Linux-evocation

Linux-evocation is a conversion of the historic 0.99.12 linux kernel so that it compiles under gcc 5, cross compiles under MacOS and runs with qemu. Why this kernel? This was the one used by the first linux distribution, Slackware 1.01 back in 1993.

Prerequisites

The kernel can be built using either of these OSSes:

• Ubuntu 16.04 xenial 32 bit
• Ubuntu 20.04 focus 64 bit
• MacOS Monterey

The following has been known to work when compiling the kernel

• gcc 5.4, 9.4.0, 12.2.0
• binutils 2.26.1, 2.34, 2.40
• make 3.8.1, 4.1, 4.2.1
• as86 and ld86 0.16.17

Testing was done with qemu i386 2.8.0 on MacOS Sierra using the slackware 1.01 disk image from the 2014 QEMU Advent Calendar.

A Vagrantfile has been included that sets up Ubuntu using the ubuntu/xenial32 base image. Just cd to the linux directory in the ubuntu home dir.

Compilation

make config
make dep
make

The Makefile detects if you are using MacOS and if so, sets the CROSS_COMPILE variable to i386-elf-.

To cleanup

# Wipe build, but keep config
make clean

# Wipe everything, including config
make mrproper

Running it

Just booting the kernel

To see the kernel boot, run it like this:

qemu-system-i386 -m 8 -kernel zImage -curses

The -curses parameter makes qemu use the curses library to emulate a VGA screen. Press esc-2-q to exit.

Running it with a slackware's 1.01 disk image

The kernel isn't particularly useful without a root filesystem, so to see something more realistic, use the QEMU slackware 1.01 disk image.

Download and extract the slackware 1.01 disk image from the 2014 QEMU Advent Calendar with

wget http://www.qemu-advent-calendar.org/2014/download/qemu-xmas-slackware.tar.xz
tar xfv qemu-xmas-slackware.tar.xz
unxz qemu-xmas-slackware/slackware.qcow2.xz
mkdir images
mv qemu-xmas-slackware/slackware.qcow2 images/
rm -Rf qemu-xmas-slackware qemu-xmas-slackware.tar.xz

The kernel needs to have a major/minor device for the root device written into it. This is done by creating a floppy image and booting off of that.

First create an empty floppy

# Create an empty floppy
dd bs=512 count=2880 if=/dev/zero of=floppy

Then copy the kernel to the floppy and set the root to /dev/hda1.

dd bs=512 if=zImage of=floppy conv=notrunc
echo -ne "\x01\x03" | dd ibs=1 obs=1 count=2 seek=508 of=floppy conv=notrunc

Boot qemu with the floppy and the slackware 1.01 image on /dev/hda:

qemu-system-i386 -m 8 -curses -drive if=floppy,format=raw,file=floppy -drive if=ide,format=qcow2,file=images/slackware.qcow2 -boot a

The results:

...
Mounting remote file systems...
INET: syslogd inetd
May  7 14:16:54 init[1]: Entering runlevel: 6

Welcome to Linux 0.99pl12.

slack login: root
May  7 14:16:56 login[32]: no shadow password for `root' on `tty1'

Linux 0.99pl12. (Posix).
No mail.
May  7 14:16:56 login[32]: ROOT LOGIN ON TTY `tty1'

slack:/# uname -a
Linux slack 0.99.12 #6 Sun Aug 8 16:02:35 CDT 1993 i586

Running it with networking

Add the following to the qemu command line to enable SLIRP networking:

-device ne2k_isa,netdev=slirp -netdev user,id=slirp

Then run this inside slackware:

/etc/ifconfig eth0 10.0.2.15 netmask 255.255.255.0 broadcast 10.0.2.255 up
/etc/route add 10.0.2.0 eth0
/etc/route add default gw 10.0.2.2

Put these command lines in /etc/rc.d/rc.inet1 to make them take effect on boot.

That results in:

slack:/# ping 10.0.2.2 -c 1
PING 10.0.2.2 (10.0.2.2): 56 data bytes
64 bytes from 10.0.2.2: icmp_seq=0 ttl=185 time=3 ms

--- 10.0.2.2 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 3/3/3 ms

Changes

• sys_idle() and panic() use the hlt instruction for better performance under qemu.
• Search for a ramdisk filesystem on more blocks on the floppy, to allow for using the 1 MB image from linux-0.11-lab.
• Adapted ramdisk code to also look for an ext2 filesystem in addition to minix.
• Reduced calibrate_delay() loop time from 100 ticks to 10 ticks for faster booting.
• Added /* wait for "start of" clock tick */ from a later kernel to calibrate_delay().
• Fixed spurious keyboard "7"s when running under qemu and moving the mouse, or detaching from the UI and then pressing a key.

Compiler Warnings

Although -Wall is switched on, a considerable amount of warnings have been ignored by using -Wno-* parameters to gcc. I preferred having clean output and didn't want to spend too much time fixing warnings. I'll leave it as an exercise to a motivated reader to fix them.

Why

The main purpose was to learn about how the linux kernel works by reading, compiling and running the code. Inspired by Mariuz's Blog, I decided to make whatever changes were needed to compile the kernel with a modern toolchain. This allowed for a quick edit/build/run cycle and also provided a migration path.

Known Issues

• It's got Y2k bugs
• The keyboard fix for the qemu "7"s (see commit f39c093) isn't perfect. There must be a better way of doing this.
• There's something not right about the networking code. qemu sometimes goes to 100% cpu usage after pinging 10.0.2.2 for a while
• Pings sometimes come back with negative times