u-root

Description

u-root embodies four different projects.

• Go versions of many standard Linux tools, such as ls,
  cp, or shutdown. See
  cmds/core for most of these.

• Go bootloaders that use kexec to boot Linux or multiboot kernels such as
  ESXi, Xen, or tboot. They are meant to be used with
  LinuxBoot. With that, parsers for
  GRUB config files or
  syslinux config files are to make transition to
  LinuxBoot easier.

• A way to create very small Go programs using
  busybox mode or source mode (see below).

• A way to create initramfs (an archive of files) to use with Linux kernels.

Creating Initramfs Archives

u-root can create an initramfs in two different modes:

• source mode includes Go toolchain binaries + simple shell + Go source files
  in the initramfs archive. Tools are compiled from source on the fly by the
  shell.

  When you try to run a command that is not built, it is compiled first and
  stored in tmpfs. From that point on, when you run the command, you get the
  one in tmpfs. Don't worry: the Go compiler is pretty fast.

• bb mode: One busybox-like binary comprising all the Go tools you ask to
  include. See here for how it works.

  In this mode, u-root copies and rewrites the source of the tools you asked
  to include to be able to compile everything into one busybox-like binary.

SystemBoot

SystemBoot is a set of bootloaders written in Go. It is meant to be a
distribution for LinuxBoot to create a system firmware + bootloader. All of
these use kexec to boot. The commands are in cmds/boot.

• pxeboot: a network boot client that uses DHCP and HTTP or TFTP to get a
  boot configuration which can be parsed as PXELinux or iPXE configuration
  files to get a boot program.

• fbnetboot: a network boot client that uses DHCP and HTTP to get a boot
  program based on Linux, and boots it. To be merged with pxeboot.

• localboot: a tool that finds bootable kernel configurations on the local
  disks and boots them.

• boot2: similar to localboot, finds a bootable kernel configuration on
  disk (GRUB or syslinux) and boots it. To be merged into localboot.

• systemboot: a wrapper around fbnetboot and localboot that just mimicks
  a BIOS/UEFI BDS behaviour, by looping between network booting and local
  booting. Use -uinitcmd argument to the u-root build tool to make it the
  boot program.

This project started as a loose collection of programs in u-root by various
LinuxBoot contributors, as well as a personal experiment by
Andrea Barberio that has since been merged
in. It is now an effort of a broader community and graduated to a real project
for system firmwares.

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 10,
LinuxBoot using coreboot, u-root and systemboot.

You can build systemboot like this:

u-root -build=bb -uinitcmd=systemboot core github.com/u-root/u-root/cmds/boot/{systemboot,localboot,fbnetboot}

Usage

Make sure your Go version is 1.12. Make sure your GOPATH is set up correctly.

Download and install u-root:

go get github.com/u-root/u-root

You can now use the u-root command to build an initramfs. Here are some
examples:

# Build a bb-mode cpio initramfs of all the Go cmds in ./cmds/core/...
u-root -build=bb

# Generate a bb-mode archive with bootloaders
u-root -build=bb core boot

# Generate a cpio archive named initramfs.cpio.
u-root -format=cpio -build=source -o initramfs.cpio

# Generate a bb-mode archive with only these given commands.
u-root -format=cpio -build=bb ./cmds/core/{init,ls,ip,dhclient,wget,cat}

-format=cpio and -build=source are the default flag values. The default set
of packages included is all packages in
github.com/u-root/u-root/cmds/core/....

In addition to using paths to specify Go source packages to include, you may
also use Go package import paths (e.g. golang.org/x/tools/imports) to include
commands. Only the main package and its dependencies in those source
directories will be included. For example:

You can build the initramfs built by u-root into the kernel via the
CONFIG_INITRAMFS_SOURCE config variable or you can load it separately via an
option in for example Grub or the QEMU command line or coreboot config variable.

Testing in QEMU

A good way to test the initramfs generated by u-root is with qemu:

qemu-system-x86_64 -nographic -kernel path/to/kernel -initrd /tmp/initramfs.linux_amd64.cpio

Note that you do not have to build a special kernel on your own, it is
sufficient to use an existing one. Usually you can find one in /boot.

