Linux porting on ARM based board: Read only root filesystem using AUFS

Overview:

My prototype based on ti8168 evaluation board encounters frequent filesystem corruption due power outrage. After looking on resources available on the web it was better if we have a read only root file system.articles on raspberry pi sited gave me a useful pointers to start my work on AUFS.my experience with AUFS mailing list has been overwhelming,would appreciate the maintainers for his prompt reply to all my query.

The idea is to implement a Read only root file-system using a AUFS support in kernel, AUFS is a filesystem hosts read-only and read-write directories as AUFS file-system branches. currently the system incorporates two layers,the read-write layer will happen on tmpfs and read-only layer will host the ubifs root file system.My board support Linux kernel 2.6.37,so the discussion is with respect to this source.

Steps involved.

1. create a initramfs.
2. patch the kernel with AUFS support

About initramfs

This is basic minimal root file system embedded in kernel, the kernel checks for the presence of the initramfs, if found, mounts it as / and runs /init. The init program is a shell script, in our case do the preparation to mount the read-write and read-only branches on Unification File System and finally give the complete control to real root file system.

The initramfs has a complete set of directories similar to a normal root file system. The busybox binary is statically build and copied to /bin directory of initramfs. This is eventually available as cpio archive placed in the kernel source folder, this initial file system gets loaded into memory during the Linux boot process, the path of the archive is notified in the .config file of the kernel.

Initramfs creation.


#create initramfs folder to start the activity
mkdir initramfs
cd initramfs

# create required directories
mkdir -p aufs bin dev etc lib proc rootfs rw sbin sys usr/{bin,sbin}

# populate dev
mknod -m 622 dev/console c 5 1
mknod -m 622 dev/tty0 c 4 0

# Copy statically build busybox to initramfs 
cp /bin/busybox bin/
ln -s busybox bin/sh

# create init file
cat > init <#!/bin/sh

echo "Waiting"
sleep 1

echo "Mounting dev proc sys"

mount -t devtmpfs none /dev
mount -t proc proc /proc
mount -t sysfs sysfs /sys

echo "Mount rootfs om mtd4  of raw NAND on /rootfs"
ubiattach /dev/ubi_ctrl  -m 4 -O 2048
mount -t ubifs -o ro,noatime  ubi0:rootfs  /rootfs

echo "Mounting rw on tmpfs"
mount    -o    rw    -t    tmpfs    aufs-tmpfs    /rw

# Remove old stuff
rm -rf /rw/tmp
rm -rf /rw/dev

#The upper is writable tmpfs /rw which should be invisible under the root dir, 
#and the lower is readonly ubifs /rootfs which should be visible. 
mount -t aufs -o noatime,br=/rw=rw:/rootfs=ro none /aufs


mkdir -p  /aufs/rootfs
mount --move /rootfs /aufs/rootfs


umount /sys
umount /proc
mount -t devtmpfs none /aufs/dev
exec switch_root /aufs /sbin/init
EOF

# make init executable
chmod a+x init

# create the initramfs image
find . | cpio -H newc -o > ../initramfs.cpio

#Copy the archive file to TI linux-2.6.x
cp ../initramfs.cpio /'PATH OF LINUX SOURCE'/

Patch the kernel with AUFS support

  
cd  /'Working kernel source Dir'/

# download aufs source
git clone git://git.code.sf.net/p/aufs/aufs2-standalone aufs2-standalone
cd aufs2-standalone
git checkout origin/aufs2.2-37

#Copy aufs related files to kernel source tree
cp -a aufs2-standalone/fs .
cp aufs2-standalone/include/linux/aufs_type.h include/linux/

#Patch the kernel with AUFS related updates
patch -p1 < aufs2-standalone/aufs2-base.patch
patch -p1 < aufs2-standalone/aufs2-kbuild.patch
patch -p1 < aufs2-standalone/aufs2-standalone.patch
patch -p1 < aufs2-standalone/loopback.patch
patch -p1 < aufs2-standalone/proc_map.patch

Kernel menuconfig changes reflected in .config file.


CONFIG_INITRAMFS_SOURCE="initramfs.cpio"                                                CONFIG_INITRAMFS_ROOT_UID=0
CONFIG_INITRAMFS_ROOT_GID=0
CONFIG_INITRAMFS_COMPRESSION_NONE=y 
CONFIG_AUFS_FS=y
CONFIG_AUFS_BRANCH_MAX_127=y
CONFIG_AUFS_SBILIST=y
CONFIG_AUFS_HNOTIFY=y
CONFIG_AUFS_HFSNOTIFY=y
CONFIG_AUFS_PROC_MAP=y
CONFIG_AUFS_SP_IATTR=y
CONFIG_AUFS_SHWH=y  
CONFIG_AUFS_BDEV_LOOP=y                                              
CONFIG_AUFS_DEBUG=y  
CONFIG_AUFS_MAGIC_SYSRQ=y

Recompile the kernel with the above changes and replace the current Linux image with this new build uImage.

Changing the root file-system for package updates

mount -o remount,rw /rootfs
mount -o remount,udba=notify /
mount -o remount,mod:/rootfs=rw /
#Now make changes in the '/', finally manual copy the changes to '/rootfs/' path
#reboot the system after copy
reboot