uimage

uimage builds initramfs images composed of arbitrary Go commands and files.

uimage optimizes for space by utilizing gobusybox to compile many arbitrary Go commands into one binary.

uimage can be easily used with u-root, which contains many Go coreutils-like commands as well as bootloaders. However, uimage supports compilation of any Go command, and the use of u-root is not required.

Getting Started

Make sure your Go version is >= 1.21. If your Go version is lower,

$ go install golang.org/dl/go1.21.5@latest
$ go1.21.5 download
$ go1.21.5 version
# Now use go1.21.5 in place of go

Download and install mkuimage either via git:

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

Or install directly with go:

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

Note

The mkuimage command will end up in $GOPATH/bin/mkuimage, so you may need to add $GOPATH/bin to your $PATH.

Examples

Here are some examples of using the mkuimage command to build an initramfs.

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

Build gobusybox binaries of these two commands and add to initramfs:

$ cd ./u-root
$ mkuimage ./cmds/core/{init,gosh}

$ cpio -ivt < /tmp/initramfs.linux_amd64.cpio
...
-rwxr-x---   0 root     root      4542464 Jan  1  1970 bbin/bb
lrwxrwxrwx   0 root     root            2 Jan  1  1970 bbin/gosh -> bb
lrwxrwxrwx   0 root     root            2 Jan  1  1970 bbin/init -> bb
...

Add symlinks for shell and init:

$ mkuimage -initcmd=init -defaultsh=gosh ./cmds/core/{init,gosh}

$ cpio -ivt < /tmp/initramfs.linux_amd64.cpio
...
lrwxrwxrwx   0 root     root           12 Jan  1  1970 bin/defaultsh -> ../bbin/gosh
lrwxrwxrwx   0 root     root           12 Jan  1  1970 bin/sh -> ../bbin/gosh
...
lrwxrwxrwx   0 root     root            9 Jan  1  1970 init -> bbin/init
...

Builds with globs and exclusion

Build everything from core without ls and losetup:

$ mkuimage ./cmds/core/* -./cmds/core/{ls,losetup}

Multi-module workspace builds

Important

mkuimage works when go build and go list work as well.

There are 2 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

$ mkuimage ./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.

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

GBB_PATH is a place that mkuimage will look for commands. Each colon-separated GBB_PATH element is concatenated with patterns from the command-line and checked for existence. For example:

GBB_PATH=$(pwd)/u-root:$(pwd)/cpu mkuimage \
    cmds/core/{init,gosh} \
    cmds/cpud

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

To ease usability, the goanywhere tool can create one Go workspaces 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 -- go build -o $(pwd)
$ goanywhere ./u-root/cmds/core/{init,gosh} ./cpu/cmds/cpud -- mkuimage

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.

goanywhere supports GBB_PATH, exclusions, globs, and curly brace expansions as well.

Caution

While workspaces are good for local compilation, they are not meant to be checked in to version control systems. See below for the recommended way.

Multi-module go.mod builds

You may also create a go.mod with the commands you intend to compile.

To depend on commands outside of ones own repository, the easiest way to depend on Go commands is the following:

mkdir mydistro
cd mydistro
go mod init mydistro

Create a file with some unused build tag like this to create dependencies on commands:

//go:build tools

package something

import (
        _ "github.com/u-root/u-root/cmds/core/ip"
        _ "github.com/u-root/u-root/cmds/core/init"
        _ "github.com/hugelgupf/p9/cmd/p9ufs"
)

You can generate this file for your repo with the gencmddeps tool from gobusybox:

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

gencmddeps -o deps.go -t tools -p something \
    github.com/u-root/u-root/cmds/core/{ip,init} \
    github.com/hugelgupf/p9/cmd/p9ufs

The unused build tag keeps it from being compiled, but its existence forces go mod tidy to add these dependencies to go.mod:

go mod tidy

mkuimage \
  github.com/u-root/u-root/cmds/core/ip \
  github.com/u-root/u-root/cmds/core/init \
  github.com/hugelgupf/p9/cmd/p9ufs

# Works for offline vendored builds as well.
go mod vendor

mkuimage \
  github.com/u-root/u-root/cmds/core/ip \
  github.com/u-root/u-root/cmds/core/init \
  github.com/hugelgupf/p9/cmd/p9ufs

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.

$ mkuimage -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:

$ mkuimage -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:

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

Cross Compilation (targeting different architectures and OSes)

Just like standard Go tooling, cross compilation is easy and supported.

To cross compile for an ARM, on Linux:

GOARCH=arm mkuimage ./u-root/cmds/core/*

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

GOOS=linux GOARCH=amd64 ./u-root/cmds/core/*

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.

To automate testing, you may use the same vmtest framework that we use as well. It has native uimage support.

Build Modes

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

bb 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, mkuimage 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.