init-bottom_overlay

#!/bin/sh
# based on https://gist.github.com/mutability/6cc944bde1cf4f61908e316befd42bc4
# 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