Include initial init script and build script

.gitignore

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+target
+busybox-cache
+

README.md

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 # initramfs-overlay
-Initramfs focussed on mounting a read only image with writeable overlay using overlayfs
+Initramfs focussed on mounting a read only image with writeable overlay using overlayfs.
+
+## Disk partitions
+The following disk partitions are expected, currently located by label:
+* The image partition (ext4,LABEL=img): Contains the OS squashfs image to mount and boot.
+* The overlay partition (ext4,LABEL=overlay): Writeable partition overlaid with the OS image, cleared each boot.
+* The boot partition (vfat,LABEL=boot): Optional for initframfs, contains the kernel plus squashfs module (if not in kernel).
+* The new image partition (ext4,LABEL=newimg): Optional, can be used to provide an updated OS image.
+
+### The boot partition
+If the partition is detected, the squashfs module will be loaded from /modules/<kernel-version>/squashfs.ko if the module existed. Otherwise it is assumed the kernel has build-in support for squashfs.
+
+### The overlay partition
+This is used as the writeable area of the overlay and consists of three directories which are created if not preset:
+* /upper: Used for the 'upperdir' of the overlayfs. This is where all deleted/modified/created files are stored. This is cleared on each boot.
+* /work: Used for the 'workdir' of the overlayfs.
+* /persistent: Used as an additional 'lowerdir' of the overlayfs. As the 'upperdir' is cleared, this read-only lower dir allows customisations to be made compared to the read-only OS image.
+
+### The image partition
+This must contain a single file, root-squashfs.img which contains the full operating system.
+
+### The new image partition.
+If the partition is detected and contains an image file with the same name as the one that will be mounted from the image partition it will be moved into the image partition. The previous image will be renamed <name>-old.img
+
+## Project structure
+The build.sh script will generate two initramfs files per architecture into a 'target' directory.
+* init-<version>-<arch>.gz - The 'standard' initramfs. This should be used in most cases.
+* init-<version>-<arch>-debug.gz - The 'debug' initramfs. This will load a shell before booting to allow any checks to made, type 'exit' to boot. Also it will load a shell if an error occurs, rather than exiting immediately resulting in a shutdown.
+
+Currently three architectures are built:
+* armv6l
+* i686
+* x86_64
+
+The build will include all files under 'src' in the intramfs. A busybox binary (cached into 'busybox-cache') is also placed under /bin/busybox and corresponding symlinks for all commands supported by busybox.

build.sh

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+#!/bin/bash
+set -e
+
+PROJECT_VERSION="0.1.0"
+
+BUSYBOX_VERSION="1.21.1"
+BUSYBOX_BASE_URL="https://busybox.net/downloads/binaries/$BUSYBOX_VERSION"
+declare -A BUSYBOX_MD5=(
+	["armv6l"]="d9b9d2245c2df526a8e373340f1e3b5a"
+	["i686"]="130e3ccda88aa313faee5be1af8ec21b"
+	["x86_64"]="d1a32f63fef8e639a63d85e46753c0ae" )
+
+function verifyBusybox {
+	arch=$1
+	md5=($(md5sum busybox-cache/busybox-$arch))
+	if [[ $md5 != ${BUSYBOX_MD5[$arch]} ]]; then
+		return 1
+	fi
+}
+
+# Clean
+if [ -e target ]; then
+    rm -rf target
+fi
+mkdir target
+mkdir -p busybox-cache
+
+# Build image for each arch
+for arch in ${!BUSYBOX_MD5[@]}; do
+	# Download/verify busybox binaries
+	if ! verifyBusybox $arch; then
+		echo "File busybox-$arch does not exist or failed checksum validation, downloading..."
+		wget $BUSYBOX_BASE_URL/busybox-$arch -O busybox-cache/busybox-$arch
+		if ! verifyBusybox $arch; then
+			echo "Checksum validation failed for busybox-$arch" >&2
+			exit 1
+		fi
+	fi
+	
+	# Create base
+	INIT_DIR=target/$arch
+	mkdir -p $INIT_DIR/{boot,bin,dev,etc,lib,mnt,proc,sbin,sys,tmp}
+	cp -rf src/* $INIT_DIR
+	cp -f busybox-cache/busybox-$arch $INIT_DIR/bin/busybox
+	chmod 755 $INIT_DIR/bin/busybox
+	for bbcmd in $($INIT_DIR/bin/busybox --list); do
+	    ln -s busybox $INIT_DIR/bin/$bbcmd
+	done
+	cd $INIT_DIR
+	find . | cpio -H newc -o | gzip > ../init-$PROJECT_VERSION-$arch.gz
+	cd ../..
+
+	# Create debug (start shell before switching root, start shell on error instead of exiting)
+	cp -r $INIT_DIR $INIT_DIR-debug
+	INIT_DIR=$INIT_DIR-debug
+	sed -i -r '/# End mounting OS filesystems/a sh' $INIT_DIR/init
+	sed -i -r 's/exit 1/sh/' $INIT_DIR/init
+	cd $INIT_DIR
+	find . | cpio -H newc -o | gzip > ../init-$PROJECT_VERSION-$arch-debug.gz
+	cd ../..
+done

