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 | 133 +++++++++++++++++++++++++++++++++
1 file changed, 133 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..40e6107f0d
--- /dev/null
+++ b/meta/classes/overlayfs.bbclass
@@ -0,0 +1,133 @@
+# 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. This has to be done in your machine configuration.
+# QA check fails to catch file existence if you redefine this variable in your
recipe!
+#
+# OVERLAYFS_MOUNT_POINT[data] ?= "/data"
+#
+# The class assumes you have a data.mount systemd unit defined in your
+# systemd-machine-units recipe 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
+# want to have a writable location on the file system, but not interested
where the data
+# survive a reboot. Then 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"
+#
+# Note: the class does not support /etc directory itself, because systemd
depends on it
+
+REQUIRED_DISTRO_FEATURES += "systemd overlayfs"
+
+inherit systemd features_check overlayfs-qa
+
+def strForBash(s):
+ return s.replace('\\', '\\\\')
+
+def unitFileList(d):
+ fileList = []
+ overlayMountPoints = d.getVarFlags("OVERLAYFS_MOUNT_POINT")
+
+ if not overlayMountPoints:
+ bb.fatal("A recipe uses overlayfs class but there is no
OVERLAYFS_MOUNT_POINT set in your MACHINE configuration")
+
+ # check that we have required mount points set first
+ requiredMountPoints = d.getVarFlags('OVERLAYFS_WRITABLE_PATHS')
+ for mountPoint in requiredMountPoints:
+ if mountPoint not in overlayMountPoints:
+ bb.fatal("Missing required mount point for
OVERLAYFS_MOUNT_POINT[%s] in your MACHINE configuration" % mountPoint)
+
+ for mountPoint in overlayMountPoints:
+ for path in d.getVarFlag('OVERLAYFS_WRITABLE_PATHS',
mountPoint).split():
+ fileList.append(mountUnitName(path))
+ fileList.append(helperUnitName(path))
+
+ return fileList
+
+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
+"""
+ 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
--
2.28.0
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