Hi Vyacheslav
It would be very nice to have such a feature in Yocto.
If I remember correctly I tried to do that once with an overlay on
/etc. However, that did not work with systemd. Did you ever try to add
/etc as a mount point?
On Thu, 2021-06-03 at 16:21 +0200, Vyacheslav Yurkov wrote:
> It's often desired in Embedded System design to have a read-only rootfs.
> But a lot of different applications might want to have a read-write access
> to some parts of a filesystem. It can be especially useful when your update
> mechanism overwrites the whole rootfs, but you want your application data
> to be preserved between updates. This class provides a way to achieve that
> by means of overlayfs and at the same time keeping the base rootfs read-only.
>
> Signed-off-by: Vyacheslav Yurkov <uvv.m...@gmail.com>
> ---
> meta/classes/overlayfs.bbclass | 132 +++++++++++++++++++++++++++++++++
> 1 file changed, 132 insertions(+)
> create mode 100644 meta/classes/overlayfs.bbclass
>
> diff --git a/meta/classes/overlayfs.bbclass b/meta/classes/overlayfs.bbclass
> new file mode 100644
> index 0000000000..7fac3e696e
> --- /dev/null
> +++ b/meta/classes/overlayfs.bbclass
> @@ -0,0 +1,132 @@
> +# Class for generation of overlayfs mount units
> +#
> +# It's often desired in Embedded System design to have a read-only rootfs.
> +# But a lot of different applications might want to have a read-write access
> to
> +# some parts of a filesystem. It can be especially useful when your update
> mechanism
> +# overwrites the whole rootfs, but you want your application data to be
> preserved
> +# between updates. This class provides a way to achieve that by means
> +# of overlayfs and at the same time keeping the base rootfs read-only.
> +#
> +# Usage example.
> +#
> +# Set a mount point for a partition overlayfs is going to use as upper layer
> +# in your machine configuration. Underlying file system can be anything that
> +# is supported by overlayfs
> +#
> +# OVERLAYFS_MOUNT_POINT[data] ?= "/data"
> +#
> +# The class assumes you have a data.mount systemd unit defined elsewhere in
> your
> +# BSP and installed to the image.
> +#
> +# Then you can specify writable directories on a recipe base
> +#
> +# OVERLAYFS_WRITABLE_PATHS[data] = "/usr/share/my-custom-application"
> +#
> +# To support several mount points you can use a different variable flag.
> Assume we
> +# what to have a writable location on the file system, but not interested
> where the data
> +# survive a reboot. The we could have a mnt-overlay.mount unit for a tmpfs
> file system:
> +#
> +# OVERLAYFS_MOUNT_POINT[mnt-overlay] = "/mnt/overlay"
> +# OVERLAYFS_WRITABLE_PATHS[mnt-overlay] = "/usr/share/another-application"
> +
> +OVERLAYFS_WRITABLE_PATHS[data] ?= ""
> +
> +inherit systemd
> +
> +def strForBash(s):
> + return s.replace('\\', '\\\\')
> +
> +def unitFileList(d):
> + fileList = []
> + overlayMountPoints = d.getVarFlags("OVERLAYFS_MOUNT_POINT")
> + for mountPoint in overlayMountPoints:
> + for path in d.getVarFlag('OVERLAYFS_WRITABLE_PATHS',
> mountPoint).split():
> + fileList.append(mountUnitName(path))
> + fileList.append(helperUnitName(path))
> +
> + return fileList
> +
> +# this function is based on
> https://github.com/systemd/systemd/blob/main/src/basic/unit-name.c
> +def escapeSystemdUnitName(path):
> + escapeMap = {
> + '/': '-',
> + '-': "\\x2d",
> + '\\': "\\x5d"
> + }
> + return "".join([escapeMap.get(c, c) for c in path.strip('/')])
> +
> +def mountUnitName(unit):
> + return escapeSystemdUnitName(unit) + '.mount'
> +
> +def helperUnitName(unit):
> + return escapeSystemdUnitName(unit) + '-create-upper-dir.service'
> +
> +python do_create_overlayfs_units() {
> + CreateDirsUnitTemplate = """[Unit]
> +Description=Overlayfs directories setup
> +Requires={DATA_MOUNT_UNIT}
> +After={DATA_MOUNT_UNIT}
> +DefaultDependencies=no
> +
> +[Service]
> +Type=oneshot
> +ExecStart=mkdir -p {DATA_MOUNT_POINT}/workdir{LOWERDIR} && mkdir -p
> {DATA_MOUNT_POINT}/upper{LOWERDIR}
> +RemainAfterExit=true
> +StandardOutput=journal
> +
> +[Install]
> +WantedBy=multi-user.target
The mounting of overlays should probably happen very early at boot,
before services start using the desired folders.
Regards,
Adrian
> +"""
> + MountUnitTemplate = """[Unit]
> +Description=Overlayfs mount unit
> +Requires={CREATE_DIRS_SERVICE}
> +After={CREATE_DIRS_SERVICE}
> +
> +[Mount]
> +What=overlay
> +Where={LOWERDIR}
> +Type=overlay
> +Options=lowerdir={LOWERDIR},upperdir={DATA_MOUNT_POINT}/upper{LOWERDIR},workdir={DATA_MOUNT_POINT}/workdir{LOWERDIR}
> +
> +[Install]
> +WantedBy=multi-user.target
> +"""
> +
> + def prepareUnits(data, lower):
> + args = {
> + 'DATA_MOUNT_POINT': data,
> + 'DATA_MOUNT_UNIT': mountUnitName(data),
> + 'CREATE_DIRS_SERVICE': helperUnitName(lower),
> + 'LOWERDIR': lower,
> + }
> +
> + with open(os.path.join(d.getVar('WORKDIR'), mountUnitName(lower)),
> 'w') as f:
> + f.write(MountUnitTemplate.format(**args))
> +
> + with open(os.path.join(d.getVar('WORKDIR'), helperUnitName(lower)),
> 'w') as f:
> + f.write(CreateDirsUnitTemplate.format(**args))
> +
> + overlayMountPoints = d.getVarFlags("OVERLAYFS_MOUNT_POINT")
> + for mountPoint in overlayMountPoints:
> + for lower in d.getVarFlag('OVERLAYFS_WRITABLE_PATHS',
> mountPoint).split():
> + prepareUnits(d.getVarFlag('OVERLAYFS_MOUNT_POINT', mountPoint),
> lower)
> +}
> +
> +# we need to generate file names early during parsing stage
> +python () {
> + unitList = unitFileList(d)
> + for unit in unitList:
> + d.appendVar('SYSTEMD_SERVICE_' + d.getVar('PN'), ' ' + unit);
> + d.appendVar('FILES_' + d.getVar('PN'), strForBash(unit))
> +
> + d.setVar('OVERLAYFS_UNIT_LIST', ' '.join([strForBash(s) for s in
> unitList]))
> +}
> +
> +do_install_append() {
> + install -d ${D}${systemd_system_unitdir}
> + for unit in ${OVERLAYFS_UNIT_LIST}; do
> + install -m 0444 ${WORKDIR}/${unit} ${D}${systemd_system_unitdir}
> + done
> +}
> +
> +addtask create_overlayfs_units before do_install
>
>
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