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File name : overlayroot-chroot

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#!/bin/sh
#
#    overlayroot-chroot - chroot wrapper script for overlayroot
#    Copyright (C) 2012 Dustin Kirkland <kirkland@ubuntu.com>
#
#    Authors: Dustin Kirkland <kirkland@ubuntu.com>
#
#    This program is free software: you can redistribute it and/or modify
#    it under the terms of the GNU General Public License as published by
#    the Free Software Foundation, version 3 of the License.
#
#    This program is distributed in the hope that it will be useful,
#    but WITHOUT ANY WARRANTY; without even the implied warranty of
#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#    GNU General Public License for more details.
#
#    You should have received a copy of the GNU General Public License
#    along with this program.  If not, see <http://www.gnu.org/licenses/>.

set -e
set -f # disable path expansion
REMOUNTS=""

error() {
	printf "ERROR: $@\n" 1>&2
}
fail() { [ $# -eq 0 ] || error "$@"; exit 1; }

info() {
	printf "INFO: $@\n" 1>&2
}

get_lowerdir() {
	local overlay=""
	overlay=$(awk \
		'$1 == "overlayroot" && $2 == "/" { print $0 }' /proc/mounts)
	if [ -n "${overlay}" ]; then
		lowerdir=${overlay##*lowerdir=}
		lowerdir=${lowerdir%%,*}
		if mountpoint "${lowerdir}" >/dev/null; then
			_RET="${lowerdir}"
		else
			fail "Unable to find the overlayroot lowerdir"
		fi
	else
		fail "Unable to find an overlayroot filesystem"
	fi
}

clean_exit() {
	local mounts="$1" rc=0 d="" lowerdir="" mp=""
	for d in ${mounts}; do
		if mountpoint ${d} >/dev/null; then
			umount ${d} || rc=1
		fi
	done
	for mp in $REMOUNTS; do
		mount -o remount,ro "${mp}" ||
			error "Note that [${mp}] is still mounted read/write"
	done
	[ "$2" = "return" ] && return ${rc} || exit ${rc}
}

# Try to find the overlayroot filesystem
get_lowerdir
lowerdir=${_RET}

recurse_mps=$(awk '$1 ~ /^\/dev\// && $2 ~ starts { print $2 }' \
	starts="^$lowerdir/" /proc/mounts)

mounts=
for d in proc run sys; do
	if ! mountpoint "${lowerdir}/${d}" >/dev/null; then
		mount -o bind "/${d}" "${lowerdir}/${d}" || fail "Unable to bind /${d}"
		mounts="$mounts $lowerdir/$d"
		trap "clean_exit \"${mounts}\" || true" EXIT HUP INT QUIT TERM
	fi
done

# Remount with read/write
for mp in "$lowerdir" $recurse_mps; do
	mount -o remount,rw "${mp}" &&
		REMOUNTS="$mp $REMOUNTS" ||
		fail "Unable to remount [$mp] writable"
done
info "Chrooting into [${lowerdir}]"
chroot ${lowerdir} "$@"

# Clean up mounts on exit
clean_exit "${mounts}" "return"
trap "" EXIT HUP INT QUIT TERM

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