  • mkdir var/db/repos/gentoo (Gentoo example: package manager db)
• Mount the required directories
  • mount -t proc /proc proc
  • mount -R /sys sys
  • mount -R /dev dev (Steam requires /dev/shm)
  • mount -R /run run (For accessing D-Bus; essential for some optional convenience features of Steam)
  • mount -R /var/db/repos/gentoo var/db/repos/gentoo (Gentoo example)
  • If using systemd, do mount --make-rslave path for sys, dev and run

• chroot dir : Enter chroot
  • Use linux64 chroot dir if on a 32-bit OS and need to run a 64 bit program (linux64 is an alias to setarch linux64)
• env update && source /etc/profile : Load environment variables
• export PS1="(chroot) $PS1" : Set shell prompt
• Perform other configurations
  • Set locale
  • Set timezone
  • Install essential software like Mesa3D
• Exit
• Unmount mounted directories
  • umount -l proc
  • umount -l sys
  • umount -l dev
  • umount -l run
  • umount -l var/db/repos/gentoo
• Optional (For Steam on X11)
  • For Steam
    • Run id -u from outside the chroot to get the local user's UID
    • useradd -u <UID> -m -G audio,video steam : Create steam user with the same UID as the user
  • For X11 Forwarding
    • Install xhosts on the host and restart the display manager to allow X11 forwarding to chroot
    • This allows the DM or xinit to grant permissions according to xhosts to the local UID
    • A more insecure method is to run xhosts +local, which will allow any user to access X Server. Revoke this by xhosts -local

Other considerations

• Copy /lib/modules/$(uname -r) to lib/modules: For kernel modules
• Re-mount vs Bind mount
• /dev/pts
• Files like /etc/passwd, /etc/shadow, /etc/groups, sshd server keys

One thing I missed was setting up users, etc.

Example Wrapper Script for Steam

This simply makes a chroot and runs steam

#!/bin/sh

# steam chroot bits
chroot_bits="64"

# steam chroot directory
chroot_dir="/usr/local/steam64/"

# check if chroot bits is valid
if [ "${chroot_bits}" = "32" ] ; then
  chroot_arch="linux32"
elif [ "${chroot_bits}" = "64" ] ; then
  chroot_arch="linux64"
else
  printf "Invalid chroot bits value '%s'. Permitted values are '32' and '64'.\n" "${chroot_bits}"
  exit 1
fi

# check if the chroot directory exists
if [ ! -d "${chroot_dir}" ] ; then
  printf "The chroot directory '%s' does not exist!\n" "${chroot_dir}"
  exit 1
fi

# mount the chroot directories
mount -v -t proc /proc "${chroot_dir}proc"
mount -vR /sys "${chroot_dir}sys"
mount --make-rslave "${chroot_dir}sys"
mount -vR /dev "${chroot_dir}dev"
mount --make-rslave "${chroot_dir}dev"
mount -vR /run "${chroot_dir}run"
mount --make-rslave "${chroot_dir}run"
mount -vR /var/db/repos/gentoo "${chroot_dir}var/db/repos/gentoo"
# the --make-rslave flags are needed for systemd support (not needed in runit)

# chroot, substitute user, and start steam
"${chroot_arch}" chroot "${chroot_dir}" su -c 'steam' steam

# unmount the chroot directories when steam exits
umount -vl "${chroot_dir}proc"
umount -vl "${chroot_dir}sys"
umount -vl "${chroot_dir}dev"
umount -vl "${chroot_dir}run"
umount -vl "${chroot_dir}var/db/repos/gentoo"