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.

• A way to compile many Go programs into a single binary with busybox mode.

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

• 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.

Requirements

For u-root on Linux, certain Kconfig options are necessary. Basic defconfigs are in configs/. See also the configs README.

Demo

Below is a demo of what it looks like, on OSX (but tested on Linux too). To recreate this demo:

git clone --depth 1 https://github.com/u-root/u-root
cd u-root/tools/testramfs
go build
GOARCH=amd64 GOOS=linux ./testramfs ../.. # ../.. is the path to u-root source

It goes by quickly, but the process of rewriting 115 commands into Go packages, via the Go AST package (i.e. compiler front end); and recompiling all that into one command and generating an initramfs, takes 7 seconds. For arm64 and riscv64, the time is also about 7 seconds. The entire set of commands, including VM boot and exit, is 31 seconds. Note: you do not need to supply an amd64, arm64, or riscv64 kernel; it is built-in to the package. GOARCH=arm64 and GOARCH=riscv64 work too.

View Demo

Usage

Make sure your Go version is >= 1.21.

Download and install u-root either via git:

git clone https://github.com/u-root/u-root
cd u-root
go install

Or install directly with go:

go install github.com/u-root/u-root@latest

[!NOTE] The u-root command will end up in $GOPATH/bin/u-root, so you may need to add $GOPATH/bin to your $PATH.

Examples

Here are some examples of using the u-root command to build an initramfs.

git clone https://github.com/u-root/u-root
cd u-root

# Build an initramfs of all the Go cmds in ./cmds/core/... (default)
u-root

# Generate an archive with bootloaders
#
# core and boot are templates that expand to sets of commands
u-root core boot

# Generate an archive with only these given commands
u-root ./cmds/core/{init,ls,ip,dhclient,wget,cat,gosh}

# Generate an archive with all of the core tools with some exceptions
u-root core -cmds/core/{ls,losetup}

[!IMPORTANT]

u-root works exactly when go build and go list work as well.

See also the section below discussing the AMD64 architecture level.

[!NOTE]

The u-root tool is the same as the mkuimage tool with some defaults applied.

In the near future, uimage will replace u-root.

[!TIP]

To just build Go busybox binaries, try out gobusybox's makebb tool.

Multi-module workspace builds

There are several ways to build multi-module command images using standard Go tooling.

$ mkdir workspace
$ cd workspace
$ git clone https://github.com/u-root/u-root
$ git clone https://github.com/u-root/cpu

$ go work init ./u-root
$ go work use ./cpu

$ u-root ./u-root/cmds/core/{init,gosh} ./cpu/cmds/cpud

$ cpio -ivt < /tmp/initramfs.linux_amd64.cpio
...
-rwxr-x---   0 root     root      6365184 Jan  1  1970 bbin/bb
lrwxrwxrwx   0 root     root            2 Jan  1  1970 bbin/cpud -> bb
lrwxrwxrwx   0 root     root            2 Jan  1  1970 bbin/gosh -> bb
lrwxrwxrwx   0 root     root            2 Jan  1  1970 bbin/init -> bb
...

# Works for offline vendored builds as well.
$ go work vendor # Go 1.22 feature.

$ u-root ./u-root/cmds/core/{init,gosh} ./cpu/cmds/cpud

When creating a new Go workspace is too much work, the goanywhere tool can create one on the fly. This works only with local file system paths:

$ go install github.com/u-root/gobusybox/src/cmd/goanywhere@latest

$ goanywhere ./u-root/cmds/core/{init,gosh} ./cpu/cmds/cpud -- u-root

goanywhere creates a workspace in a temporary directory with the given modules, and then execs u-root in the workspace passing along the command names.

[!TIP]

While workspaces are good for local compilation, they are not meant to be checked in to version control systems.

For a non-workspace way of building multi-module initramfs images, read more in the mkuimage README. (The u-root tool is mkuimage with more defaults applied.)

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 by default. See below for how to disable.

$ u-root -files /bin/bash

$ cpio -ivt < /tmp/initramfs.linux_amd64.cpio
...
-rwxr-xr-x   0 root     root      1277936 Jan  1  1970 bin/bash
...
drwxr-xr-x   0 root     root            0 Jan  1  1970 lib/x86_64-linux-gnu
-rwxr-xr-x   0 root     root       210792 Jan  1  1970 lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
-rwxr-xr-x   0 root     root      1926256 Jan  1  1970 lib/x86_64-linux-gnu/libc.so.6
lrwxrwxrwx   0 root     root           15 Jan  1  1970 lib/x86_64-linux-gnu/libtinfo.so.6 -> libtinfo.so.6.4
-rw-r--r--   0 root     root       216368 Jan  1  1970 lib/x86_64-linux-gnu/libtinfo.so.6.4
drwxr-xr-x   0 root     root            0 Jan  1  1970 lib64
lrwxrwxrwx   0 root     root           42 Jan  1  1970 lib64/ld-linux-x86-64.so.2 -> /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
...

