02 Jan 2017

Rust on the WiFi Pineapple (and OpenWrt)

Over the holidays I wanted to get a very simple Rust application running on my WiFi Pineapple Nano. Since I don’t have much experience with embedded systems and cross-compilation, it sounded like something fun to do and I was sure I might learn a thing or two. Many hours later and lots of frustration, I ended up with a simple solution that I’d like to share in this post.

The major obstacle to overcome is building your own standard library, since Rust doesn’t ship with one for our target out of the box. I first tried it “the old way” by checking out the source and running through all the steps described in rust-cross. While I got it to build in the end it still didn’t work because of some version issues (I compiled with rustc 1.16.0-dev (4ecc85beb 2016-12-28) and the same version but rustc 1.16.0-nightly (4ecc85beb 2016-12-28) rejected running it).

Close to giving up at this point after spending hours on it, I found the xargo wrapper around cargo, an awesome project by Jorge Aparicio. With this tool I got it to work fairly quickly and this is also the approach we’ll be following in this post. We still need to handle a couple of extra steps not outlined in the xargo documentation, so I hope this post provides extra value to some of you.

Which target?

Before we get into the weeds we need to do our homework first and figure out the compilation target for cross compilation. Once you SSH into your pineapple and run uname -a you’ll see something like this:

[email protected]:~# uname -a
Linux Pineapple 3.18.36 #40 Fri Oct 28 05:42:22 UTC 2016 mips GNU/Linux

This is the output from my WiFi Pineapple NANO running the 1.1.3 firmware. The important part here is mips, which gives us a clue which platform the pineapple is running on. The device is built on top of OpenWRT, a linux distribution for embedded devices.

The /etc/openwrt_release file provides more information about the OpenWrt release itself:

[email protected]:~# cat /etc/openwrt_release
DISTRIB_ID='OpenWrt'
DISTRIB_RELEASE='Chaos Calmer'
DISTRIB_REVISION='r49403'
DISTRIB_CODENAME='chaos_calmer'
DISTRIB_TARGET='ar71xx/generic'
DISTRIB_DESCRIPTION='OpenWrt Chaos Calmer 15.05.1'
DISTRIB_TAINTS='no-all no-ipv6 busybox'

Our target is ‘ar71xx/generic’, running OpenWrt ChaosCalmer 15.05.1. Looking for more info here, we can see that its a big endian platform based on the Atheros AR71xx/AR724x/913x chipset.

That gives us enough information to locate this download page for our chipset and especially we need to download the correct SDK.

OpenWrt only provides the SDK for Linux as a host platform, so if you are running on Windows or OSX you’ll need a virtual machine to follow the steps.

OpenWRT SDK Setup

To make sure our toolchain works we can compile and run a simple “Hello World” from C code by using the SDK directly. This will make sure that we have everything set up properly, since Rust builds on those binaries at a later stage.

Unpack the archive and add these environment variables to your .bashrc (or equivalent) so that you don’t run into path issues.

export PATH=$PATH:~/openwrt/staging_dir/toolchain-mips_34kc_gcc-4.8-linaro_uClibc-0.9.33.2/bin/
export STAGING_DIR=~/openwrt/staging_dir/

We are adding the SDK gcc binaries to our PATH and also provide the STAGING_DIR environment variable that the SDK needs while building. Note that I’ve renamed the OpenWrt-SDK-15.05.1-ar71xx-generic_gcc-4.8-linaro_uClibc-0.9.33.2.Linux-x86_64 (wow…) directory after extraction simply to openwrt so its easier to read the command line. Note: make sure to reload/source your .bashrc file if you want to stay in the current terminal or open a new one so the changes take effect.

Here is a very simple hello world in C (hello.c):

#include<stdio.h>

main() {
        printf("Hello, World!\n");
}

You can compile it with:

$ mips-openwrt-linux-gcc hello.c -o hello

If it compiles without issues, copy it to the device and run it!

$ scp hello [email protected]:~
$ ssh [email protected]

[email protected]:~# ./hello
Hello, World!
[email protected]:~#

Cross-Compiling Rust

At this point we know that the SDK is set up correctly and we can compile and run C code on the pineapple just fine.

