redhook is a helper crate for writing interposition libraries (LD_PRELOAD, DYLD_INSERT_LIBRARIES, etc.) in Rust.

To use redhook, add the lines

#[macro_use]
extern crate redhook;

to your library, then use the hook! macro to declare the function you want to hook, and the name you want to give to your hook function:

hook! {
    unsafe fn existing_function(x: i32) -> i32 => my_function {
        42
    }
}

To access the underlying function, use real!(existing_function).

Compile your library as a dylib, which can be done via rustc --crate-type=dylib, or by adding

[lib]
name = "mylib"
crate_type = ["dylib"]

to your Cargo.toml. Then find the resulting .so file (./mylib.so for rustc, or inside target for Cargo) and set the LD_PRELOAD environment variable to that.

There are a few examples included, as separate crates:

geofft@cactuar:~/src/rust/redhook/examples/readlinkspy$ cargo build
   Compiling redhook v0.0.1 (file:///home/geofft/src/rust/redhook/examples/readlinkspy)
   Compiling redhook_ex_readlinkspy v0.0.1 (file:///home/geofft/src/rust/redhook/examples/readlinkspy)
geofft@cactuar:~/src/rust/redhook/examples/readlinkspy$ LD_PRELOAD=target/debug/libreadlinkspy-7cb7c673fbb62878.so ls -l /bin/sh
readlink("/bin/sh")
lrwxrwxrwx 1 root root 4 Feb 19  2014 /bin/sh -> dash

geofft@cactuar:~/src/rust/redhook/examples/fakeroot$ cargo build
geofft@cactuar:~/src/rust/redhook/examples/fakeroot$ LD_PRELOAD=target/debug/libfakeroot-1be6565b67838e2e.so id
uid=0(root) gid=1001(geofft) euid=1001(geofft) groups=1001(geofft),27(sudo),111(sbuild)

redhook currently supports building interposition libraries for LD_PRELOAD on glibc (GNU/Linux) and DYLD_INSERT_LIBRARIES on Apple's libc (Mac OS X) from the same source code. If you're interested in support for other platforms, please file an issue or pull request.

redhook is named after the Red Hook neighborhood in Brooklyn, New York. Portions of the implementation borrow heavily from concepts in @Kimundi's lazy_static! macro.

redhook is free software, available under the terms of the 2-clause BSD license.

Please note that no attempt is made to catch panics. Once the code changes for RFC 1236 land and are stable, I'll release a version that depends on them. Until then, your code must ensure that it never panics.