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0.3.0 | Nov 18, 2021 |
#35 in FFI
163 downloads per month
79KB
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 that have other purposes (such as
rustix
, origin
, c-scape
, and c-gull
) 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.
Mustang currently runs on Rust Nightly 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:
- ripgrep
- coreutils, including the "unix" feature set
- async-std
- tokio
- bat, including git support with libgit2
- cargo-watch
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
.
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 the
mustang/target-specs
directory, so that you can name mustang
targets with
--target=…
. For example, within a mustang repo:
$ export RUST_TARGET_PATH="$PWD/mustang/target-specs"
Then, in your own crate, add a dependency on mustang
:
[dependencies]
mustang = "<current version>"
And add a mustang::can_run_this!();
to your top-level module (eg. main.rs).
This does nothing in non-mustang
-target builds, but in mustang
-target
builds arranges for mustang
's libraries to be linked in.
mustang::can_run_this!();
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-mustang-linux-gnu --example hello
Hello, world!
$
That's a Rust program built entirely from Rust saying "Hello, world!"!
For more detail, mustang has an env_logger
feature, which you can enable, and set
RUST_LOG
to see various pieces of mustang in action:
$ RUST_LOG=trace cargo +nightly run --quiet -Z build-std --target=x86_64-mustang-linux-gnu --example hello --features log,env_logger
[2021-06-28T06:28:31Z TRACE origin::program] Program started
[2021-06-28T06:28:31Z TRACE origin::threads] Main Thread[916066] initialized
[2021-06-28T06:28:31Z TRACE origin::program] Calling `.init_array`-registered function `0x5555558fb480(1, 0x7fffffffdb98, 0x7fffffffdba8)`
[2021-06-28T06:28:31Z TRACE origin::program] Calling `main(1, 0x7fffffffdb98, 0x7fffffffdba8)`
Hello, world!
[2021-06-28T06:28:31Z TRACE origin::program] `main` returned `0`
[2021-06-28T06:28:31Z TRACE origin::program] Program exiting
$
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-mustang-linux-gnu/debug/examples/hello
$
C Runtime interop
To compile C code with a *-mustang-*
target, you may need to
tell the cc
crate which C compiler to use; for example, for i686-mustang-linux-gnu
,
set the environment variable CC_i686-mustang-linux-gnu
to
i686-linux-gnu-gcc
.
panic = "abort"
When using panic = "abort"
in your Cargo.toml, change the -Z build-std
to
-Z build-std=panic_abort,std
. See here for background.
Known Limitations
Known limitations in mustang
include:
- Dynamic linking isn't implemented yet.
- Many libc C functions that aren't typically needed by most Rust programs aren't implemented yet.
Alternatives
Eyra uses the same underlying libraries as Mustang, but doesn't use a custom
target, and doesn't need -Z build-std
.
Using Origin directly is another option; there are examples showing how
to do this. Using origin directly can produce very small binary sizes; the
tiny
example can produce a 408-byte executable. The big downside of using
Origin directly is that it doesn't provide a std
implementation.
Origin Studio is a demo of what a std
-like environment on top of Origin
might look like. It has things like println!
, but it's also currently lots
of features, such as File
.
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 rustix
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
rustix
to the architecture, adding assembly sequences for making syscalls. - Port
origin
to the architecture, adding assembly sequences for program and thread primitives. - Create a target file in
mustang/target-specs
, by first following these instructions to generate a specification of a built-in target, and then:- change
is-builtin
to false - change
dynamic-linking
to false - add
-nostartfiles
and-Wl,--undefined=_Unwind_Backtrace
to pre-link-args - add
"vendor": "mustang"
See other targets in themustang/target-specs
directory for examples.
- change
- Compile some of the programs in the
examples
directory, using the new target. Trynm -u
on the binaries to check for undefined symbols which need to be implemented. - Add the architecture to tests/tests.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.
Using mustang in non-mustang programs
In non-*-mustang-*
targets, importing this crate and using the
mustang::can_run_this!()
macro has no effect, does not depend on nightly
Rust, and has no extra dependencies.
See the mustang-example example for more details.
Similar crates
c-scape has some similarities to relibc, but has a different focus. Relibc is
aiming to be a complete libc replacement, while c-scape is just aiming to cover
the things used by Rust's std
and popular crates. Some parts of Relibc are
implemented in C, while c-scape is implemented entirely in Rust.
c-scape is also similar to steed. See the Background for details.
The most significant thing that makes c-scape unique though is its design as a set of wrappers around Rust crates with Rust interfaces. C ABI compatibility is useful for getting existing code working, but once things are working, we can simplify and optimize by changing code to call into the Rust interfaces directly. This can eliminate many uses of raw pointers and C-style NUL-terminated strings, so it can be much safer.
Other similar projects are tiny-std and veneer. Similar to steed, they contain their own implementations of std.
Dependencies
~0–9MB
~94K SLoC