Mustang is a system for building programs built entirely in Rust, meaning they do not depend on any part of libc or crt1.o, and do not link in any C code.

Why? For fun! And to exercise some components built for other purposes (such as rsix) but which happen to also be part of what's needed to do what Mustang is doing. And in the future, possibly also for experimenting with new kinds of platform ABIs and new forms of process argument passing.

Mustang isn't about making anything safer, for the foreseeable future. The major libc implementations are extraordinarily well tested and mature. Mustang for its part is experimental and has lots of unsafe.

This also isn't about building a complete libc. It currently includes some things with libc-compatible interfaces, just enough to allow it to slide in underneath std, however even this may not always be necessary. We'll see.

Mustang currently runs on Rust Nightly on Linux on x86-64, aarch64, and x86.

Usage

To use it, first install rust-src, which is needed by -Z build-std:

$ rustup component add rust-src --toolchain nightly

Then, set the RUST_TARGET_PATH environment variable to a path to mustang's specs directory, so that you can name mustang targets with --target=.... For example, within a mustang repo:

$ export RUST_TARGET_PATH="$PWD/specs"

Then, in your own crate, add a dependency on mustang:

[dependencies]
mustang = { git = "https://github.com/sunfishcode/mustang" }

And add an extern crate declaration for mustang to your top-level module (eg. main.rs). This is needed even in Rust 2018 Edition, to ensure that mustang is linked in even though no functions in it are explicitly called:

extern crate mustang;

Then, compile with Rust nightly, using -Z build-std and --target=<mustang-target>. For example:

$ cargo +nightly run --quiet -Z build-std --target=x86_64-unknown-linux-mustang --example hello
.｡oO(This process was started by origin! 🎯)
.｡oO(Environment variables initialized by c-scape! 🌱)
.｡oO(I/O performed by c-scape using rsix! 🌊)
Hello, world!
.｡oO(This process will be exited by c-scape using rsix! 🚪)
$

That's a Rust program built entirely from Rust saying "Hello, world!"!

Those .｡oO lines are just debugging output to confirm everything is set up properly. Once mustang is more stable, we'll stop printing them.

A simple way to check for uses of libc functions is to use nm -u, since the above commands are configured to link libc dynamically. If mustang has everything covered, there should be no output:

$ nm -u target/x86_64-unknown-linux-mustang/debug/examples/hello
$

Known Limitations

Known limitations in mustang include:

• Lots of stuff in std doesn't work yet. Hello world works, but lots of other stuff doesn't yet.
• Linking to C libraries is not supported. There doesn't appear to be a robust way to initialize the C runtime without letting the C runtime start up and shutdown the process.
• No support for dynamic linking yet.
• No support for stack smashing protection (ssp) yet.
• The ELF init function is not supported, however the more modern .init_array mechanism is supported.
• The stdio descriptors are not sanitized, however for Rust programs, Rust itself already does this.

Background

Mustang is partly inspired by similar functionality in steed, but a few things are different. cargo's build-std is now available, which makes it much easier to work with custom targets. And Mustang is starting with the approach of starting by replacing libc interfaces and using std as-is, rather than reimplementing std. This is likely to evolve, but whatever we do, a high-level goal of Mustang is to avoid ever having to reimplement std.

Where does mustang go from here? Will it support feature X, platform Y, or use case Z? If origin can do program startup in Rust, and rsix can do system calls in Rust, what does it all mean?

And could mustang eventually support new ABIs that aren't limited to passing C-style argc/argv(/envp) convention, allowing new kinds of program argument passing?

Let's find out! Come say hi in the chat or an issue.

How does one port mustang to a new architecture?

• Port rsix to the architecture, adding assembly sequences for making syscalls on the architecture.
• Add assembly code to the _start function in src/lib.rs to call origin::rust.
• Create a target file in specs/, by first following these instructions to generate a default target file, and then:
  • change dynamic-linking to false
  • add -nostdlib, -Wl,--require-defined=start, and -Wl,--require-defined=environ to pre-link-args See other targets in the specs/ directory for examples.
• Add the architecture to example/test/test.rs.
• Add CI testing to .github/workflows/main.yml, by copying what's done for other architectures.

How does one port mustang to a new OS?

One probably needs to do similar things as for a new architecture, and also write a new origin::rust implementation to handle the OS's convention for arguments, environment variables, and initialization functions.