chroot.sh

#!/bin/bash
set -e

# this is mostly copied from arch-chroot

BASE="$(realpath $(dirname "$0"))"
LFS="$BASE/root"

die() {
    printf "ERROR: "
    echo "$@"
    exit 1
}

add_mount() {
    mount "$@" && CHROOT_ACTIVE_MOUNTS=("$2" "${CHROOT_ACTIVE_MOUNTS[@]}")
}

add_mount_lazy() {
    mount "$@" && CHROOT_ACTIVE_LAZY=("$2" "${CHROOT_ACTIVE_LAZY[@]}")
}

bind_device() {
    touch "$2" && CHROOT_ACTIVE_FILES=("$2" "${CHROOT_ACTIVE_FILES[@]}")
    add_mount "$1" "$2" --bind
}

add_link() {
    ln -sf "$1" "$2" && CHROOT_ACTIVE_FILES=("$2" "${CHROOT_ACTIVE_FILES[@]}")
}

pid_unshare="unshare --fork --pid"
mount_unshare="$pid_unshare --mount --map-auto --map-root-user --setuid 0 --setgid 0"

setup() {
    CHROOT_ACTIVE_MOUNTS=()
    CHROOT_ACTIVE_LAZY=()
    CHROOT_ACTIVE_FILES=()
    [[ $(trap -p EXIT) ]] && die cannot set EXIT trap
    trap 'cleanup' EXIT

    add_mount_lazy "$1" "$1" --bind
    add_mount proc "$1/proc" -t proc -o nosuid,noexec,nodev
    add_mount_lazy /sys "$1/sys" --rbind
    add_link "/proc/self/fd" "$1/dev/fd"
    add_link "/proc/self/fd/0" "$1/dev/stdin"
    add_link "/proc/self/fd/1" "$1/dev/stdout"
    add_link "/proc/self/fd/2" "$1/dev/stderr"
    bind_device /dev/full "$1/dev/full"
    bind_device /dev/null "$1/dev/null"
    bind_device /dev/random "$1/dev/random"
    bind_device /dev/tty "$1/dev/tty"
    bind_device /dev/urandom "$1/dev/urandom"
    bind_device /dev/zero "$1/dev/zero"
    add_mount run "$1/run" -t tmpfs -o nosuid,nodev,mode=0755
    add_mount tmp "$1/tmp" -t tmpfs -o mode=1777,strictatime,nodev,nosuid
    add_mount shm "$1/dev/shm" -t tmpfs -o mode=1777,nosuid,nodev

    add_mount "$BASE/sources" "$1/sources" --bind
    add_mount "$BASE/packages" "$1/packages" --bind
    add_mount "$BASE/pkgspecs" "$1/pkgspecs" --bind
}

cleanup() {
    if (( ${#CHROOT_ACTIVE_MOUNTS[@]} )); then
        umount "${CHROOT_ACTIVE_MOUNTS[@]}"
    fi
    unset CHROOT_ACTIVE_MOUNTS

    if (( ${#CHROOT_ACTIVE_LAZY[@]} )); then
        umount --lazy "${CHROOT_ACTIVE_LAZY[@]}"
    fi
    unset CHROOT_ACTIVE_LAZY

    if (( ${#CHROOT_ACTIVE_FILES[@]} )); then
        rm "${CHROOT_ACTIVE_FILES[@]}"
    fi
    unset CHROOT_ACTIVE_FILES
}

declare_all() {
    declare -p | grep -Fvf <(declare -rp)
    declare -pf
}

do_chroot() {
    (( EUID == 0 )) || die do_chroot must be run as root
    [[ -d $chrootdir ]] || die path $chrootdir is not a directory

    setup "$chrootdir"

    chroot_args=()
    [[ $userspec ]] && chroot_args+=(--userspec "$userspec")

    user=$(echo "$userspec" | cut -d: -f1)
    [[ "$user" ]] || user="root"
    # FIXME uids don't work here
    if [[ "$user" == "root" ]]; then
        homedir="/root"
    else
        homedir="/home/$user"
    fi

    $pid_unshare /sbin/chroot "${chroot_args[@]}" -- "$chrootdir" /usr/bin/env -i HOME="$homedir" TERM="$TERM" PS1='(lfs chroot) \u:\w\$ ' PATH=/usr/bin:/usr/sbin "${args[@]}"
}

while getopts ':hu:' flag; do
    case $flag in
        h)
            cat <<EOF
usage: $(basename "$0") [newroot] [command ...]

    -h                  Print this help message
    -u <user>[:group]   Specify non-root user and optional group to use

If 'command' is unspecified, $(basename "$0") will launch /bin/bash.

If 'newroot' is unspecified, chroot.sh will use:
$LFS

EOF
            exit 0
            ;;
        u)
            userspec=$OPTARG
            ;;
        :)
            die option requires an argument -- $OPTARG
            ;;
        ?)
            die invalid option -- $OPTARG
            ;;
    esac
done
shift $(( OPTIND - 1 ))

chrootdir="$LFS"
(( $# )) && chrootdir=$1 && shift

args=("/bin/bash" "--login" "+h")
[[ "$@" ]] && args=("$@")

$mount_unshare bash -c "$(declare_all); do_chroot"