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Cross compiling Rust on Mac OS for an ARM Linux router

by sigmaris on February 4th, 2019

Wanting to compile a small program I’d written in Rust to run on my home router, I found this guide to cross compilation of Rust code. The router is a Netgear R7000 with an ARM processor, running FreshTomato, a distribution of Linux for ARM and MIPS architecture consumer routers. The top of that guide shows an example of installing the cross-compilation toolchain for ARM on Ubuntu, but it required some work to adapt to Mac OS High Sierra, my desktop environment.

The guide suggests rustup can be used to install extra cross compilation targets. I already have rustup which I’ve used to install Rust for Mac OS and keep it up-to-date, so that’s handy. So I ran “rustup target list” to list all the installable targets:

aarch64-apple-ios
aarch64-fuchsia
aarch64-linux-android
aarch64-pc-windows-msvc
aarch64-unknown-cloudabi
aarch64-unknown-linux-gnu
aarch64-unknown-linux-musl
arm-linux-androideabi
arm-unknown-linux-gnueabi
arm-unknown-linux-gnueabihf
arm-unknown-linux-musleabi
arm-unknown-linux-musleabihf
armebv7r-none-eabi
armebv7r-none-eabihf
armv5te-unknown-linux-gnueabi
armv5te-unknown-linux-musleabi
armv7-apple-ios
armv7-linux-androideabi
armv7-unknown-linux-gnueabihf
armv7-unknown-linux-musleabihf
armv7r-none-eabi
armv7r-none-eabihf
armv7s-apple-ios
asmjs-unknown-emscripten
i386-apple-ios
(many more x86, mips, powerpc and x86_64 targets)

That’s a lot of possible targets. It looks like in the ARM space, there’s AArch64, arm-unknown, armebv7r, armv5te and armv7(r?) architectures of various variants. So, let’s google to see what kind of CPU the router has.

According to the OpenWRT wiki it’s a Broadcom BCM4709A0. So, what kind of architecture is that? Googling for “BCM4709A0” brought me to Wikidevi, which says it’s an ARM Cortex-A9. Looking at Wikipedia for the Cortex-A9 tells me:

  • It’s a 32-bit architecture, so not AArch64
  • It’s an ARMv7-A architecture

So I’d guess one of the armv7 targets is the best one. It’s probably not armv7-apple-ios or armv7-linux-androideabi, since this isn’t an iOS or Android OS. That leaves armv7-unknown-linux-gnueabihf, armv7-unknown-linux-musleabihf, armv7r-none-eabi and armv7r-none-eabihf. I know the router runs Linux, so let’s try the first two. I installed the armv7-unknown-linux-gnueabihf target with:

rustup target add armv7-unknown-linux-gnueabihf

OK, let’s try compiling a “hello world” Rust application with that target:

cargo build --target=armv7-unknown-linux-gnueabihf

That failed with a message “error: linking with cc failed: exit code: 1” and then a note showing the entire cc command, and a note saying:

= note: clang: warning: argument unused during compilation: '-pie' [-Wunused-command-line-argument]
           ld: unknown option: --as-needed
           clang: error: linker command failed with exit code 1 (use -v to see invocation)

OK, so I guess this is clang giving that error. Clang is the native C compiler for Mac OS, but I expect it can’t link an ARM executable in the way Rust wants. So, it looks like we need a linker for ARM. Reading the guide seems to suggest that Rust doesn’t have its own linker for Linux targets – it uses the linker from a C toolchain, for example the GNU C compiler. So we need to install a C toolchain targeting ARM.

My first stop when looking to install open source tools on Mac OS is Homebrew, and indeed there’s a formula on there for arm-linux-gnueabihf-binutils – it looks like that could be what we need to get a linker targeting ARM Linux. So let’s install that with:

brew install arm-linux-gnueabihf-binutils

That installs a set of tools named arm-linux-gnueabihf-addr2line, arm-linux-gnueabihf-ar and so on. I know the linker is normally invoked as “ld”, and cross-compilation toolchains by convention prefix their tool names with the target name, so the ARM Linux linker should be arm-linux-gnueabihf-ld. I know from the guide that this needs to go in ~/.cargo/config in a section like this:

[target.armv7-unknown-linux-gnueabihf]
linker = "arm-linux-gnueabihf-gcc"

But the Homebrew formula didn’t install arm-linux-gnueabihf-gcc – it only has arm-linux-gnueabihf-ld. Well, let’s try that instead, so the config is:

[target.armv7-unknown-linux-gnueabihf]
linker = "arm-linux-gnueabihf-ld"

OK, let’s try compiling again…

   Compiling rust-sandbox v0.1.0 (/Users/hugh/Source/rust-sandbox)

error: linking with `arm-linux-gnueabihf-ld` failed: exit code: 1
  |
  (...long command removed...)
  = note: arm-linux-gnueabihf-ld: cannot find -ldl
          arm-linux-gnueabihf-ld: cannot find -lrt
          arm-linux-gnueabihf-ld: cannot find -lpthread
          arm-linux-gnueabihf-ld: cannot find -lgcc_s
          arm-linux-gnueabihf-ld: cannot find -lc
          arm-linux-gnueabihf-ld: cannot find -lm
          arm-linux-gnueabihf-ld: cannot find -lrt
          arm-linux-gnueabihf-ld: cannot find -lpthread
          arm-linux-gnueabihf-ld: cannot find -lutil
          arm-linux-gnueabihf-ld: cannot find -lutil

This is more promising, but it looks like the linker can’t find all of those libraries to link with. Those look like parts of the GNU C library and other system libraries for Linux, which the Homebrew package arm-linux-gnueabihf-binutils doesn’t seem to include. These would normally be installed on a Linux system, but on Mac OS we don’t have them.

It seemed like I might need to install a more complete Linux toolchain that includes those libraries, but before trying that, let’s look at the other Rust target – armv7-unknown-linux-musleabihf. The “musl” in the name refers to the musl C library, a small C library that can be statically linked with Rust programs instead of the GNU C library. This sounds promising as it removes the need to link against libpthread, etc, which we had problems with earlier.

Let’s put the same linker configuration in ~/.cargo/config for the armv7-unknown-linux-musleabihf target:

[target.armv7-unknown-linux-musleabihf]
linker = "arm-linux-gnueabihf-ld"

And try compiling our Rust program with this target:

cargo build --target=armv7-unknown-linux-musleabihf
   Compiling rust-sandbox v0.1.0 (/Users/hugh/Source/rust-sandbox)
    Finished dev [unoptimized + debuginfo] target(s) in 1.12s

It built, so let’s copy the executable to the router:

scp target/armv7-unknown-linux-musleabihf/debug/rust-sandbox router:/tmp/

And then SSH on to the router and run it:

$ ssh router.sigmaris.info
 ========================================================
 Welcome to the Netgear R7000 [TomatoUSB]
 (output trimmed)
 ========================================================

root@router:/tmp/home/root# /tmp/rust-sandbox
Hello, world!

Great, it works! Using the musl C library and statically linking everything is somewhat less optimal than linking against the C library that’s already installed on Linux, as it means the built executable size is larger, but it’s good enough for a simple Rust program.

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