overctl

#!/bin/sh

BOOT=/boot
CMDLINE=cmdline.txt
CMDLINE_RO=cmdline.txt.overlay
CMDLINE_RW=cmdline.txt.orig

print_usage() {
	cat <<-TEXT
		Usage: ${0##*/} [-h|-r|-s|-t|-w]
		   -h, --help     This message
		   -r, --ro       Set read-only root with overlay fs
		   -s, --status   Show current state
		   -t, --toggle   Toggle between -r and -w
		   -w, --rw       Set read-write root
	TEXT
}

# Check whether user requests help
[ "$1" = "--help" -o "$1" = "-h" ] && { print_usage; exit 0; }

# Change to the working directory
cd $BOOT

# Check that the files we need are present and readable
for file in $CMDLINE $CMDLINE_RO $CMDLINE_RW; do
	[ -r $file ] || { echo "ERROR: missing file $file" >&2 && exit 1; }
done

# Process arg (if any)
case "$1" in
	--ro|-r)	action=RO
			;;
	--rw|-w)	action=RW
			;;
	--status|-s)	action=status
			;;
	--toggle|-t)	action=toggle
			;;
	*)		print_usage
			exit 1
			;;
esac

line_active=$(cat $CMDLINE)
line_ro=$(cat $CMDLINE_RO)
line_rw=$(cat $CMDLINE_RW)

# Check which CMDLINE is in use, this doubles as a sanity check
if [ "$line_active" = "$line_ro" ]; then
	state=RO
elif [ "$line_active" = "$line_rw" ]; then
	state=RW
else
	echo "ERROR: $CMDLINE matches neither $CMDLINE_RO, nor $CMDLINE_RW" >&2
	exit 1
fi

check_permission() {
	# Check that we are running as root
	[ $(id -u) -eq 0 ] || { echo "ERROR: requires root privileges" >&2; exit 1; }

	# Check that we can write to CMDLINE, otherwise try to remount BOOT
	if ! touch $CMDLINE 2>/dev/null; then
		mount -o remount,rw $BOOT 2>/dev/null
		[ $? -eq 0 ] || { echo "ERROR: failed to mount $BOOT RW" >&2; exit 1; }
		boot_remount=y
	fi

	return 0
}

# Take action
if [ "$action" = "status" ]; then
	str=$(mount | grep ' on / ')

	if echo $str | grep -q 'overlay'; then
		echo -n "/ is currently mounted RO" >&2
	elif echo $str | grep -q 'rw'; then
		echo -n "/ is currently mounted RW" >&2
	else
		echo "ERROR: unable to determine mount options for /" >&2
		exit 1
	fi

	echo " and will be mounted $state on next boot" >&2
	exit 0
elif [ "$action" = "$state" ]; then
	echo "/ is already set to be mounted $state on next boot" >&2
	exit 1
elif [ "$state" = "RO" ]; then
	check_permission && cp $CMDLINE_RW $CMDLINE
	newstate=RW
elif [ "$state" = "RW" ]; then
	check_permission && cp $CMDLINE_RO $CMDLINE
	newstate=RO
fi

echo "/ will be mounted $newstate on next boot" >&2

# Remount BOOT read-only if that was the way we found it
if [ "$boot_remount" = "y" ]; then
	mount -o remount,ro $BOOT 2>/dev/null
	[ $? -eq 0 ] || { echo "failed to mount $BOOT RO" >&2; exit 1; }
fi

# The End
exit 0