The setup is based on Wedson’s tutorial. This document describes the necessary steps to build an initrd and run a custom kernel under qemu. This custom kernel includes the patches for the PuzzleFS driver.

Prerequisites

Install the necessary tools for building the Linux kernel:

  • For Ubuntu, you can find a list here
  • For Fedora, you can find a list here
  • For Arch Linux, you can find a list here

Install qemu.

Steps

  1. Get the PuzzleFS branch 
     git clone https://github.com/ariel-miculas/linux.git --branch puzzlefs

    We’ll call this path KERNEL_PATH.

  2. Follow the rust quickstart guide

  3. Make sure make LLVM=1 rustavailable is successful

    This is especially important because CONFIG_RUST=y will be silently ignored if the rust toolchain is not available.

  4. Configure and build the kernel 
     $ make LLVM=1 allnoconfig qemu-busybox-min.config puzzlefs.config
 $ make LLVM=1 -j$(nproc)
  5. Setup busybox 
     git clone git://git.busybox.net/busybox
 cd busybox
 make menuconfig # enable 'Build static binary' config
 make
 make install

    This will create the _install directory with the rootfs inside it. We’ll call the busybox path BUSYBOX_PATH.

  6. Create a home directory in the rootfs and build a puzzlefs image inside ($BUSYBOX_PATH/_install/home/puzzlefs_oci)

    To build a puzzlefs image:

    • install puzzlefs using cargo: cargo install puzzlefs or clone the puzzlefs repository and run make release

    • create a simple filesystem structure with a few directories and files (e.g. in /tmp/simple_rootfs) 
        $ tree simple_rootfs
  simple_rootfs
  ├── dir-1
  ├── dir-2
  ├── dir-3
  ├── dir-4
  ├── file1
  └── file2

  5 directories, 2 files

    • build a puzzlefs oci image at $BUSYBOX_PATH/_install/home/puzzlefs_oci with the tag first_try:

        $ puzzlefs build /tmp/simple_rootfs \
  $BUSYBOX_PATH/_install/home/puzzlefs_oci first_try

    • get first_try’s image manifest from puzzlefs_oci/index.json

        $ jq ".manifests[] | .digest" index.json
        "sha256:c43e5ab9d0cee1dcfbf442d18023b34410de3deb0f6dbffcec72732b6830db09"

  7. Add the following init script in the busybox rootfs (defaults to $BUSYBOX_PATH/_install):

     #!/bin/sh
 mount -t devtmpfs none /dev
 mkdir -p /proc
 mount -t proc none /proc

 ifconfig lo up
 udhcpc -i eth0

 mkdir /mnt
 mount -t puzzlefs -o oci_root_dir="/home/puzzlefs_oci" -o \
 image_manifest="c43e5ab9d0cee1dcfbf442d18023b34410de3deb0f6dbffcec72732b6830db09" \
 none /mnt

 setsid sh -c 'exec sh -l </dev/ttyS0 >/dev/ttyS0 2>&1'

    Make sure to replace the image_manifest with your own digest. This init script will be passed to rdinit in the kernel command line.

  8. Generate the initramfs

     cd $BUSYBOX_PATH/_install && find . | cpio -H newc -o | gzip > ../ramdisk.img

    This will generate a compressed ramdisk image in $BUSYBOX_PATH/ramdisk.img.

  9. Run with qemu: 
     qemu-system-x86_64 \
     -accel kvm \
     -cpu host \
     -m 4G \
     -initrd $BUSYBOX_PATH/ramdisk.img \
     -kernel $KERNEL_PATH/arch/x86/boot/bzImage \
     -nographic \
     -append 'console=ttyS0 nokaslr debug rdinit=/init' \
     -nic user,model=rtl8139 \
     -no-reboot
  10. Check whether puzzlefs has been successfully mounted: 
    ~ # grep puzzlefs /proc/filesystems
nodev   puzzlefs
~ # mount | grep puzzlefs
none on /mnt type puzzlefs (rw,relatime)
~ # ls /mnt/
dir-1  dir-2  dir-3  dir-4  file1  file2