You can determine placement with colons:

$ u-root -files "/bin/bash:sbin/sh"

$ cpio -ivt < /tmp/initramfs.linux_amd64.cpio
...
-rwxr-xr-x   0 root     root      1277936 Jan  1  1970 sbin/sh
...

For example on Debian, if you want to add two kernel modules for testing, executing your currently booted kernel:

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

Use -skip-ldd to not automatically include ldd dependencies for binary files. This can be useful for

• Reproducible Builds: Ensures builds don't depend on the host system's libraries
• Cross-compilation: When host libraries are incompatible with target architecture
• Controlled Dependencies: When you want to manually specify exact library versions

Init and Uinit

u-root has a very simple (exchangable) init system controlled by the -initcmd and -uinitcmd command-line flags.

• -initcmd determines what /init is symlinked to. -initcmd may be a u-root command name or a symlink target.

• -uinitcmd is run by the default u-root init after some basic file system setup. There is no default, users should optionally supply their own. -uinitcmd may be a u-root command name with arguments or a symlink target with arguments.

• After running a uinit (if there is one), init will start a shell determined by the -defaultsh argument.

We expect most users to keep their -initcmd as init, but to supply their own uinit for additional initialization or to immediately load another operating system.

All three command-line args accept both a u-root command name or a target symlink path. Only -uinitcmd accepts command-line arguments, however. For example,

u-root -uinitcmd="echo Go Gopher" ./cmds/core/{init,echo,gosh}

cpio -ivt < /tmp/initramfs.linux_amd64.cpio
# ...
# lrwxrwxrwx   0 root     root           12 Dec 31  1969 bin/uinit -> ../bbin/echo
# lrwxrwxrwx   0 root     root            9 Dec 31  1969 init -> bbin/init

qemu-system-x86_64 -kernel $KERNEL -initrd /tmp/initramfs.linux_amd64.cpio -nographic -append "console=ttyS0"
# ...
# [    0.848021] Freeing unused kernel memory: 896K
# 2020/05/01 04:04:39 Welcome to u-root!
#                              _
#   _   _      _ __ ___   ___ | |_
#  | | | |____| '__/ _ \ / _ \| __|
#  | |_| |____| | | (_) | (_) | |_
#   \__,_|    |_|  \___/ \___/ \__|
#
# Go Gopher
# ~/>

Passing command line arguments like above is equivalent to passing the arguments to uinit via a flags file in /etc/uinit.flags, see Extra Files.

Additionally, you can pass arguments to uinit via the uroot.uinitargs kernel parameters, for example:

u-root -uinitcmd="echo Gopher" ./cmds/core/{init,echo,gosh}

cpio -ivt < /tmp/initramfs.linux_amd64.cpio
# ...
# lrwxrwxrwx   0 root     root           12 Dec 31  1969 bin/uinit -> ../bbin/echo
# lrwxrwxrwx   0 root     root            9 Dec 31  1969 init -> bbin/init

qemu-system-x86_64 -kernel $KERNEL -initrd /tmp/initramfs.linux_amd64.cpio -nographic -append "console=ttyS0 uroot.uinitargs=Go"
# ...
# [    0.848021] Freeing unused kernel memory: 896K
# 2020/05/01 04:04:39 Welcome to u-root!
#                              _
#   _   _      _ __ ___   ___ | |_
#  | | | |____| '__/ _ \ / _ \| __|
#  | |_| |____| | | (_) | (_) | |_
#   \__,_|    |_|  \___/ \___/ \__|
#
# Go Gopher
# ~/>

Note the order of the passed arguments in the above example.

The command you name must be present in the command set. The following will not work:

u-root -uinitcmd="echo Go Gopher" ./cmds/core/{init,gosh}
# 21:05:57 could not create symlink from "bin/uinit" to "echo": command or path "echo" not included in u-root build: specify -uinitcmd="" to ignore this error and build without a uinit

You can also refer to non-u-root-commands; they will be added as symlinks. We don't presume to know whether your symlink target is correct or not.

This will build, but not work unless you add a /bin/foobar to the initramfs.

u-root -uinitcmd="/bin/foobar Go Gopher" ./cmds/core/{init,gosh}

This will boot the same as the above.

u-root -uinitcmd="/bin/foobar Go Gopher" -files /bin/echo:bin/foobar -files your-hosts-file:/etc/hosts ./cmds/core/{init,gosh}

The effect of the above command:

• Sets up the uinit command to be /bin/foobar, with 2 arguments: Go Gopher
• Adds /bin/echo as bin/foobar
• Adds your-hosts-file as etc/hosts
• builds in the cmds/core/init, and cmds/core/gosh commands.

