fluxflop - floppy-sized Linux

It's 2024 and floppies are still cool. Maintain that coolness by building your own floppy-sized Linux distro like it's 1999.

This project is buildable by a gcc toolchain provided by your distribution or by building a musl toolchain. For the smallest binaries sizes, musl is recommended.

Pre-reqs

gcc-i686 build: Package:

apt-get install build-essential gcc-i686-linux-gnu g++-i686-linux-gnu

i486/i686-musl build:

git clone https://github.com/richfelker/musl-cross-make
cd musl-cross-make
make TARGET=i486-linux-musl
make install

arm64-musl build:

make TARGET=i486-linux-musl
make install

Add the musl toolchain to your path: export PATH=~/musl-cross-make/bin:$PATH

Prepare the kernel:

wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.9.3.tar.xz
tar -xf linux-6.9.3.tar.xz

# .config - default i686 config
# .config-x86-fb - i486 musl with framebuffer support
# .config-x86-serial - i486 musl with serial support

cp ./configs/linux/.config-<config-name> linux-6.9.3/.config

The smallest kernel produced is using .config-x86-fb (XZ compression) = 592KB

Root filesystem layout:

mkdir -p root/{bin,proc,dev,sys,usr}



# Create a simple init script
# or compile tools/init.c and place in root/
cat > root/init <<EOF
#!/bin/sh
# Mount essential filesystems
mount -t proc none /proc
mount -t sysfs none /sys
mount -t devtmpfs none /dev

# Start a shell
exec /bin/sh -i 

# Infinite loop to prevent the script from exiting
while true; do
    sleep 1
done
EOF

chmod a+x root/init

# you will need root privileges to make these nodes
mknod -m 660 ./root/dev/console c 5 1
mknod -m 666 ./root/dev/ttyS0 c 4 64
mknod -m 666 ./root/dev/tty c 5 0

Output image directories:

mkdir -p bootable_image/{boot,isolinux}

Build

Linux:

cd linux-6.9.3
make ARCH=x86 CROSS_COMPILE=i686-linux-gnu- -j8 #use bzImage for x86, Image.gz for arm64

Toybox:

wget https://landley.net/toybox/bin/toybox-i486 -O ./root/bin/toybox-i486
./scripts/toybox-symlinks.sh

Toybox provides many common Linux utilities including a shell. For the sake of simplicity, the static builds provided by Toybox is used. You can optionally compile Toybox yourself:

wget https://landley.net/toybox/downloads/toybox-0.8.11.tar.gz
tar xvf toybox-0.8.11.tar.gz
cp ./configs/toybox/.config toybox-0.8.11
cd toybox-0.8.11
make LDFLAGS="-static -s" ARCH=x86 CROSS_COMPILE=i486-linux-musl- -j8
make install
cp -R install/* ./root

Strip debug symbols in the root filesystem:

./scripts/stripfs.sh

Make the initramfs:

./scripts/make-cpio.sh

(Optional): Syslinux for ISOs:

wget https://mirrors.edge.kernel.org/pub/linux/utils/boot/syslinux/syslinux-6.03.tar.gz
tar xvf syslinux-6.03.tar.gz
cd syslinux-6.03

cp syslinux/bios/core/isolinux.bin bootable_image/isolinux
cp syslinux/bios/com32/elflink/ldlinux/ldlinux.c32 bootable_image/isolinux

# copy config
cp configs/isolinux/isolinux.cfg bootable_image/isolinux

Boot

Make a bootable ISO:

./scripts/make-iso.sh

and boot with:

qemu-system-i386 -cdrom out.iso -m 32 -nographic 

Make a bootable floppy: Edit the Linux .config and set CONFIG_INITRAMFS_SOURCE to your rootfs cpio archive path.

git clone https://github.com/snacsnoc/tiny-floppy-bootloader

# copy your kernel to the current dir
cp ../linux-6.9.3/arch/x86/boot/bzImage .

bash build.sh

Boot with qemu: qemu-system-i386 -fda disk.img

Boot with kernel and initial ramdisk with qemu:

qemu-system-i386 -kernel ./linux-6.8.11/arch/x86/boot/bzImage -initrd ./rootfs.cpio.gz -append "init=/init rdinit=/init console=ttyS0,115200n8" -m 32 -nographic

or, if you built with simple framebuffer:

qemu-system-i386 -kernel ./bzImage -initrd ./rootfs.cpio.gz -append "init=/init rdinit=/init console=tty0" -m 32

Extra

Use a simple C program to use as init:

gcc -static -o init tools/init.c

Use as the initramfs:

echo init | cpio -ov --format=newc > initramfs.img

Kernel image comparison with printk support:

with:

stat -c %s arch/arm64/boot/Image
2809864

without:

stat -c %s arch/arm64/boot/Image
2283528

-fomit-frame-pointer:

easto@debian-build:~/testing-linux/linux-6.8.11$ ls -ls arch/x86/boot/bzImage 
728 -rw-r--r-- 1 easto easto 741888 May 30 02:27 arch/x86/boot/bzImage

easto@debian2:/tmp/linux-6.9.3$ stat -c %s vmlinux
1919144

make KCFLAGS="-fomit-frame-pointer" ARCH=x86 CROSS_COMPILE=i686-linux-gnu- bzImage -j8
 
easto@debian-build:~/testing-linux/linux-6.8.11$ ls -ls arch/x86/boot/bzImage 
716 -rw-r--r-- 1 easto easto 729600 May 30 02:30 arch/x86/boot/bzImage

easto@debian2:/tmp/linux-6.9.3$ stat -c %s vmlinux
1886332

-fno-inline-small-functions:

easto@debian2:/tmp/linux-6.9.3$ stat -c %s vmlinux
1919144

make KCFLAGS="-fno-inline-small-functions" ARCH=x86 CROSS_COMPILE=i486-linux-musl- -j8

easto@debian2:/tmp/linux-6.9.3$ stat -c %s vmlinux
1919528

find largest objects:

nm --size-sort --print-size vmlinux | tail -n 20

or using Bloaty

bloaty -d symbols -n 100 vmlinux

Resources

https://landley.net/toybox/

https://justine.lol/sizetricks/

https://elinux.org/images/7/72/Linux_tiny.pdf

https://git.openembedded.org/meta-micro/tree/conf

https://elinux.org/Linux_Tiny

https://ops.tips/notes/booting-linux-on-qemu/

https://android.googlesource.com/platform/external/syzkaller/+/HEAD/docs/linux/setup_linux-host_qemu-vm_arm64-kernel.md

https://github.com/ahgamut/superconfigure/?tab=readme-ov-file

https://github.com/buildroot/buildroot/tree/master/board/qemu/aarch64-virt