Re: [OE-core] [PATCH v2 4/4] oe-selftest: overlayfs: Add a demo case for /etc

[Vyacheslav Yurkov <uvv.mail@gmail.com>]

On 08.01.2026 14:46, Paul Barker wrote:
> On Wed, 2025-12-24 at 14:16 +0000, Vyacheslav Yurkov via
> lists.openembedded.org wrote:
>> From: Vyacheslav Yurkov <uvv.mail@gmail.com>
>>
>> /etc is a special directory. It's possible to create a mount unit for
>> it, but many system services that start early at boot time will not
>> rescan its content when you mount overlay on top. The added test case
>> demonstrates this by adding a sample systemd service and enabling it in
>> overlay.
>>
>> Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>
>> ---
>>   meta/lib/oeqa/selftest/cases/overlayfs.py | 98 +++++++++++++++++++++++
>>   1 file changed, 98 insertions(+)
>>
>> diff --git a/meta/lib/oeqa/selftest/cases/overlayfs.py b/meta/lib/oeqa/selftest/cases/overlayfs.py
>> index 3e55e97927..426b4ccee3 100644
>> --- a/meta/lib/oeqa/selftest/cases/overlayfs.py
>> +++ b/meta/lib/oeqa/selftest/cases/overlayfs.py
>> @@ -263,6 +263,104 @@ EOT
>>   
>>           self._test_correct_image('systemd-machine-units', systemd_machine_unit_append)
>>   
>> +    @skipIfNotMachine("qemux86-64", "tests are qemux86-64 specific currently")
>> +    def test_etc_mount(self):
>> +        """
>> +        Summary:   /etc is not supposed to be used with overlayfs.bbclass
>> +        Expected:  Observe inconsistencies after using etc overlay with a mount unit
>> +        Author:    Vyacheslav Yurkov <uvv.mail@gmail.com>
>> +        """
>> +        systemd_machine_unit_append = """
>> +SYSTEMD_SERVICE:${PN} += " \
>> +    data.mount \
>> +    etc.mount \
>> +"
>> +
>> +do_install:append() {
>> +    install -d ${D}${systemd_system_unitdir}
>> +    install -d ${D}${ROOT_HOME}
>> +    cat <<EOT > ${D}${systemd_system_unitdir}/etc.mount
>> +[Unit]
>> +Description=OverlayFS mount for /etc directory
>> +DefaultDependencies=no
>> +RequiresMountsFor=/data
>> +
>> +[Mount]
>> +What=overlay
>> +Where=/etc
>> +Type=overlay
>> +Options=lowerdir=/etc,upperdir=/data/overlay/etc,workdir=/data/overlay-workdir/etc
>> +[Install]
>> +WantedBy=local-fs.target
>> +EOT
>> +
>> +    cat <<EOT >${D}${systemd_system_unitdir}/data.mount
>> +[Unit]
>> +Description=Persistent storage partition
>> +
>> +[Mount]
>> +What=/dev/sda3
>> +Where=/data
>> +Type=ext4
>> +Options=defaults
>> +
>> +[Install]
>> +WantedBy=local-fs.target
>> +EOT
>> +    cat <<EOT > ${D}${ROOT_HOME}/test-daemon.service
>> +[Unit]
>> +Description=My one-shot task
>> +After=local-fs.target
>> +
>> +[Service]
>> +Type=oneshot
>> +ExecStart=/usr/bin/echo test
>> +RemainAfterExit=yes
>> +
>> +[Install]
>> +WantedBy=multi-user.target
>> +EOT
>> +}
>> +
>> +FILES:${PN} += "${ROOT_HOME}"
>> +"""
>> +
>> +        config = """
>> +IMAGE_INSTALL:append = " systemd-machine-units"
>> +DISTRO_FEATURES:append = " overlayfs"
>> +
>> +# Use systemd as init manager
>> +INIT_MANAGER = "systemd"
>> +
>> +# enable overlayfs in the kernel
>> +KERNEL_EXTRA_FEATURES:append = " features/overlayfs/overlayfs.scc"
>> +
>> +IMAGE_FSTYPES += "wic"
>> +WKS_FILE = "overlayfs_etc.wks.in"
>> +OVERLAYFS_ROOTFS_TYPE = "ext4"
>> +"""
>> +
>> +        self.write_config(config)
>> +
>> +        machine_inc = """
>> +OVERLAYFS_MOUNT_POINT[etc] = "/etc"
>> +OVERLAYFS_QA_SKIP[mnt-overlay] = "mount-configured"
>> +"""
>> +        self.set_machine_config(machine_inc)
>> +        self.write_recipeinc('systemd-machine-units', systemd_machine_unit_append)
>> +        bitbake('core-image-minimal')
>> +        with runqemu('core-image-minimal', image_fstype='wic', discard_writes=False) as qemu:
>> +            test_daemon_path = get_bb_var('ROOT_HOME') + '/test-daemon.service'
>> +            status, output = qemu.run_serial('cp -f ' + test_daemon_path + ' /etc/systemd/system/')
>> +            status, output = qemu.run_serial("systemctl enable test-daemon")
>> +            status, output = qemu.run_serial("sync")
>> +        with runqemu('core-image-minimal', image_fstype='wic') as qemu:
>> +            # Check the test service status. It's enabled and supposed to start, but it didn't
>> +            status, output = qemu.run_serial("systemctl is-enabled test-daemon")
>> +            self.assertTrue("enabled" in output, msg=output)
>> +            status, output = qemu.run_serial("systemctl is-active test-daemon")
>> +            self.assertTrue("inactive" in output, msg=output)
>> +
> What is actually being tested here? It looks like we're just confirming
> that incorrect behaviour occurs. Why do we want to ensure that the
> behaviour stays incorrect?
>
> If someone is installing an 'etc.mount' unit into the rootfs that
> behaves like this then there's probably nothing we can do within Yocto
> Project to help them. I suppose you could use overlayfs for /etc if you
> set it up from an initramfs before pivoting to the main rootfs.
>
> Best regards,
>

Correct, and this is what overlayfs-etc class is for. I've seen attempts 
to use the mount unit instead and this is just a proof of concept that 
it doesn't work.

Slava