Hi Adrian,
I don't think using /etc with overlay is a wise idea. You'd like to keep
system configuration updatable, which wouldn't be simple if the whole
/etc is an overlay (TBH, I'm not even sure it's possible that way. You'd
probably need to write custom scripts and add initramfs here). But you
can export to overlay individual user-specific configuration, for
example /etc/wpa_supplicant.
Vyacheslav
On 05.06.2021 09:37, Adrian Freihofer wrote:
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 <[email protected]>
---
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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