Go Busybox

Contact: best bet to reach us is the #u-root-dev channel on the Open Source Firmware Slack (Sign Up Link).

Go Busybox is a set of Go tools that allow you to compile many Go commands into one binary. The resulting binary uses its invocation arguments (os.Args) to determine which command is being called.

| Feature | Support status | | ---------- | ----------------------------------------------------- | | Go version | Tested are 1.20-1.22 | | Packaging | Go workspaces, Go modules, Go vendoring | | GOOS | any (linux is tested) | | GOARCH | any (amd64, arm, arm64, riscv64 are tested) | | CGO | Not supported |

An example:

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

git clone github.com/u-root/u-root
cd u-root
makebb ./cmds/core/dmesg ./cmds/core/strace

A binary named bb should appear. It can be invoked in one of two ways -- either with a symlink or using a second argument.

./bb dmesg
./bb strace echo "hi"

It is meant to be used with symlinks for convenience:

# Make a symlink dmesg -> bb
ln -s bb dmesg
# Symlink means that argv[0] is the command name.
./dmesg

# Make a symlink strace -> bb
ln -s bb strace
./strace echo "hi"

Go Busybox does this by copying all the source for these Go commands and rewriting it in a temporary directory.

Go Busybox can be used with any Go commands across multiple Go modules:

mkdir workspace
cd workspace

git clone https://github.com/hugelgupf/p9
git clone https://github.com/u-root/cpu

go work init ./p9
go work use ./cpu

makebb ./cpu/cmds/* ./p9/cmd/*

[!IMPORTANT] makebb works any time go build or go list also work.

For multi-module compilation, use Go workspaces or read below.

Path resolution & multi-module builds

makebb and the APIs mentioned below all accept commands from file system paths, Go package paths, and globs matching path.Match thereof.

makebb with globs and exclusions

In addition to the standard syntaxes supported by go list and go build, makebb also accepts globs and exclusions.

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

makebb ./cmds/core/ip ./cmds/core/init

# Escaping the * to show that the Go APIs know how to resolve this as well
makebb ./cmds/core/\*
makebb ./cmds/core/i\* ./cmds/boot/pxeboot

# All core commands except ip.
makebb ./cmds/core/\* -./cmds/core/ip

makebb with Go workspaces & GBB_PATH.

To compile commands from multiple modules, you may use workspaces.

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

makebb \
    ./u-root/cmds/core/init \
    ./u-root/cmds/core/elvish \
    ./cpu/cmds/cpud

# Also works for offline builds with `go work vendor` (Go 1.22 feature):
go work vendor

makebb \
    ./u-root/cmds/core/init \
    ./u-root/cmds/core/elvish \
    ./cpu/cmds/cpud

For a shortcut to specify many commands sharing common path elements (e.g. from the same repository), the GBB_PATH environment variable exists. Paths are concatenated with every colon-separated element of GBB_PATH from left to right and checked for existence.

GBB_PATH=$(pwd)/u-root:$(pwd)/cpu makebb \
    cmds/core/init \
    cmds/core/elvish \
    cmds/cpud

# matches:
#   $(pwd)/u-root/cmds/core/init
#   $(pwd)/u-root/cmds/core/elvish
#   $(pwd)/cpu/cmds/cpud

goanywhere

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

goanywhere creates a Go workspace temporarily on the fly from the packages' modules in the given local file paths.

goanywhere then executes the command given after "--" in the workspace directory and amends the packages as args.

For example,

# -o $(pwd) is needed since goanywhere executes the command in the workspace
# directory, and by default the Go binary is created in the current wd.
goanywhere ./u-root/cmds/core/{init,gosh} -- go build -o $(pwd)
goanywhere ./u-root/cmds/core/{init,gosh} -- makebb -o $(pwd)
goanywhere ./u-root/cmds/core/{init,gosh} -- mkuimage [other args]
goanywhere ./u-root/cmds/core/{init,gosh} -- u-root [other args]

Like makebb, goanywhere supports GBB_PATH, globs and shell expansions.

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

makebb with multiple Go modules

makebb supports Go workspaces for locally checked out sources, as shown above.

For multiple Go module command dependencies that aren't checked out locally, we apply some Go module tricks.

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

TMPDIR=$(mktemp -d)
cd $TMPDIR
go mod init foobar

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:

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

[!IMPORTANT] gencmddeps does not support file paths or exclusions, as these rely on a go.mod already being present to resolve the Go package name.

The input to gencmddeps must be full Go package paths.

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

go mod tidy

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

# Also works with vendored, offline builds:
go mod vendor

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

APIs

Besides the makebb CLI command, there is a Go API at src/pkg/bb.

Shortcomings

• Any imported packages' init functions are run for every command.

  For example, if some command imports and uses the testing package, all commands in the busybox will have testing's flags registered as a side effect, because testing's init function runs with every command.

  While Go busybox handles every main commands' init functions, it does not handle dependencies' init functions. Done properly, it would rewrite all non-standard-library packages as well as commands. This has not been necessary to implement so far. It would likely be necessary if, for example, two different imported packages register the same flag unconditionally globally.

How It Works

src/pkg/bb implements a Go source-to-source transformation on pure Go code (no cgo).

This AST transformation does the following:

• Takes a Go command's source files and rewrites them into Go package files (almost) without global side effects.
• Writes a main.go file with a main() that calls into the appropriate Go command package based on argv[0] or argv[1].

This allows you to take two Go commands, such as Go implementations of dmesg and strace and compile them into one binary.

Which command is invoked is determined by argv[0] or argv[1] if argv[0] is not recognized. Let's say bb is the compiled binary; the following are equivalent invocations of dmesg and strace:

(cd ./src/cmd/makebb && go install)
(cd ./test/nested && makebb ./cmd/dmesg ./cmd/strace)

# Make a symlink dmesg -> bb
ln -s bb dmesg
./dmesg

# Make a symlink strace -> bb
ln -s bb strace
./strace echo "hi"

./bb dmesg
./bb strace echo "hi"

Command Transformation

Principally, the AST transformation moves all global side-effects into callable package functions. E.g. main becomes registeredMain, each init becomes initN, and global variable assignments are moved into their own initN. A registeredInit calls each initN function in the correct init order.

Then, these registeredMain and registeredInit functions can be registered with a global map of commands by name and used when called upon.

Let's say a command github.com/org/repo/cmds/sl contains the following main.go:

package main

import (
  "flag"
  "log"
)

var name = flag.String("name", "", "Gimme name")

func init() {
  log.Printf("init %s", *name)
}

func main() {
  log.Printf("train")
}

This would be rewritten to be:

package sl // based on the directory name

import (
  "flag"
  "log"

  "../bb/pkg/bbmain" // generated import path
)

// Type has to be inferred through type checking.
var name *string

func init0() {
  log.Printf("init %s", *name)
}

func init1() {
  name = flag.String("name", "", "Gimme name")
}

func registeredInit() {
  // Order is determined by go/types.Info.InitOrder.
  init1()
  init0()
}

func registeredMain() {
  log.Printf("train")
}

func init() {
  bbmain.Register("sl", registeredInit, registeredMain)
}

Generated main.go

The main.go file is generated from ./src/pkg/bb/bbmain/cmd/main.go.

package