Chroot

This material is work-in-progress ...

grsec info needs to be removed, we don't use that any more in Alpine Linux.
(Last edited by Zcrayfish on 13 Jan 2024.)

"Changing root" or "chrooting" is a method for zooming in on part of your filesystem, so that, for example, /path will refer to what was formerly accessible at /mnt/path. The "root" in the expression "chroot" refers to the root filesystem /, not to the root user. (Though typically you will need root user privileges in order to chroot.)

A common reason for chrooting is to perform maintenance on existing systems where booting and/or logging in no longer works. One has to boot the hardware somehow, such as with an Installation or Rescue CD or USB; then one mounts the broken system and chroots into it and performs the repairs. Common examples are:

reinstalling the bootloader
rebuilding the initramfs image
upgrading or downgrading packages
resetting a forgotten password

Chrooting can also be used to create and host a separate virtualized installation of a system. This can be useful for:

Testing and development, with software that's too risky to deploy on a production system.
Software can be developed, built and tested in a chroot populated only with its expected dependencies.
Can run legacy software or other software whose supporting libraries or data files can't be installed in the host system.
Open file descriptors (for files, pipelines and network connections) can be "carried into" the chroot by running programs, making it unnecessary for those programs' working files to be visible inside the chroot directory. This can help separate privileges or design sandboxes. Note that chroot is not a secure way to contain a hostile process with root privileges. See Wikipedia's discussion of some of chroot's limits.

Scripts for installing Alpine Linux that use chroot:

Preparation

All the steps in this guide will have to be performed as root user.
If you are using chroot to repair an existing Linux system, it will need to be mounted first. If you don't know the location and/or filetype of the disk(s) where it resides, read the output of fdisk -l or lsblk. We will suppose the broken system is at /dev/sdx1 and its filetype is ext3. Create a directory for mounting this disk, and mount it:
mkdir -p $CHROOT
mount -t ext3 /dev/sdx1 $CHROOT

where $CHROOT evaluates to the path you want to chroot into (for example, /mnt).
If the system directory is encrypted or in a LVM group or anything like that, we'll leave it to you to figure out what steps to take to prepare it and mount it.
If there are separate filesystems for other directories (such as /boot, /var, /usr, /home), mount them too:
mount /dev/sdx2 $CHROOT/boot
mount /dev/sdx3 $CHROOT/var
mount /dev/sdx4 $CHROOT/usr

For many repair tasks, you may not need to mount the broken system's /home directory.
If you do have /boot on a separate partition, it's essential to have it mounted before working with GRUB, performing a kernel upgrade, or anything of that sort. It's also possible to mount some of these filesystems after you've chrooted, and in some cases mounting /boot in particular after chrooting has worked better with GRUB. If you can, though, it's smarter to mount these beforehand. The reason is that if you do it from inside the chroot, the outside/host environment won't know about the mounted filesystems, so if you forget to umount them before exiting the chroot, the system won't know to umount them when it shuts down, either. That could damage those filesystems.
The script presented below will take care of generating or remounting other system directories like /dev, /proc, /sys, /tmp and so on.
The architecture of the host system you're booted into (e.g., if it's a 32bit LiveCD, that's x86) and the system you want to chroot into must match. You can determine what architecture you're booted into using uname -m.
Before chrooting, be sure any kernel modules you'll need when working inside the chroot have been loaded. For example:
modprobe dm-mod
modprobe dm-crypt
Set up your network if you'll need it inside the chroot, e.g., to install updated packages. The script presented below will copy the /etc/resolv.conf from the host system, but will prompt before overwriting an existing etc/resolv.conf inside the chroot. If you like, you can generate one from scratch by doing:
echo 'nameserver 8.8.8.8' > $CHROOT/etc/resolv.conf

Performing the chroot

The most reliable way to chroot is to use the following script:

Contents of /root/start-chroot

#!/bin/sh -e

if [ 0 -ne `id -u` ]; then
  echo "This script needs root access" >&2
  exit 1
fi

if ! [ -d "$1" ] || [ x-h = x"$*" ] || [ x--help = x"$*" ]; then
  echo "Usage: ${0##*/} <chroot_directory>" >&2
  exit 1
fi

cd "$1"
if ! [ -d ./etc ]; then
  echo "No etc directory inside $1" >&2
  exit 1
fi
shift

MOUNTED=
umount_all() {
  case $MOUNTED in
  shm\ *) if [ -L ./dev/shm ]; then
            umount ./`readlink ./dev/shm`
          else
            umount ./dev/shm
          fi
          MOUNTED=${MOUNTED#shm };;
  esac
  case $MOUNTED in
  run\ *) umount ./run
          MOUNTED=${MOUNTED#run };;
  esac
  case $MOUNTED in
  tmp\ *) umount ./tmp
          MOUNTED=${MOUNTED#tmp };;
  esac
  case $MOUNTED in
  proc\ *) umount ./proc
          MOUNTED=${MOUNTED#proc };;
  esac
  case $MOUNTED in
  sys\ *) umount ./sys
          MOUNTED=${MOUNTED#sys };;
  esac
  case $MOUNTED in
  pts\ *) umount ./dev/pts
          MOUNTED=${MOUNTED#pts };;
  esac
  case $MOUNTED in
  dev\ *) umount ./dev
          MOUNTED=${MOUNTED#dev };;
  esac
}
trap 'umount_all' EXIT

