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

Let Bruce help you put the BusyBox to work.

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:

  • RAM disk support (in the Block Devices menu)

  • Initial RAM disk (initrd) support (also in the Block Devices menu)

  • ROM file system support (in the File Systems menu)

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
make xconfig
make dep
make bzImage

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

Building a Static-Linked Busybox

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)
It's important to get this right; if you install a dynamic-linked version of Busybox, your system won't run because we aren't installing the runtime dynamic linker and its libraries on the floppy disk for this example.

Creating a ROM Root File System

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
Enter the tiny-linux/dev directory:
cd dev
Create the generic terminal devices:
mknod tty c 5 0
mknod console c 5 1
chmod 666 tty console
#Allow anyone to open and write terminals.
Create the virtual terminal device for the VGA display:
mknod tty0 c 4 0
chmod 666 tty0
Create the RAM disk device:
mknod ram0 b 1 0
chmod 600 ram0
Create the null device, used to discard unwanted output:
mknod null c 1 3
chmod 666 null
Change directory to tiny-linux/etc/init.d, where startup scripts are stored:
cd ../etc/init.d
Use an editor to create this shell script in tiny-linux/etc/init.d/rcS. It will be executed when the system boots:
#! /bin/sh
mount -a # Mount the default file systems mentioned in /etc/fstab.
Make the script executable:
chmod 744 rcS
Change directory to tiny-linux/etc:
cd ..
Use an editor to create the file tiny-linux/etc/fstab, which says what file systems should be mounted at boot time:
proc  /proc      proc    defaults     0      0
none  /var/shm   shm     defaults     0      0
Set the mode of tiny-linx/etc/fstab:
chmod 644 fstab
Use an editor to create the file tiny-linux/etc/inittab, which tells /bin/init, the system startup program, what processes to start:
The above example runs the script /etc/init.d/rcS at boot time and runs an interactive shell on the console device.

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
Copy your static-linked version of Busybox from wherever you built it into tiny-linux/bin/busybox with a command similar to this one:
cp ~/busybox-0.46/busybox busybox
Add another command name ls to Busybox using the ln command:
ln busybox ls
Run ls, and the result should look like this:
-rwxr-xr-x    2 root     root       580424 Sep 12 15:17 busybox
-rwxr-xr-x    2 root     root       580424 Sep 12 15:17 ls
Repeat the above ln command for all of these names:
[, 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,
Are you tired yet? Well, now is a good time to take a break--you've finished creating your ROM root file system.



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How Keyboard is working when no support provided in Linux

Anonymous's picture

How Keyboard is working when no support provided in Linux ?
When i booted Linux from floppy and used the ramdisk as the rootfs, everything is working fine.

But i was wondering how the keyboard is working ? No loadable drivers provided.
Also no support in Linux provided>

I am not getting the # prompt instead i am getting the $ prompet

K.RajaSekhar's picture

Hai Bruce Perens,

I have followed each and every step you told in this article but i am not getting the # prompt after booting it from the floppy. Instead i am getting the $ prompt.

How to get the # prompt can you tell me the error i am donig

Thanking you,

Re: Building Tiny Linux Systems with Busybox, Part 2: Building t

Anonymous's picture

Luckily he provided the list of names in almost the right format for python.
An easier way to create those links to busybox:

import os
names=["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", "csyslogd", "tail", "tar", "tee", "telnet", "test", "touch", "tri", "true", "tty", "umount", "uname", "uniq", "update", "uptime", "usleep", "uudecode", "uuencode", "wc", "which", "whoami", "yes", "zcat"]
for name in names:
os.system("ln busybox "+name)

Re: Building Tiny Linux Systems with Busybox, Part 2: Building t

Thorsten's picture

I did this do create the links with a debian 'busybox-static'

# Thorsten Strusch 2005-01-06
BUSYBOX=$(which busybox)
Liste=$( $BUSYBOX 2>&1 | grep -A20 Currently | grep -v Curren | sed -e '{s/\t//}' -e '{s/\n//}' -e '{s/ //}' | grep -v ^$)
export IFS=','
for i in $Liste
do ln -s $BUSYBOX $i
unset IFS

Re: Building Tiny Linux Systems with Busybox, Part 2: Building t

Anonymous's picture

Or even easier:

from the busybox build dir

$ make PREFIX=../tiny-linux install-hardlinks

Re: Building Tiny Linux Systems with Busybox, Part 2: Building t

dragoran's picture

all went ok, kernel uncompressed, but then..kernel panic :(

kernel panic: unable to mount root fs on 01:00, what that is.. i gave my full memory, but .. nothing.. why? can u send me reply?

P.S.: best article(s).. thanx...

Re: Building Tiny Linux Systems with Busybox, Part 2: Building t

Anonymous's picture

Im trying to make a mini floppy disk linux distro thing (if thats the right name) , i used a differant tutorial looks almost the same as this BUT .... i got the same boot error! that and something about VFS and "cramfs: wrong magic" so yeah. somethings up there, maybe i enabled some wrong options in the kernel and they are breaking each other.

Re: Building Tiny Linux Systems with Busybox, Part 2: Building t

Anonymous's picture

did you enable ROM filesystem support in the kernel? if not you can try mkcramfs instead

ROM filesystem

Anonymous's picture

I had a similar problem with romfs. In my case, I had to gzip the file system without "-9".