gorepro

README

gorepro

go install github.com/capnspacehook/gorepro@latest

Reproduce Go binaries by recreating and running the commands that produced them.

gorepro [flags] binary

Requirements

Binaries to reproduce must have been built with Go 1.18 or later. The latest stable release of Go is also required to be present on the system you will run gorepro on. Additionally, Docker may be required depending on how the binary you want to reproduce was originally built.

Purpose

Verifying pre-compiled binaries of open source projects is important from a security prospective. While you can easily inspect the source code for backdoors or malicious flaws, it can be difficult to verify the validity of distributed binaries. By manually creating a binary and comparing it to the official binary, you can verify that the official binary was indeed created from the published source code.

Go makes it fairly easy to reproduce binaries, but it is still not always simple. Gorepro aims to make the process as easy as possible. Simply give it a Go binary and it will attempt to recreate the exact same binary, bit for bit.

Mechanism

Gorepro recreates the go build command that produced a specified binary by reading the embedded build metadata that Go 1.18 and newer includes by default. Gorepro will build in a docker container if the version of Go on your system wasn't what was used to build the specified binary. If the binary also has embedded version control system (VCS) information, gorepro will try to ensure your build environment is compatible with what the binary was created with. After the new binary is built, gorepro hashes both the original and new binaries with SHA-256 and compares the hashes to ensure they are exactly the same.

Gorepro assumes and requires that the directory it is run in contains the source code used to build the specified binary. The binary to reproduce does not need to be in any specified directory though.

Troubleshooting

If gorepro detects a problem that prevents reproducibility that it can't fix, it will notify you and tell you how to fix it. However, gorepro can't detect everything, and some fixes aren't intuitive (looking at you, unset -trimpath).

Manually passing Go files

Gorepro will fail if the binary you're trying to reproduce had Go files passed to the go build command that created it and you aren't in the same directory as those files. Yes, go build -o mybin and go build -o mybin main.go will produce slightly different binaries for some reason. To fix, simply run gorepro in the directory of the file(s) gorepro specified.

Undetected build flags

Currently -buildmode isn't included in build metadata, though that should change for Go 1.20. In any case, you may find that the command gorepro generates to reproduce a binary is missing flags that were included in the original build. If that's the case, you can pass the missing flags like so: gorepro -b="-buildmode=pie -ldflags=-s" mybin.

Caveats

• Reproducing binaries built with CGO_ENABLED=1 is not supported. Reproducing C/C++ code is a whole can of worms I'm not even going to attempt to try.
• Git is the only VCS that gorepro supports. Reproducing binaries built that use other VCSes is possible using gorepro, but build environment checks will not be run.
• Only reproducing binaries built by the official Go compiler gc and the official Go toolchain is supported. I may support the gccgo compiler in the future if the need arises, but I'm unsure whether I'd want to support TinyGo or other Go implementations.

Creating easily reproducible binaries

• If at all possible, set CGO_ENABLED=0 when building. Pure Go binaries are much easier to reproduce.
• Always set -trimpath. There is no reason not to for production builds. Not doing so requires users who wish to reproduce your binary to build from the exact same directory path you did, and set the GOROOT and GOMODCACHE environmental variables to the same values that were used when the binary was built.
• Document the process used to create the binary, or better yet publish the code that produced it. Gorepro can detect most flags or environmental variables used, but cannot detect all depending on what version of Go was used to build the binary.