Building a minimal RootFS with Busybox, GLIBC and DropBear

BusyBox is a collection of cut down versions of common UNIX utilities compiled into a single small executable. This makes BusyBox an ideal foundation for resource constrained systems.

Prerequisites

Install the following prerequisites (assuming an Ubuntu 14.04 built machine):

apt-get install gcc-arm-linux-gnueabi
apt-get install libncurses5-dev
apt-get install gawk

BusyBox

BusyBox can be built either as a single static binary requiring no external libraries, or built requiring shared libraries such as GLIBC (default). This setting can be found under BusyBox Settings -> Build Options -> Build BusyBox as a static binary (no shared libs).

I generally choose to build BusyBox to require GLIBC as it is highly likely you will want to run additional applications that will require GLIBC.

wget http://busybox.net/downloads/busybox-1.22.1.tar.bz2
tar -xjf busybox-1.22.1.tar.bz2
cd busybox-1.22.1/
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- defconfig
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- menuconfig

At the menu, you can configure BusyBox options. Once configured, you can build BusyBox:

make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- 
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- install CONFIG_PREFIX=/home/export/rootfs

GLIBC

GLIBC is the GNU C Library and includes common system calls required by executables running on your system.

Download, build and install GLIBC:

wget http://ftp.gnu.org/gnu/libc/glibc-2.19.tar.gz
tar -xzf glibc-2.19.tar.gz
mkdir glibc-build
cd glibc-build/
../glibc-2.19/configure arm-linux-gnueabi --target=arm-linux-gnueabi --prefix= --enable-add-ons
make
make install install_root=/home/export/rootfs 

If you get an error resembling that below with cross-rpcgen, then it would appear cross-rpcgen was built for arm, but is trying to run on your x86 based build system. To alleviate the problem I pre-build a copy for x86 and place it in /glibc-build/sunrpc/cross-rpcgen and then restart the arm build.

CPP='arm-linux-gnueabi-gcc -E -x c-header' /.../glibc-build/sunrpc/cross-rpcgen -Y ../scripts -c rpcsvc/bootparam_prot.x -o /.../glibc-build/sunrpc/xbootparam_prot.T
/.../glibc-build/sunrpc/cross-rpcgen: 1: /.../glibc-build/sunrpc/cross-rpcgen: Syntax error: word unexpected (expecting ")")
make[2]: *** [/.../glibc-build/sunrpc/xbootparam_prot.stmp] Error 2

Preparing RootFS

Once BusyBox and GLIBC has been cross-compiled, you will want to create the remainder of the root file system. Start by creating the necessary directory structure:

mkdir proc sys dev etc/init.d

Now we must mount the /proc & /sys filesystem and populate the /dev nodes. This can be done at runtime by creating a file called etc/init.d/rcS and adding:

#!bin/sh
mount -t proc none /proc
mount -t sysfs none /sys
echo /sbin/mdev > /proc/sys/kernel/hotplug
/sbin/mdev -s

and make executable:

chmod +x etc/init.d/rcS 

You should now have a basic, yet quite functional, BusyBox root file system.

DropBear (Optional)

DropBear is a small SSH server and client and is useful to allow remote shell access to your system.

Download, build and install DropBear:

wget https://matt.ucc.asn.au/dropbear/releases/dropbear-2014.63.tar.bz2
tar -xjf dropbear-2014.63.tar.bz2
cd dropbear-2014.63
./configure --host=arm-linux-gnueabi --prefix=/ --disable-zlib CC=arm-linux-gnueabi-gcc LD=arm-linux-gnueabi-ld
make
make install DESTDIR=/home/export/rootfs

DropBear requires RSA and DSS (Digital Signature Standard) encryption keys to be generated. I normally do this on the target, but you could generate the keys on the host if you have the dropbearkey executable installed.

To generate your keys:

mkdir /etc/dropbear
dropbearkey -t dss -f /etc/dropbear/dropbear_dss_host_key  
dropbearkey -t rsa -f /etc/dropbear/dropbear_rsa_host_key 

You will also require users and passwords to validate login credentials:

touch /etc/passwd
touch /etc/group
adduser root -u 0

Unless otherwise specified, root will be given a default home directory of /home/root. However as this doesn't exist, DropBear will close your connection immediately after successfully logging in. To address this, simply create a home directory for root:

mkdir /home /home/root

DropBear can now be started by running:

dropbear

and you should be able to remotely login to your system using the root user.