rootfs: keep /var writable, even if the rootfs is read-only

Having a read-only root filesystem is always a source of pain and trouble.
Many applications and tools expect to be able to store their state or
caching data or at least their logs somewhere in the filesystem.

The '/var' directory tree has a well known structure according to the
"File System Hierarchy Standard" and is used by all carefully designed
programs. Thus, this change provides a way to have this '/var' directory
tree writable, even if the main root filesystem is mounted read-only. It
uses an overlay filesystem and by default a RAM disk to store changed and
added data to this directory tree in a non persistent manner.

Due to the nature of the overlay filesystem the underlaying files from the
main root filesystem can still be accessed.

This approach requires the overlay filesystem support from the Linux
kernel. In order to use it, the feature CONFIG_OVERLAY_FS must be enabled.

The ugly details to establish the required overlaying filesystem are hidden
behind a "mount helper" for a dummy filesystem (here called 'varoverlayfs').
Thus, a BSP can change the overlaying filesystem by providing its own
'run-varoverlay.mount' in order to restrict the default RAM disk
differently or to switch to a different local storage.

The '/etc/fstab' file gets touched in this change, to enable some already
used RAM disks on demand, to gain backward compatibility if no overlay
approach is used.

Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>

4 files changed, 34 insertions, 3 deletions

diff --git a/projectroot/etc/fstab b/projectroot/etc/fstab
index 0121c3076..364b495a9 100644
--- a/projectroot/etc/fstab
+++ b/projectroot/etc/fstab
@@ -11,6 +11,6 @@ debugfs	/sys/kernel/debug	debugfs	noauto					0 0
 # ramdisks
 tmpfs	/tmp			tmpfs	nosuid,nodev,mode=1777,size=20%		0 0
 tmpfs	/run			tmpfs	nosuid,nodev,strictatime,mode=0755	0 0
-tmpfs	/var/log		tmpfs	nosuid,nodev,noexec,mode=0755,size=10%	0 0
-tmpfs	/var/lock		tmpfs	nosuid,nodev,noexec,mode=0755,size=1M	0 0
-tmpfs	/var/tmp		tmpfs	nosuid,nodev,mode=1777,size=20%		0 0
+@VAR_OVERLAYFS@tmpfs	/var/log		tmpfs	nosuid,nodev,noexec,mode=0755,size=10%	0 0
+@VAR_OVERLAYFS@tmpfs	/var/lock		tmpfs	nosuid,nodev,noexec,mode=0755,size=1M	0 0
+@VAR_OVERLAYFS@tmpfs	/var/tmp		tmpfs	nosuid,nodev,mode=1777,size=20%		0 0
diff --git a/projectroot/usr/lib/systemd/system/run-varoverlayfs.mount b/projectroot/usr/lib/systemd/system/run-varoverlayfs.mount
new file mode 100644
index 000000000..c067b9b96
--- /dev/null
+++ b/projectroot/usr/lib/systemd/system/run-varoverlayfs.mount
@@ -0,0 +1,9 @@
+[Unit]
+Description=Overlay for '/var'
+Before=local-fs.target
+
+[Mount]
+Where=/run/varoverlayfs
+What=tmpfs
+Type=tmpfs
+Options=size=20%
diff --git a/projectroot/usr/lib/systemd/system/var.mount b/projectroot/usr/lib/systemd/system/var.mount
new file mode 100644
index 000000000..bd6350237
--- /dev/null
+++ b/projectroot/usr/lib/systemd/system/var.mount
@@ -0,0 +1,11 @@
+[Unit]
+Description=Writable support for '/var'
+After=run-varoverlayfs.mount
+Before=local-fs.target
+
+[Mount]
+Where=/var
+# note: this is a dummy filesystem only to trigger the corresponding mount helper
+What=varoverlayfs
+Type=varoverlayfs
+Options=metacopy=on
diff --git a/projectroot/usr/sbin/mount.varoverlayfs b/projectroot/usr/sbin/mount.varoverlayfs
new file mode 100644
index 000000000..f8fc8c88f
--- /dev/null
+++ b/projectroot/usr/sbin/mount.varoverlayfs
@@ -0,0 +1,11 @@
+#!/bin/sh -e
+# Mount helper tool to mount some kind of writable filesystem over '/var'
+# (which might be read-only).
+# What kind of filesystem is used to mount over '/var' can be controlled via
+# the 'run-varoverlayfs.mount' mount unit and is usually a RAM disk.
+
+mkdir -p /run/varoverlayfs/upper
+mkdir -p /run/varoverlayfs/work
+mount -t overlay -olowerdir=/var,upperdir=/run/varoverlayfs/upper,workdir=/run/varoverlayfs/work "${@}"
+systemctl stop run-varoverlayfs.mount
+rmdir /run/varoverlayfs