Sunday, May 19, 2013

Build a minimalist root filesystem

During my research, I have seen many different tutorials to create a minimalist root filesystem. Some of them advise to copy files from the host's root filesystem and then proceed to customization, others use very handy tools like Buildroot and finally some people create it manually.

It is this last option that we will experiment here thanks to this twiki written by T.I.

Our root filesystem is based on BusyBox which offers the main linux command line utilities in a single executable.

Build BusyBox

Download the latest release of busybox on the official website and extract it to your working directory.
cd busybox-1.21.0
make menuconfig
Follow the instructions as detailed in the T.I. twiki to customize your binary. I have made some different choices here and there to add additional commands like fdisk. Do as you like, it does not really matter...

Build ...
ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- make
... and install it on the second partition of your SD Card :
make CONFIG_PREFIX=/media/<path_to_your_sd_card_p2> install
Here is what you should see on your partition after the last step :
bin  linuxrc  lost+found  sbin  usr

Create compulsory directories

The official rules to build a root filesystem are detailed in the Filesystem Hierarchy Standard (FHS). Here is a complete list of the top level directories :

bin : essential user command binaries
boot : files used by the bootloader
dev : devices files
etc : system configuration files
home : users home directory
lib : essential libraries (C library, kernel modules,...)
media : mount points for removable medias
mnt : mount points for temporarily mounted filesystems
opt : Add-on software packages
proc : virtual filesystem for kernel and process information
root : root user's home directory
sbin : essntial system administration binaries
sys : virtual filesystem for system information and control
tmp : temporary files
usr : secondary hierarchy containing most applications and documents useful to most users
var : variable data stored by daemons and utilities

We don't need all of them as we won't provide a multi-user environment. /home, /mnt, and /opt can be ommited.

Create the directories (as root)

cd /media/<path_to_your_sd_card_p2>
sudo mkdir dev dev/pts etc etc/init.d lib mnt opt proc root sys tmp var var/log
Optional : debug is needed by debugfs, add it if your kernel has been built with this option
sudo mkdir debug

Create a static node for console

Like Unix systems, every object in Linux is visible as a file (except for networking interfaces). As such, /dev contains device files (nodes) that represent the system devices. This directory needs to be populated either statically (old fashion way, every node has to be created manually with mknode) or dynamically (common way thanks to udev or mdev).

In our case, we will only add a node for the console.
mknod dev/console c 5 1
The rest of the nodes will be added by mdev which is part of BusyBox.
cd etc
gedit mdev.conf
audio       0:5 0666
console     0:5 0600
control.*   0:0 0660 @/bin/mv /dev/$MDEV /dev/snd/
dsp         0:5 0666
event.*     0:0 0600 @/bin/mv /dev/$MDEV /dev/input/
fb          0:5 0666
nfs         0:5 0770
null        0:0 0777
pcm.*       0:0 0660 @/bin/mv /dev/$MDEV /dev/snd/
rtc         0:0 0666
tty         0:5 0660
tty0*       0:5 0660
tty1*       0:5 0660
tty2*       0:5 0660
tty3*       0:5 0660
tty4*       0:5 0660
tty5*       0:5 0660
tty6*       0:5 0660
ttyS*       0:5 0640
urandom     0:0 0444
zero        0:0 0666

Create an fstab file to mount /proc and /dev/pts at boot

fstab is system configuration file used to list the available disks and disk partitions and describes how they are initialized. The mount command relies on this file to determine which option should be used when mounting a specific device.
# still in /etc
gedit fstab
proc            /proc           proc    defaults        0 0
none            /dev/pts        devpts  mode=0622       0 

Login utilities

/etc must contain the files group, passwd and hosts for login. For the moment, root only needs to be defined in group and hosts only needs to have the localhost registered.
gedit group
gedit passwd
gedit hosts       localhost

Create inittab

Read this post for information about inittab.
gedit inittab

# /bin/ash
# Start an "askfirst" shell on the serial port

# Stuff to do when restarting the init process

# Stuff to do before rebooting
::shutdown:/bin/umount -a -r
::shutdown:/sbin/swapoff -a

Create init script

gedit init.d/rcS
#   ---------------------------------------------
#   Common settings
#   ---------------------------------------------

hostname $HOSTNAME

#   ---------------------------------------------
#   Prints execution status.
#   arg1 : Execution status
#   arg2 : Continue (0) or Abort (1) on error
#   ---------------------------------------------
status ()
       if [ $1 -eq 0 ] ; then
               echo "[SUCCESS]"
               echo "[FAILED]"

               if [ $2 -eq 1 ] ; then
                       echo "... System init aborted."
                       exit 1


#   ---------------------------------------------
#   Get verbose
#   ---------------------------------------------
echo ""
echo "    System initialization..."
echo ""
echo "    Hostname       : $HOSTNAME"
echo "    Filesystem     : v$VERSION"
echo ""
echo ""
echo "    Kernel release : `uname -s` `uname -r`"
echo "    Kernel version : `uname -v`"
echo ""

#   ---------------------------------------------
#   MDEV Support
#   (Requires sysfs support in the kernel)
#   ---------------------------------------------
echo -n " Mounting /proc             : "
mount -n -t proc /proc /proc
status $? 1

echo -n " Mounting /sys              : "
mount -n -t sysfs sysfs /sys
status $? 1

echo -n " Mounting /dev              : "
mount -n -t tmpfs mdev /dev
status $? 1

echo -n " Mounting /dev/pts          : "
mkdir /dev/pts
mount -t devpts devpts /dev/pts
status $? 1

echo -n " Enabling hot-plug          : "
echo "/sbin/mdev" > /proc/sys/kernel/hotplug
status $? 0

echo -n " Populating /dev            : "
mkdir /dev/input
mkdir /dev/snd

mdev -s
status $? 0

#   ---------------------------------------------
#   Disable power management
#   (Requires sysfs support in the kernel)
#   ---------------------------------------------
# echo -n " Disabling Power mgmt       : "
# echo -n "1" > /sys/power/cpuidle_deepest_state
# status $? 1

#   ---------------------------------------------
#   Turn off LCD after 1 hour of inactivity
#   (Requires sysfs support in the kernel)
#   ---------------------------------------------
# echo -n " Turn off LCD after 1 hour  : "
# echo -n "3600" > /sys/power/fb_timeout_value
# status $? 1

#   ---------------------------------------------
#   Mount the default file systems
#   ---------------------------------------------
echo -n " Mounting other filesystems : "
mount -a
status $? 0

#   ---------------------------------------------
#   Set PATH
#   ---------------------------------------------
export PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin

#   ---------------------------------------------
#   Start other daemons
#   ---------------------------------------------
echo -n " Starting syslogd           : "
status $? 0

echo -n " Starting telnetd           : "
status $? 0

#   ---------------------------------------------
#   Done!
#   ---------------------------------------------
echo ""
echo "System initialization complete."

make it executable
chmod +x rcS

Copy the dependencies 

We built BusyBox with the ARM cross toolchain as a C program without the -static option, which means that it will look for C libraries at runtime.We have to add them !

Note : Use ldd to list the dependencies of a binary

cd /media/<path_to_your_sd_card_p2>/lib
# Copy the C libraries
copy -r /usr/arm-linux-gnueabi/lib/* .
# Remove the debug informations from the libraries to save space

At this point, you have a valid root filesystem with the bare minimum. It does not include the kernel modules yet and if you want to add more executables, you'll also need to add their dependencies. The filesystem will get larger when these files are added.

First boot

Here is my display at first boot :

This time, there is no panic displayed. The kernel finds the init process and executes our script.
To access BusyBox press Enter :


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