If you quickly need to obtain a kernel, for example, when you are on a non-Linux
system, you can assemble a URL to download one through Arch Linux's
iPXE menu file. It
would download from ${mirrorurl}iso/${release}/arch/boot/x86_64/vmlinuz, so
just search for a mirror URL you prefer and a release version, for example,
http://mirror.rackspace.com/archlinux/iso/2019.10.01/arch/boot/x86_64/vmlinuz.

Framebuffer

For framebuffer support, append a VESA mode via the vga kernel parameter:

qemu-system-x86_64 \
  -kernel path/to/kernel \
  -initrd /tmp/initramfs.linux_amd64.cpio \
  -append "vga=786"

For a list of modes, refer to the
Linux kernel documentation.

Entropy / Random Number Generator

Some utilities, e.g., dhclient, require entropy to be present. For a speedy
virtualized random number generator, the kernel should have the following:

CONFIG_VIRTIO_PCI=y
CONFIG_HW_RANDOM_VIRTIO=y
CONFIG_CRYPTO_DEV_VIRTIO=y

Then you can run your kernel in QEMU with a virtio-rng-pci device:

qemu-system-x86_64 \
    -device virtio-rng-pci \
    -kernel vmlinuz \
    -initrd /tmp/initramfs.linux_amd64.cpio

In addition, you can pass your host's RNG:

qemu-system-x86_64 \
    -object rng-random,filename=/dev/urandom,id=rng0 \
    -device virtio-rng-pci,rng=rng0 \
    -kernel vmlinuz \
    -initrd /tmp/initramfs.linux_amd64.cpio

Compression

You can compress the initramfs. However, for xz compression, the kernel has some
restrictions on the compression options and it is suggested to align the file to
512 byte boundaries:

xz --check=crc32 -9 --lzma2=dict=1MiB \
   --stdout /tmp/initramfs.linux_amd64.cpio \, dd conv=sync bs=512 \
   of=/tmp/initramfs.linux_amd64.cpio.xz

Extra Files

You may also include additional files in the initramfs using the -files flag.
If you add binaries with -files are listed, their ldd dependencies will be
included as well. As example for Debian, you want to add two kernel modules for
testing, executing your currently booted kernel:

NOTE: these files will be placed in the $HOME dir in the initramfs.

u-root -files "$HOME/hello.ko $HOME/hello2.ko"
qemu-system-x86_64 -kernel /boot/vmlinuz-$(uname -r) -initrd /tmp/initramfs.linux_amd64.cpio

To specify the location in the initramfs, use <sourcefile>:<destinationfile>.
For example:

u-root -files "root-fs/usr/bin/runc:usr/bin/run"

Getting Packages of TinyCore

Using the tcz command included in u-root, you can install tinycore linux
packages for things you want.

You can use QEMU NAT to allow you to fetch packages. Let's suppose, for example,
you want bash. Once u-root is running, you can do this:

% tcz bash

The tcz command computes and fetches all dependencies. If you can't get to
tinycorelinux.net, or you want package fetching to be faster, you can run your
own server for tinycore packages.

You can do this to get a local server using the u-root srvfiles command:

% srvfiles -p 80 -d path-to-local-tinycore-packages

Of course you have to fetch all those packages first somehow :-)

Build an Embeddable U-root

You can build this environment into a kernel as an initramfs, and further embed
that into firmware as a coreboot payload.

In the kernel and coreboot case, you need to configure ethernet. We have a
dhclient command that works for both ipv4 and ipv6. Since v6 does not yet work
that well for most people, a typical invocation looks like this:

% dhclient -ipv4 -ipv6=false

Or, on newer linux kernels (> 4.x) boot with ip=dhcp in the command line,
assuming your kernel is configured to work that way.

Updating Dependencies

# The latest released version of dep is required:
curl https://raw.githubusercontent.com/golang/dep/master/install.sh, sh
dep ensure

Hardware

If you want to see u-root on real hardware, this
board is a good start.

Contributions

For information about contributing, including how we sign off commits, please
see CONTRIBUTING.md.

Improving existing commands (e.g., additional currently unsupported flags) is
very welcome. In this case it is not even required to build an initramfs, just
enter the cmds/ directory and start coding. A list of commands that are on the
roadmap can be found here.