Building Tiny Linux Systems with Busybox, Part 2: Building the Kernel

For this example I use Linux kernel version 2.2.17. The 2.4.0-test8 kernel that I tried did not size the RAM disk for the root file system properly, leading to a ``not enough memory'' message at boot time. That bug will probably be repaired in the 2.4 series of kernels by the time you read this.

We will build our example to run on an i386-architecture PC-compatible system with PC keyboard and VGA display, booting from a floppy disk and running the root file system entirely in RAM once the system is booted. This example should also boot from IDE disks and from FLASH EEPROM devices that masquerade as IDE disks. It can also be configured to boot from a CD-ROM.

Build a bzImage-style kernel with all of the facilities needed for the application, plus these three:

Don't use kernel modules, because this example system doesn't support them. Don't put any facilities in the kernel that you don't need, as they will use up space that you need on the floppy disk. A kernel with the facilities you need should be around half a megabyte in size and should fit easily on a floppy along with the ROM root file system. A kernel with many unnecessary bells and whistles will be a megabyte or more and won't leave sufficient room for your ROM root file system.

If you're not familiar with building and installing kernels on a normal Linux PC, you'll need to study up on that. In short, I placed the kernel sources in /usr/src/linux and ran:

xhost +localhost
su
make xconfig
make dep
make bzImage

This created a compiled Linux kernel in /usr/src/linux/arch/i386/boot/bzImage.

In the busybox source directory, edit the Makefile, changing the variable DOSTATIC from false to true. Then run make. That will create a static-linked version of busybox. Confirm that it is static-linked by running this command:

ldd busybox

This should print something like:

statically linked (ELF)

We're going to go through all of the steps for creating a minimal root file system by hand so that you will understand just how little is necessary to boot your system rather than copying all of the files from the root of your Linux distribution and then being afraid to remove anything because you don't know whether it's necessary. You will need to become root (the superuser) to perform the following steps because the mknod command requires superuser privilege.

Create the tiny-linux directory and change directory into it:

mkdir tiny-linux
cd tiny-linux

Create the standard directories in it:

mkdir dev etc etc/init.d bin proc mnt tmp var var/shm
chmod 755 . dev etc etc/init.d bin proc mnt tmp var var/shm

cd dev

mknod tty c 5 0
mknod console c 5 1
chmod 666 tty console
#Allow anyone to open and write terminals.

mknod tty0 c 4 0
chmod 666 tty0

mknod ram0 b 1 0
chmod 600 ram0

mknod null c 1 3
chmod 666 null

cd ../etc/init.d

#! /bin/sh
mount -a # Mount the default file systems mentioned in /etc/fstab.

chmod 744 rcS

cd ..

proc  /proc      proc    defaults     0      0
none  /var/shm   shm     defaults     0      0

chmod 644 fstab

::sysinit:/etc/init.d/rcS
::askfirst:/bin/sh

Set the modes of tiny-linux/etc/inittab:

chmod 644 inittab

That's everything necessary to create your root file system, except for the installation of the programs. Change directory to tiny-linux/bin:

cd ../bin

cp ~/busybox-0.46/busybox busybox

ln busybox ls

-rwxr-xr-x    2 root     root       580424 Sep 12 15:17 busybox
-rwxr-xr-x    2 root     root       580424 Sep 12 15:17 ls

[, ar, basename, cat, chgrp, chmod, chown,
chroot, chvt, clear, cp, cut, date, dc, dd,
deallocvtdf, dirname, dmesg, du, dumpkmap dutmp,
echo, false, fbset, fdflush, find, free,
freeramdisk, fsck.minix, grep, gunzip, gzip, halt,
head, hostid, hostname, id, init, insmod, kill,
killall, length, linuxrc, ln, loadacm, loadfont,
loadkmap, logger, logname, lsmod, makedevs, md5sum,
mkdir, mkfifo, mkfs.minix, mknod, mkswap, mktemp,
more, mount, mt, mv, nc, nslookup, ping, poweroff,
printf, ps, pwd, reboot, rm, rmdir, rmmod, sed,
setkeycodes, sh, sleep, sort, swapoff, swapon, syn, c
syslogd, tail, tar, tee, telnet, test, touch, tri,
true, tty, umount, uname, uniq, update, uptime,
usleep, uudecode, uuencode, wc, which, whoami, yes,
zcat