no-std eyra

Rust programs written entirely in Rust

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new 0.16.7 Dec 5, 2023
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0.16.3 Oct 31, 2023
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Rust programs written entirely in Rust

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Eyra is a package that supports building Rust programs implemented entirely in Rust.

It uses Origin for program and thread startup and shutdown, and c-gull for ABI-compatible libc function implementations. It currently works on Nightly Rust on Linux on x86-64, x86, aarch64, and riscv64.

Quick start

Running a Rust program under Eyra needs two steps. First, a Cargo.toml dependency, which we can add with:

cargo add eyra --rename=std

And, a build.rs file to add -nostartfiles to the link flags to disable the host startup code, so that Eyra can provide its own. build.rs:

fn main() {

With that, cargo build, cargo run, cargo test (with Nightly) and so on will work normally with any *-unknown-linux-gnu* target.

Under the covers, it's using Origin to start and stop the program, c-ward to handle libc calls from std, and rustix to do the printing, so it's completely implemented in Rust.


For an example of the above steps, check out this hello world example.

Other examples include


Why use Eyra?

  • It fixes Rust's set_var unsoundness issue. The environment-variable implementation leaks memory internally (it is optional, but enabled by default), so setenv etc. are thread-safe.

  • Whole-program LTO, including the libc. This sometimes produces smaller static binaries, and sometimes produces faster code (though on the other hand, sometimes it doesn't, though on the first hand, there are still low-hanging fruit, so consider trying it and filing issues).

    For even more code-size reductions, see the techniques in the hello-world-small example.

  • Support for compiling programs with alternate calling conventions, using Eyra and -Zbuild-std to build a program completely from source.

  • Fully static linking that supports the platform NSS/DNS config. "Is such a thing even possible?", "Yes it is."

  • Or, bring your own reason! Be creative and do your own thing, and tell us about it!

Why not use Eyra?

  • It's not as mature as the major libc implementations.

  • It's not as complete as the major libc implementations. It can run most Rust code, and some popular C libraries, but still lacks a lot of things used by typical C code.

  • It currently depends on Rust Nightly and only runs on Linux, and currently only on x86-64, x86, aarch64, and riscv64.

  • It can't currently run under Miri because Miri doesn't currently recognize syscalls made from assembly code. That said, Eyra does strive to adhere to strict provenance and to avoid undefined behavior throughout, so if Miri were to gain support for such syscalls, Eyra should be well-positioned.

  • No support for dynamic linking.

It might seem like "memory safety" might be a reason to use Eyra, and Eyra does have a lot of code written in safe Rust, so it does benefit some from Rust's memory safety. However, Eyra also has a lot of unsafe code (it's unavoidable for implementing a libc). Until this code has been more throughly proven, it's not realistic to consider it more safe than mature C code.

Fully static linking

Eyra executables don't depend on any dynamic libraries, however by default they do still depend on a dynamic linker (eg. "/lib64/ld-linux-x86-64.so.2").

For fully static linking, there are currently two options:

  • Build with RUSTFLAGS=-C target-feature=+crt-static -C relocation-model=static. This disables Position-Independent Executable (PIE) mode, which is straightforward, however it loses the security benefits of Address-Space Layout Randomization (ASLR).

  • Build with RUSTFLAGS=-C target-feature=+crt-static and enable the experimental-relocate feature. This allows PIE mode and ASLR to work, however it does so by enabling an experimental implementation of relocations. This code seems to be working in practice so far, however it involves Rust code patching itself as it runs, which is outside of any Rust semantics.

Optional logging

Eyra has a log feature to enable Rust log tracing of program and thread startup and shutdown, and an env_logger feature to install env_logger as the logger, which can be enabled in Cargo.toml:

std = { package = "eyra", version = "<current-version>", features = ["log", "env_logger"] }

With this, and setting the RUST_LOG environment variable to "trace", the hello world program output like this:

[TRACE origin::program] Program started
[TRACE origin::thread] Main Thread[51383] initialized
[TRACE origin::program] Calling `.init_array`-registered function `0x55e86306bb80(1, 0x7ffd0f76aad8, 0x7ffd0f76aae8)`
[TRACE origin::program] Calling `origin_main(1, 0x7ffd0f76aad8, 0x7ffd0f76aae8)`
Hello, world!
[TRACE origin::program] `origin_main` returned `0`
[TRACE origin::thread] Thread[51383] calling `at_thread_exit`-registered function
[TRACE origin::thread] Thread[51383] calling `at_thread_exit`-registered function
[TRACE origin::program] Program exiting with status `0`

Compatibility with -Zbuild-std

Eyra works with -Zbuild-std, however the --rename=std trick used above doesn't work, so it's necessary to instead use this cargo add invocation:

cargo add eyra

and to also add this line to the program's main.rs file:

extern crate eyra;

to ensure that the Eyra libraries are linked in.

Reducing code size

Eyra can be used with the techniques in min-sized-rust to produce very small statically-linked binaries. Check out the hello-world-small example.

Relationship to Mustang

Eyra is similar to Mustang and uses the same underlying code, but instead of using a custom target and -Z build-std, Eyra just needs users to add -nostartfiles to their link line, such as via build.rs in the example.

Like Mustang, Eyra currently runs on Nightly Rust on Linux on x86-64, x86, aarch64, and riscv64. It aims to support all Linux versions supported by Rust, though at this time it's only tested on relatively recent versions. It's complete enough to run:

Design philosophy

Eyra and the libraries it uses have some design goals.

Normal Rust, all the way down

Sometimes in libc implementation code, there's a temptation to say "it's ok if some things are technically Undefined Behavior, because this is Low Level Code and We Know What We're Doing".

Origin, c-scape, c-gull, rustix, and the others strive to resist this temptation, and follow the Rust rules, including strict provenance, I/O safety, and all the rest, all the way down to the syscalls.

It's just normal Rust code, as far down as we can go in userspace, and when we eventually do have to switch to inline asm, we do as little of it as we can.

Currently there is only one known place where this goal is not achieved. In a "static PIE" executable (eg. built with RUSTFLAGS="-C target-feature=+crt-static"), the dynamic linker isn't used, so the executable has to handle all its relocations itself. However, that means storing to memory locations that wouldn't otherwise be considered mutable. Origin's code for doing this is currently disabled by default, and can be enabled with the "experimental-relocate" cargo feature.

C compatibility as a layer on top of Rust, not vice versa

Eyra is built on a collection of Rust crates with idiomatic Rust APIs, and two crates, c-scape and c-gull, which are relatively thin layers on top that implement the libc-compatible C ABI.

It's sometimes more work to write the code as separate layers like this, but it has the advantage of clearly separating out the unsafe associated with things like C pointers and strings in libc APIs from the essential unsafe needed to implement things like system calls, thread primitives, and other features. And it means that Rust programs that don't want to go through the C compatibility layer can use the underlying crates directly.


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