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Creating Custom rootfs and kernel Images

Creating a kernel Image

Currently, Firecracker supports only uncompressed, ELF kernel images. You can build an uncompressed Linux kernel image with:

make vmlinux

Here's a quick step-by-step guide to building your own kernel that Firecracker can boot:

  1. Get the Linux source code:

    git clone https://github.com/torvalds/linux.git linux.git
    cd linux.git
  2. Check out the Linux version you want to build (e.g. we'll be using v4.20 here):

    git checkout v4.20
  3. You will need to configure your Linux build. You can start from our recommended config - just copy it to .config (under the Linux sources dir). You can make interactive config adjustments using:

    make menuconfig

    Note: there are many ways of building a kernel config file, other than menuconfig. You are free to use whichever one you choose.

  4. Build the uncompressed kernel image:

    make vmlinux
  5. Upon a successful build, you can find the uncompressed kernel image under ./vmlinux.

Creating a rootfs Image

A rootfs image is just a file system image, that hosts at least an init system. For instance, our getting started guide uses an EXT4 FS image with OpenRC as an init system. Note that, whichever file system you choose to use, support for it will have to be compiled into the kernel, so it can be mounted at boot time.

To build an EXT4 image:

  1. Prepare a properly-sized file. We'll use 50MiB here, but this depends on how much data you'll want to fit inside:

    dd if=/dev/zero of=/rootfs.ext4 bs=1M count=50
  2. Create an empty file system on the file you created:

    mkfs.ext4 rootfs.ext4

You now have an empty EXT4 image in rootfs.ext4, so let's prepare to populate it. First, you'll need to mount this new file system, so you can easily access its contents:

mkdir /tmp/my-rootfs
sudo mount rootfs.ext4 /tmp/my-rootfs

The minimal init system would be just an ELF binary, placed at /sbin/init. The final step in the Linux boot process executes /sbin/init and expects it to never exit. More complex init systems build on top of this, providing service configuration files, startup / shutdown scripts for various services, and many other features.

For the sake of simplicity, let's set up an Alpine-based rootfs, with OpenRC as an init system. To that end, we'll use the official Docker image for Alpine Linux:

  1. First, let's start the Alpine container, bind-mounting the EXT4 image created earlier, to /my-rootfs:

    docker run -it --rm -v /tmp/my-rootfs:/my-rootfs alpine
  2. Then, inside the container, install the OpenRC init system, and some basic tools:

    apk add openrc
    apk add util-linux
  3. And set up userspace init (still inside the container shell):

    # Set up a login terminal on the serial console (ttyS0):
    ln -s agetty /etc/init.d/agetty.ttyS0
    echo ttyS0 > /etc/securetty
    rc-update add agetty.ttyS0 default
    
    # Make sure special file systems are mounted on boot:
    rc-update add devfs boot
    rc-update add procfs boot
    
    # Then, copy the newly configured system to the rootfs image:
    for d in bin etc lib root sbin usr; do tar c "/$d" | tar x -C /my-rootfs; done
    for dir in dev proc run sys var; do mkdir /my-rootfs/${dir}; done
    
    # All done, exit docker shell
    exit
  4. Finally, unmount your rootfs image:

    sudo umount /tmp/my-rootfs

You should now have a kernel image (vmlinux) and a rootfs image (rootfs.ext4), that you can boot with Firecracker.

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