readonly root via overlayfs

etc_initramfs-tools_hooks_overlay

#!/bin/sh
# goes in /etc/initramfs-tools/hooks/overlay

# Ensure that overlayfs is loaded.

PREREQ=""
prereqs()
{
    echo "$PREREQ"
}

case $1 in
    prereqs)
        prereqs
        exit 0
        ;;
esac

. /usr/share/initramfs-tools/hook-functions

manual_add_modules overlay

etc_initramfs-tools_scripts_init-bottom_overlay

#!/bin/sh
# goes in /etc/initramfs-tools/scripts/init-bottom/overlay

PREREQ=""
prereqs()
{
    echo "$PREREQ"
}

case $1 in
    prereqs)
        prereqs
        exit 0
        ;;
esac

. /scripts/functions

RW_SIZE=256m

# Should we run at all?

OVERLAY="no"
for arg in $(cat /proc/cmdline)
do
    case "$arg" in
        overlay=*)
            OVERLAY=${arg#overlay=}
            ;;
        *)
            ;;
    esac
done

if [ "$OVERLAY" != "yes" ]
then
    log_warning_msg "overlay not enabled, leaving the root fs alone"
    exit 0
fi

log_begin_msg "Remounting root fs in overlay mode"

if ! modprobe -qb overlay
then
    log_failure_msg "can't load overlay module"
    exit 0
fi

# initially we have:

#  /           initramfs
#  ${rootmnt}  readonly-mounted real root FS  (mounted from device ${ROOT})
#
# We want:
#
#  /              initramfs
#  ${rootmnt}     overlayfs using ${rootmnt}/ro and ${rootmnt}/rw
#  ${rootmnt}/ro  readonly-mounted real root FS  (mounted from device ${ROOT})
#  ${rootmnt}/rw  tmpfs that will be used for changes
#  ${rootmnt}/rw/upper  overlayfs upper dir
#  ${rootmnt}/rw/work   overlayfs work dir
#

# create and mount /ro and /rw on the initramfs, we will move them later
for dir in /ro /rw
do
  [ -d "${dir}" ] || mkdir -p "${dir}"
  if [ $? -ne 0 ]
  then
    log_failure_msg "can't create ${dir}"
    exit 0
  fi
done

# set up /rw
if ! mount -t tmpfs -o "size=${RW_SIZE}" overlay-rw /rw
then
    log_failure_msg "can't mount tmpfs on /rw"
    exit 0
fi

for dir in /rw/upper /rw/work
do
  if ! mkdir -p ${dir}
  then
      log_failure_msg "overlay: can't create ${dir}"
      exit 0
  fi
done

# set up /ro
if ! mount -o move "${rootmnt}" /ro
then
    log_failure_msg "can't move root fs to /ro"
    exit 0
fi

# set up an overlayfs on rootmnt
if ! mount -t overlay -o lowerdir=/ro,upperdir=/rw/upper,workdir=/rw/work overlay-root "${rootmnt}"
then
    log_failure_msg "can't move root fs to /ro"

    # try to recover
    if ! mount -o move /ro "${rootmnt}"
    then
        panic "recovering the old root fs failed, panicking"
        exit 0
    fi

    log_warning_msg "overlay: moved regular rootfs back into place and continuing"
    exit 0
fi

for dir in "${rootmnt}/ro" "${rootmnt}/rw"
do
  [ -d "${dir}" ] || mkdir -p "${dir}"
  if [ $? -ne 0 ]
  then
    log_failure_msg "can't create ${dir}"
    exit 0
  fi
done

# move /ro and /rw into place so we can access them later
if ! mount -o move /ro "${rootmnt}/ro"
then
    log_failure_msg "can't move /ro to ${rootmnt}/ro"
    exit 0
fi

if ! mount -o move /rw "${rootmnt}/rw"
then
    log_failure_msg "can't move /rw to ${rootmnt}/rw"
    exit 0
fi

# populate fstab on the new root

{
    cat <<EOF
# overlayfs mount is in use; changes to the root FS are
# written to tmpfs and will be discarded on reboot.
#
# to disable, pass "overlay=no" on the kernel command line
# (in /boot/cmdline.txt)
#
# the underlying root FS is mounted readonly on /ro
# temporary changes are written to /rw

EOF

    {
        while read dev dir fstype fsopts freq pass
        do
            if [ "${dir}" = "${rootmnt}" ]
            then
                echo "${dev} / ${fstype} ${fsopts} ${freq} ${pass}"
            fi
        done
    } </proc/mounts

    echo "# original fstab follows"

    {
        while read dev dir fstype fsopts freq pass
        do
            if [ "${dir}" != "/" ]
            then
                echo "${dev} ${dir} ${fstype} ${fsopts} ${freq} ${pass}"
            fi
        done
    } <"${rootmnt}/ro/etc/fstab"
} >"${rootmnt}/etc/fstab"

log_end_msg
exit 0

OVERLAYFS

- install the two shellscripts into the appropriate places under /etc/initramfs-tools
- run update-initramfs
- put "overlay=yes" on the kernel command line
- reboot

With the overlay in place, the real root is mounted readonly on /ro.
Only the root fs is changed, other filesystems are mounted normally.

Remove "overlay=yes" (or change it to something other than yes) and reboot to go back to readwrite.
(This probably means that you want the commandline config to live somewhere other than on the root fs, e.g. under /boot)