#mkdir -p ./etc ./dev/pts ./sys ./proc ./tmp ./run ./boot ./root

cp -iL /etc/resolv.conf ./etc/ || true  # if ^C, will cancel script

mount --bind /dev ./dev
MOUNTED="dev $MOUNTED"

mount -t devpts devpts ./dev/pts -o nosuid,noexec
MOUNTED="pts $MOUNTED"

mount -t sysfs sys ./sys -o nosuid,nodev,noexec,ro
MOUNTED="sys $MOUNTED"

mount -t proc proc ./proc -o nosuid,nodev,noexec
MOUNTED="proc $MOUNTED"

mount -t tmpfs tmp ./tmp -o mode=1777,nosuid,nodev,strictatime
MOUNTED="tmp $MOUNTED"
mount -t tmpfs run ./run -o mode=0755,nosuid,nodev
MOUNTED="run $MOUNTED"
if [ -L ./dev/shm ]; then
  mkdir -p ./`readlink ./dev/shm`
  mount -t tmpfs shm ./`readlink ./dev/shm` -o mode=1777,nosuid,nodev
else
  #mkdir -p ./dev/shm
  mount -t tmpfs shm ./dev/shm -o mode=1777,nosuid,nodev
fi
MOUNTED="shm $MOUNTED"

case $1 in
  -l) shift;;
  -l*) one=${1#-l}; shift; set -- -"$one" "$@";;
esac
chroot . /usr/bin/env -i SHELL=/bin/sh HOME=/root TERM="$TERM" \
  PATH=/usr/sbin:/usr/bin:/sbin:/bin PS1='chroot # ' /bin/sh -l "$@"

# FIXME
# are USER and LOGNAME set automatically?
# perhaps: source /etc/profile && export PS1="chroot $PS1"

Copy that script to some location on your system, such as /root/start-chroot, run chmod +x on it, and then invoke it as:

/root/start-chroot $CHROOT

Todo: Comment on Environment variables, extra arguments to start-chroot

It is certainly possible to perform a chroot by hand, without using this script. For some purposes, you can get away with a simpler procedure than the script follows. But it's hard to catalogue in advance when taking shortcuts will get you in trouble, so it's best to just get in the habit of using this script (or doing the same things it does). As you read around in various wikis (perhaps including this one), you will encounter a variety of instructions for how to chroot by hand. These instructions often differ in small details from each other and are usually not as thorough as the above script.

Troubleshooting

If you see the error:

chroot: cannot run command '/bin/sh': Exec format error, it is likely that the architectures of the host environment and the chroot environment do not match. For example, you booted with a 32bit Live CD and are trying to chroot into an x86_64 system.
chroot: '/bin/sh': Permission denied, try remounting the system with exec permissions: mount -o remount,exec $CHROOT

Inside the chroot

At this point, you're still running the kernel you booted with, but all paths /path will refer to what used to be $CHROOT/path.

If you have multiple terminals open on or connected to the running system, the chroot will only be effective in the terminal that you performed it in.

Now you can perform whatever troubleshooting (or do whatever development work) you need to do:

resintall GRUB to your disk's MBR
rebuild your initramfs image
fix your /etc/fstab
perform a kernel upgrade (or downgrade)
(re)install other packages using your package manager
reset a forgotten password

or whatever.

If you'll be doing anything with GRUB, you should first make sure that sure your /etc/mtab file is up-to-date:

grep -v rootfs /proc/mounts > /etc/mtab

Run graphical applications from the chroot

If you have an X server running on your outside/host system, you can start graphical applications from inside the chroot. To allow the chroot environment to connect to the X server, you have to do this in the host system (either before chrooting, or in a terminal other than the one you performed the chroot in):

xhost +local:

Then, to direct applications from the chroot to the X server, set the DISPLAY environment variable inside the chroot to match its value in the host environment (for the user who owns the X server). So for example, inside the chroot you might say:

export DISPLAY=:0

If you're running a Grsec enabled kernel and the graphical application fails to connect, you may need to allow abstract unix domain sockets outside of chroot.

Run the following command on the host as root user:

sysctl -w kernel.grsecurity.chroot_deny_unix=0

Cleaning up

When you're finished, ensure that all running programs have stopped. Then exit the chroot:

exit

If you started the chroot using the start-chroot script, it will take care of umounting /dev, /proc, /sys, /tmp, and so on. Else you should do this by hand. In any case, you should umount any partitions like /boot that you mounted yourself, and then umount the system partition at $CHROOT if appropriate.

If you get an error saying that $CHROOT (or any partition inside it) is busy, this can mean one of two things:

A program was left running inside of the chroot.
Or more frequently, a mountpoint still exists on this mount. For example, $CHROOT/usr is still mounted when trying to umount $CHROOT.

In the latter case, simply unmount the offending mountpoint first. To get a reminder of all the current mountpoints, run mount with no parameters.

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