systemboot

Note: systemboot has been merged into u-root. This repository is now read-only and staying only for historical reasons, but you should build your bootloader entirely from u-root. How? The CLI tools have been moved under u-root/cmds/boot{fbnetboot,localboot,uinit}, the libraries under [u-root/pkg/]{bootconfig,booter,checker,crypto,recovery,rng,storage,vpd}. u-root/tools/vpdbootmanager and u-root/examples/fixmynetboot.

SystemBoot is a distribution for LinuxBoot to create a system firmware + bootloader. It is based on u-root. The provided programs are:

• netboot: a network boot client that uses DHCP and HTTP to get a boot program based on Linux, and uses kexec to run it
• localboot: a tool that finds bootable kernel configurations on the local disks and boots them
• uinit: a wrapper around netboot and localboot that just mimicks a BIOS/UEFI BDS behaviour, by looping between network booting and local booting. The name uinit is necessary to be picked up as boot program by u-root.

This work is similar to the pxeboot and boot commands that are already part of u-root, but approach and implementation are slightly different. Thanks to Chris Koch and Jean-Marie Verdun for pioneering in this area.

This project started as a personal experiment under github.com/insomniacslk/systemboot but it is now an effort of a broader community and graduated to a real project for system firmwares.

The next sections go into further details.

netboot

The netboot client has the duty of configuring the network, downloading a boot program, and kexec’ing it. Optionally, the network configuration can be obtained via SLAAC and the boot program URL can be overridden to use a known endpoint.

In its DHCP-mode operation, netboot does the following:

• bring up the selected network interface (eth0 by default)
• make a DHCPv6 transaction asking for network configuration, DNS, and a boot file URL
• extract network and DNS configuration from the DHCP reply and configure the interface
• extract the boot file URL from the DHCP reply and download it. The only supported scheme at the moment is HTTP. No TFTP, sorry, it’s 2018 (but I accept pull requests)
• kexec the downloaded boot program

There is an additional mode that uses SLAAC and a known endpoint, that can be enabled with -skip-dhcp, -netboot-url, and a working SLAAC configuration.

localboot

The localboot program looks for bootable kernels on attached storage and tries to boot them in order, until one succeeds. In the future it will support a configurable boot order, but for that I need Google VPD support, which will come soon.

In the current mode, localboot does the following:

• look for all the locally attached block devices
• try to mount them with all the available file systems
• look for a GRUB configuration on each mounted partition
• look for valid kernel configurations in each GRUB config
• try to boot (via kexec) each valid kernel/ramfs combination found above

In the future I will also support VPD, which will be used as a substitute for EFI variables, in this specific case to hold the boot order of the various boot entries.

uinit

The uinit program just wraps netboot and localboot in a forever-loop logic, just like your BIOS/UEFI would do. At the moment it just loops between netboot and localboot in this order, but I plan to make this more flexible and configurable.

How to build systemboot

• Install a recent version of Go, we recommend 1.10 or later
• make sure that your PATH points appropriately to wherever Go stores the go-get’ed executables
• Then build it with the u-root ramfs builder using the following commands:

go get -u github.com/u-root/u-root
go get -u github.com/systemboot/systemboot/{uinit,localboot,netboot}
u-root -build=bb core github.com/systemboot/systemboot/{uinit,localboot,netboot}

The initramfs will be located in /tmp/initramfs_${platform}_${arch}.cpio.

More detailed information about the build process for a full LinuxBoot firmware image using u-root/systemboot and coreboot can be found in the LinuxBoot book chapter 11, LinuxBoot using coreboot, u-root and systemboot.

Example: LinuxBoot with coreboot

One of the ways to create a LinuxBoot system firmware is by using coreboot do the basic silicon and DRAM initialization, and then run Linux as payload, with u-root and systemboot as initramfs. See the following diagram:

(images from coreboot.org and wikipedia.org, diagram generated with draw.io)

Build and run as a fully open source bootloader in Qemu

Systemboot is one of the parts of a bigger picture: running Linux as firmware. We call this LinuxBoot, and it can be achieved in various ways. One of these is by combining coreboot, Linux, u-root and systemboot. Check out the instructions on the LinuxBoot using coreboot, u-root and systemboot chapter of the LinuxBoot Book.

TODO

• verified and measured boot
• a proper GRUB config parser
• backwards compatibility with BIOS-style partitions

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