To proceed, we need a nightly rust version, ideally set up through rustup which is the preferred way to manage your rust environments by now.

$ rustup install nightly
$ rustup default nightly

Don’t forget to add the source component, otherwise it won’t work down the road:

$ rustup component add rust-src

Finally, you should end up with a version similar to mine:

$ rustc -Vv
rustc 1.16.0-nightly (4ecc85beb 2016-12-28)
binary: rustc
commit-hash: 4ecc85beb339aa8089d936e450b0d800bdf580ae
commit-date: 2016-12-28
host: x86_64-unknown-linux-gnu
release: 1.16.0-nightly
LLVM version: 3.9

Now, install xargo and wait for it to compile:

$ cargo install xargo

We are pretty much set now, but we haven’t figured out which rustc target to use. Looking at the long output of rustc --print target-list, the one we need to use is mips-unknown-linux-uclibc (since we are compiling against the mips architecture and this OpenWRT version is compiled with uclibc)

Let’s create a hello world project with cargo:

$ cargo new hello --bin
     Created binary (application) `hello` project

Before we can build it, there are a couple modifications we need to make. First, we need to adapt the Cargo.toml to abort on panic. I don’t know why this is the case, but the xargo README says we should do it and it also fails to compile otherwise.

$ cat Cargo.toml
[package]
name = "hello"
version = "0.1.0"
authors = ["you"]

[dependencies]

[profile.dev]
panic = "abort"

[profile.release]
panic = "abort"

Next, we need to tell cargo to use a different linker for our mips target since the one on our host doesn’t work for our target. This is done by providing a custom config within the .cargo/config file:

$ cat .cargo/config
[build]
target = "mips-unknown-linux-uclibc"

[target.mips-unknown-linux-uclibc]
linker = "mips-openwrt-linux-gcc"

Finally, we need to add a Xargo.toml in order to tell xargo to also build the standard library for us:

$ cat Xargo.toml
[target.mips-unknown-linux-uclibc.dependencies.std]
features = []

Now we are all set! Run xargo build (not cargo build) and watch the magic happen. The first time xargo will build the standard library for you, but subsequent builds will just reuse the compiled libraries so its even quicker. The second time it will look like this:

$ xargo build
   Compiling hello v0.1.0 (file:///home/vagrant/src/hello)
    Finished debug [unoptimized + debuginfo] target(s) in 0.17 secs

Looking into our target/ directory we can see that there is now a hello binary available for our mips-unknown-linux-uclibc target!

~/src/hello$ tree target/
target/
|-- debug
|   |-- build
|   |-- deps
|   |-- examples
|   `-- native
`-- mips-unknown-linux-uclibc
    `-- debug
        |-- build
        |-- deps
        |   `-- hello-e7c80b1b0618ec37
        |-- examples
        |-- hello           <----------- this one ------
        `-- native

Of course, running it on our host won’t work:

$ ./target/mips-unknown-linux-uclibc/debug/hello
-bash: ./target/mips-unknown-linux-uclibc/debug/hello: cannot execute binary file: Exec format error

As a final step, copy the file over to your pineapple and run it (here I’m copying it to the mounted SD card just for fun):

$ scp target/mips-unknown-linux-uclibc/debug/hello [email protected]:/sd
$ ssh [email protected] '/sd/hello'
Hello, world!

That’s it! We just cross-compiled and ran a Rust program on a WiFi Pineapple.

One last thing…

While researching this topic I was made aware that there is still an open issue for the libc crate to fully support the mips-unknown-linux-uclibc target. It seems to work mostly but until the issue is resolved keep in mind that you might run into some weird issues.

Let me know if I missed anything or if you are also interested in running Rust on the WiFi Pineapple or similar OpenWrt devices! I’d like to close this post with a special thanks to Jorge Aparicio who is tirelessly working on making the cross compilation and embedded experience with Rust better every day (xargo, trust, cross,…).

comments powered by Disqus