src/init

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+#!/bin/busybox sh
+
+MAX_WAIT=10
+BOOT_DEVICE="LABEL=boot"
+IMG_DEVICE="LABEL=img"
+NEWIMG_DEVICE="LABEL=newimg"
+OVERLAY_DEVICE="LABEL=overlay"
+OS_IMAGE="root-squash.img"
+
+### Begin creating mount points
+for dir in root img newimg lower overlay; do
+	mkdir -p /mnt/$dir
+done
+### End creating mount points
+
+### Begin mounting special fs
+mount -t devtmpfs none /dev
+mount -t proc none /proc
+mount -t sysfs none /sys
+### End mounting special fs
+
+### Begin mounting OS file systems
+
+# Wait for devices to become available
+ATTEMPTS=0
+while [[ $ATTEMPTS -lt $MAX_WAIT ]] && ! findfs $OVERLAY_DEVICE > /dev/null; do
+	sleep 1
+	let ATTEMPTS=ATTEMPTS+1
+done
+if [[ $ATTEMPTS -eq $MAX_WAIT ]]; then
+	echo "***** Boot device not detected: $OVERLAY_DEVICE *****" >&2
+	echo "Exiting..." >&2
+	sleep 5
+	exit 1
+fi
+
+# Mount device containing updated image (if any)
+mount -t ext4 $NEWIMG_DEVICE /mnt/newimg
+if [[ -e /mnt/newimg/$OS_IMAGE ]]; then
+	# Mount device containing current image RW and replace with updated image
+	mount -t ext4 $IMG_DEVICE /mnt/img
+	mv -f /mnt/newimg/$OS_IMAGE /mnt/img/$OS_IMAGE-new
+	mv -f /mnt/img/$OS_IMAGE /mnt/img/$OS_IMAGE-old
+	mv /mnt/img/$OS_IMAGE-new /mnt/img/$OS_IMAGE
+	umount /mnt/img
+fi
+umount /mnt/newimg
+
+# Mount device containing current image RO
+mount -t ext4 -o ro $IMG_DEVICE /mnt/img
+
+# Mount boot device and load squashfs module if present
+mount -t vfat -o ro $BOOT_DEVICE /boot
+if [[ -e /boot/modules/$(uname -r)/squashfs.ko ]]; then
+	insmod /boot/modules/$(uname -r)/squashfs.ko
+fi
+umount /boot
+
+# Mount OS image
+mount -t squashfs -o ro /mnt/img/root-squash.img /mnt/lower
+
+# Load overlay module if present on OS image
+if [[ -e /mnt/lower/lib/modules/$(uname -r)/kernel/fs/overlayfs/overlay.ko ]]; then
+	insmod /mnt/lower/lib/modules/$(uname -r)/kernel/fs/overlayfs/overlay.ko
+fi
+
+# Mount device for writeable overlay and workdir
+mount -t ext4 $OVERLAY_DEVICE /mnt/overlay
+
+# Clear upper overlay dir
+if [[ -e /mnt/overlay/upper ]]; then
+	rm -rf /mnt/overlay/upper
+fi
+
+# Create base directories if not present
+for dir in persistent upper work; do
+	mkdir -p /mnt/overlay/$dir
+done
+
+# Mount root file system (overlay)
+mount -t overlay -o ro,lowerdir=/mnt/overlay/persistent:/mnt/lower,upperdir=/mnt/overlay/upper,workdir=/mnt/overlay/work overlay /mnt/root
+### End mounting OS filesystems
+
+### Begin unmounting special fs
+umount /proc
+umount /sys
+umount /dev
+### End mounting special fs
+
+### Begin switching root
+exec switch_root /mnt/root /sbin/init
+### End switching root