[PATCH] initramfs-framework: Add overlayroot module

[Alejandro Enedino Hernandez Samaniego <alhe@linux.microsoft.com>]

When installed, this module mounts a read-write (RW) overlay on
top of a root filesystem, which is kept read-only (RO).

It needs to be executed after the initramfs-module-rootfs since
it relies on it to mount the filesystem at initramfs startup but
before the finish module which normally switches root.

It requires rootrw=<foo> to be passed as a kernel parameter to
specify the device/partition to be used as RW by the overlay and
has a dependency on overlayfs support being present in the
running kernel.

It does not require the read-only IMAGE_FEATURE to be enabled.

Signed-off-by: Alejandro Enedino Hernandez Samaniego <alhe@linux.microsoft.com>
---
 .../initramfs-framework/overlayroot           | 93 +++++++++++++++++++
 .../initrdscripts/initramfs-framework_1.0.bb  |  9 ++
 2 files changed, 102 insertions(+)
 create mode 100644 meta/recipes-core/initrdscripts/initramfs-framework/overlayroot

diff --git a/meta/recipes-core/initrdscripts/initramfs-framework/overlayroot b/meta/recipes-core/initrdscripts/initramfs-framework/overlayroot
new file mode 100644
index 0000000000..ec5700e8fc
--- /dev/null
+++ b/meta/recipes-core/initrdscripts/initramfs-framework/overlayroot
@@ -0,0 +1,93 @@
+#!/bin/sh
+
+# Simple initramfs module intended to mount a read-write (RW)
+# overlayfs on top of /, keeping the original root filesystem
+# as read-only (RO).
+#
+# NOTE: The read-only IMAGE_FEATURE is not required for this to work
+#
+# It relies on the initramfs-module-rootfs to mount the original
+# root filesystem, and requires 'rootrw=<foo>' to be passed as a
+# kernel parameter, specifying the device/partition intended to
+# use as RW.
+#
+# It also has a dependency on overlayfs being enabled in the
+# running kernel via KERNEL_FEATURES (kmeta) or any other means.
+#
+# The RO root filesystem remains accessible by the system, mounted
+# at /rofs
+
+PATH=/sbin:/bin:/usr/sbin:/usr/bin
+
+# We get OLDROOT from the rootfs module
+OLDROOT="/rootfs"
+
+NEWROOT="${RWMOUNT}/root"
+RWMOUNT="/overlay"
+ROMOUNT="${RWMOUNT}/rofs"
+UPPER_DIR="${RWMOUNT}/upper"
+WORK_DIR="${RWMOUNT}/work"
+
+MODULES_DIR=/init.d
+
+exit_gracefully() {
+    echo $1 >/dev/console
+    echo >/dev/console
+    echo "OverlayRoot mounting failed, starting system as read-only" >/dev/console
+    echo >/dev/console
+
+    # Make sure / is mounted as read only anyway.
+    # Borrowed from rootfs-postcommands.bbclass
+    # Tweak the mount option and fs_passno for rootfs in fstab
+    if [ -f ${OLDROOT}/etc/fstab ]; then
+        sed -i -e '/^[#[:space:]]*\/dev\/root/{s/defaults/ro/;s/\([[:space:]]*[[:digit:]]\)\([[:space:]]*\)[[:digit:]]$/\1\20/}' ${OLDROOT}/etc/fstab
+    fi
+
+    # Tweak the "mount -o remount,rw /" command in busybox-inittab inittab
+    if [ -f ${OLDROOT}/etc/inittab ]; then
+        sed -i 's|/bin/mount -o remount,rw /|/bin/mount -o remount,ro /|' ${OLDROOT}/etc/inittab
+    fi
+
+    # Continue as if the overlayroot module didn't exist
+    . $MODULES_DIR/99-finish
+    eval "finish_run"
+}
+
+
+if [ -z "$bootparam_rootrw" ]; then
+    exit_gracefully "rootrw= kernel parameter doesn't exist and its required to mount the overlayfs"
+fi
+
+mkdir -p ${RWMOUNT}
+
+# Mount RW device
+if mount -n -t ${bootparam_rootfstype:-ext4} -o ${bootparam_rootflags:-defaults} ${bootparam_rootrw} ${RWMOUNT}
+then
+    # Set up overlay directories
+    mkdir -p ${UPPER_DIR}
+    mkdir -p ${WORK_DIR}
+    mkdir -p ${NEWROOT}
+    mkdir -p ${ROMOUNT}
+
+    # Remount OLDROOT as read-only
+    mount -o bind ${OLDROOT} ${ROMOUNT}
+    mount -o remount,ro ${ROMOUNT}
+
+    # Mount RW overlay
+    mount -t overlay overlay -o lowerdir=${ROMOUNT},upperdir=${UPPER_DIR},workdir=${WORK_DIR} ${NEWROOT} || exit_gracefully "initramfs-overlayroot: Mounting overlay failed"
+else
+    exit_gracefully "initramfs-overlayroot: Mounting RW device failed"
+fi
+
+# Set up filesystems on overlay
+mkdir -p ${NEWROOT}/proc
+mkdir -p ${NEWROOT}/dev
+mkdir -p ${NEWROOT}/sys
+mkdir -p ${NEWROOT}/rofs
+
+mount -n --move ${ROMOUNT} ${NEWROOT}/rofs
+mount -n --move /proc ${NEWROOT}/proc
+mount -n --move /sys ${NEWROOT}/sys
+mount -n --move /dev ${NEWROOT}/dev
+
+exec chroot ${NEWROOT}/ ${bootparam_init:-/sbin/init} || exit_gracefully "Couldn't chroot into overlay"
diff --git a/meta/recipes-core/initrdscripts/initramfs-framework_1.0.bb b/meta/recipes-core/initrdscripts/initramfs-framework_1.0.bb
index 9e8c1dc3ab..4e76e20026 100644
--- a/meta/recipes-core/initrdscripts/initramfs-framework_1.0.bb
+++ b/meta/recipes-core/initrdscripts/initramfs-framework_1.0.bb
@@ -18,6 +18,7 @@ SRC_URI = "file://init \
            file://e2fs \
            file://debug \
            file://lvm \
+           file://overlayroot \
           "
 
 S = "${WORKDIR}"
@@ -49,6 +50,9 @@ do_install() {
     # lvm
     install -m 0755 ${WORKDIR}/lvm ${D}/init.d/09-lvm
 
+    # overlayroot needs to run after rootfs module but before finish
+    install -m 0755 ${WORKDIR}/overlayroot ${D}/init.d/91-overlayroot
+
     # Create device nodes expected by some kernels in initramfs
     # before even executing /init.
     install -d ${D}/dev
@@ -64,6 +68,7 @@ PACKAGES = "${PN}-base \
             initramfs-module-rootfs \
             initramfs-module-debug \
             initramfs-module-lvm \
+            initramfs-module-overlayroot \
            "
 
 FILES:${PN}-base = "/init /init.d/99-finish /dev"
@@ -107,3 +112,7 @@ FILES:initramfs-module-debug = "/init.d/00-debug"
 SUMMARY:initramfs-module-lvm = "initramfs lvm rootfs support"
 RDEPENDS:initramfs-module-lvm = "${PN}-base"
 FILES:initramfs-module-lvm = "/init.d/09-lvm"
+
+SUMMARY:initramfs-module-overlayroot = "initramfs support for mounting a RW overlay on top of a RO root filesystem"
+RDEPENDS:initramfs-module-overlayroot = "${PN}-base initramfs-module-rootfs"
+FILES:initramfs-module-overlayroot = "/init.d/91-overlayroot"
-- 
2.25.1