Fly Init

This is a public snapshot of Fly's init code. It powers every Firecracker microvm we run for our users.

It is Rust-based and we thought making it public (even in this very limited fashion) could help as a reference for people making Rust-based init programs.

Usage

Please note that our init is tailored for firecracker microvms. These instructions might not quite work and differ from what we do in production.

• Build with cargo build --release
• Create a device for the init

fallocate -l 64M tmpinit
mkfs.ext2 tmpinit
mkdir initmount
mount -o loop,noatime tmpinit initmount
mkdir initmount/fly
cp target/x86_64-unknown-linux-musl/release/init initmount/fly/init
cp run.json initmount/fly/run.json # more on this later
umount initmount

• Attach this device as /dev/vda
• Attach your rootfs as /dev/vdb
• Attach a vsock virtio device

initrd

This init should also work packaged as an initrd. However, we're not running it as such at the time of this writing.

mkdir -p tmpcpio/fly
cp target/x86_64-unknown-linux-musl/release/init tmpcpio/fly/init
cp run.json tmpcpio/fly/run.json
cd tmpcpio
ls | cpio --null --create -V --format=newc -O ../initrd.cpio

You can then use initrd.cpio as your initrd parameter.

/fly/run.json

To configure out init, we're injecting a JSON file into the root device.

You should be able to figure out the format by looking at lib.rs