This will bypass the regular u-root init and just launch a shell:

u-root -initcmd=gosh ./cmds/core/{gosh,ls}

cpio -ivt < /tmp/initramfs.linux_amd64.cpio
# ...
# lrwxrwxrwx   0 root     root            9 Dec 31  1969 init -> bbin/gosh

qemu-system-x86_64 -kernel $KERNEL -initrd /tmp/initramfs.linux_amd64.cpio -nographic -append "console=ttyS0"
# ...
# [    0.848021] Freeing unused kernel memory: 896K
# failed to put myself in foreground: ioctl: inappropriate ioctl for device
# ~/>

(It fails to do that because some initialization is missing when the shell is started without a proper init.)

AMD64 Architecture Level

Before building an initramfs for AMD64 with u-root, verify that the command

go env GOAMD64

prints v1. A GOAMD64 setting of any higher version may produce such binaries that don't execute on old AMD64 processors (including the default CPU model of QEMU).

GOAMD64 can be reset to v1 with one of the following methods:

• through the GOAMD64 environment variable:

  export GOAMD64=v1
  

• through go env (only takes effect if the GOAMD64 environment variable is not set):

  go env -w GOAMD64=v1
  

Cross Compilation (targeting different architectures and OSes)

Cross-OS and -architecture compilation comes for free with Go. In fact, every PR to the u-root repo is built against the following architectures: amd64, x86 (i.e. 32bit), mipsle, armv7, arm64, and ppc64le.

Further, we run integration tests on linux/amd64, and linux/arm64, using several CI systems. If you need to add another CI system, processor or OS, please let us know.

To cross compile for an ARM, on Linux:

GOARCH=arm u-root

If you are on OSX, and wish to build for Linux on AMD64:

GOOS=linux GOARCH=amd64 u-root

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 don't have a kernel handy, you can also get the one we use for VM testing:

go install github.com/hugelgupf/vmtest/tools/runvmtest@latest

runvmtest -- bash -c "cp \$VMTEST_KERNEL ./kernel"

It may not have all features you require, however.

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, such as dhclient, require entropy to be present. Without sufficient entropy, they may take a while to succeed, possibly half a minute.

If your platform has a hardware random number generator, enable it with CONFIG_ARCH_RANDOM and trust it with CONFIG_RANDOM_TRUST_CPU. Otherwise, add uroot.nohwrng to your kernel command line so u-root uses a non-blocking software random number generator implementation.

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

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. Parsers are available for GRUB, syslinux, and other config files to make the transition to LinuxBoot easier.

• 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.

• boot: finds all bootable kernels on local disk, shows a menu, and boots them. Supports (basic) GRUB, (basic) syslinux, (non-EFI) BootLoaderSpec, and ESXi configurations.

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 about LinuxBoot using coreboot, u-root and systemboot.

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.

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

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 the cpio image created by u-root into a Linux kernel via the CONFIG_INITRAMFS_SOURCE config variable or coreboot config variable, and further embed the kernel image into firmware as a coreboot payload.

In the kernel and coreboot case, you may 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.

Build Modes

u-root can create an initramfs in two different modes, specified by -build:

• gbb mode: One busybox-like binary comprising all the Go tools you ask to include. See the gobusybox README 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.

• binary mode: each specified binary is compiled separately and all binaries are added to the initramfs.

Updating Dependencies

go get -u
go mod tidy
go mod vendor

Building without network access

The u-root command supports building with workspace vendoring and module vendoring. In both of those cases, if all dependencies are found in the vendored directories, the build happens completely offline.

Read more in the mkuimage README.

u-root also still supports GO111MODULE=off builds.

Hardware

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

Using with Plan 9

U-root works with Plan 9. The best distro to use for it is 9front, as the 9front cpiofs works with newc-format cpio (the format of Linux initramfs, which u-root generates).

Here is a script for Plan 9. Once this script runs, all the u-root commands appear in /bin. You will need to have go1.22 installed on Plan 9; or create the u-root initramfs on some other system and copy it to Plan 9.

#!/bin/rc
u-root '-defaultsh=' '-initcmd=' '-shellbang=true'
fs/cpiofs /tmp/initram*cpio
bind -a /n/tapefs /
bind -a /bbin /bin

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.

Website

The sources of u-root.org are inside the docs/ directory and are deployed to the gh-pages branch. The CNAME file is currently not part of the CI which deploys to the branch which shall be evaluated if this makes futures deployments easier.