Microhop - initramfs helper

You do not always need Dracut. 😉 Sometimes you want it really-really small, tiny and completely stripped from everything. This is what microhop is for: use mainline Linux Kernel straight to the point, omitting many generic moving parts.

Tutorial

Overview

Microhop when deployed consists of one binary utility and one configuration file:

1. microgen — utility, that generates your initramfs as CPIO gzipped archive
2. /etc/microhop.conf — main system boot configuration, can be also your profile

The resulting microhop binary is the very /init in the initramfs, containing all required functionality, such as mounting, block device detection, root switching etc. It will appear only inside the initramfs CPIO archive and is not intended to use anywhere else.

The microgen binary is the utility which generates the initramfs archive. This archive then you will copy into the /boot directory of your Linux image.

Building

WARNING: Do not build it directly using cargo, because you will get it all wrong!

To build Microhop, first clone this repository and then just run:

make build-release

In ./target/release you should have a binary, called microgen. This is all you need.

Note on Dependencies

You might need to adjust your setup. For example, on Debian/Ubuntu you would need to install the following packages:

• libclang-dev
• libblkid-dev

On openSUSE Leap static-pie linking is broken, and only static is available. With that in mind, only "thin" LTO for the release profile is available (unless it was fixed). Additionally, you will need the following packages on openSUSE Leap:

• libblkid-devel-static
• glibc-devel-static

Configuration

Configuration is also a profile. This is the basic start:

# The list of kernel modules
modules:
  - virtio_blk
  - xfs

# Devices
disks:
  # Directly access device:
  # /dev/vda3: xfs,/,rw
  #
  # Access device via label:
  # ROOT: xfs,/,rw
  #
  # Access device via UUID:
  24e1daee-e09b-4fd5-97f3-dde8aba6ad8a: xfs,/,rw

# Optionally, define a custom init app
init: /usr/bin/bash

# Optionally, define a temporary sysroot.
# Default: /sysroot
sysroot: /sysroot

# Optionally, set debug log output. If this option is removed, default is used.
# Choose one from:
# - debug
# - info (default)
# - quiet (errors only)
log: debug

Resulting configuration will just contain more modules (their dependencies). The rest will be passed through.

Generating initramfs

Essentially, the workflow is very simple:

1. Point which kernel you want to use and where it is
2. "Press a pedal" to get a new initramfs
3. Wait whole 0.05 seconds and you have it. 😉

To achieve this, do the following:

1. Mount your root filesystem, which you want to update with the new initramfs. As an example, setup a device with losetup and then mount one of its partitions:

   sudo mount /dev/loop1p3 /mnt

2. Let microgen generate it (NOTE: this is an example, your filenames may differ):

   sudo microgen new --root /mnt --config microhop.conf --file /mnt/boot/initrd-5.14.21-default

   This command above is analysing your root filesystem at /mnt, will use microhop.conf as a profile and will write the output CPIO archive to the path, specified by --file option.

3. Un-mount your image:

   sudo umount /mnt

That's basically it and hopefully it will even boot... 😉