Mumbling about computers

Cross compiling for OpenWRT platforms

2020-12-30 [ openwrt rust cross-compiling ]

I own a TP-Link X which is an ath79 based wireless router, which runs OpenWrt.

I've been trying to get some specific software running on the router to do wifi-based presence detection to enable some automation, and I thought it'd be nice to get some experience doing this in rust.

Compiling for the target

First, for the most basic case, I'd like to get the most minimal program possible running on the device itself - which has a MIPS processor. Let's try that.

int main()
        return 42;

Having this minimal program we can install gcc-9-mips-linux-gnu and cross compile it:

$ mips-linux-gnu-gcc-9 ./a.c

after moving a.out to the router and moving it we can see it does not work:

root@OpenWrt:~# ./a.out
-ash: ./a.out: not found
root@OpenWrt:~# ldd a.out
        /lib/ (0x77eb0000) => /lib/ (0x77eb0000)

Looking at the lib path we can indeed see we do not have The binary is trying to use the linker in this path because our cross-compiler doesn't know which dynamic linker to use in this platform - so it's using the same that we are using right now.

If we check the dynamic linker on the router itself and pass it to the compiler:

root@OpenWrt:~# ls /lib/ld*

We can now compile the binary and specify which linker to use, then run it remotely:

$ mips-linux-gnu-gcc-9 -Wl,--dynamic-linker=/lib/ ./a.c
root@OpenWrt:~# ./a.out
root@OpenWrt:~# echo $?

It works!

Using libraries

Let's try a slightly more complicated program:

#include <stdio.h>
int main()
        printf("%d\n", 1234);

Running the new program shows some errors this time though

root@OpenWrt:~# ./a.out
Error relocating ./a.out: __printf_chk: symbol not found
root@OpenWrt:~# ldd a.out
        /lib/ (0x77f36000) => /lib/ (0x77f36000)
Error relocating a.out: __printf_chk: symbol not found

This is because the libc I linked (glibc) is not compatible with the libc in the router (musl).

I could keep hacking at errors as they pop up, but I presume they will be plenty as the entire environment is different. Usually for cases like this what you have to use is the same toolchain as the developers who built the distro (in this case, the OpenWrt devs)

I got the sdk and followed some of these directions -- specifically setting the PATH and STAGING_DIR.

$ mips-openwrt-linux-gcc b.c
root@OpenWrt:~# ./a.out


Let's repeat the whole thing with our rust program now:

Compiling rust code for the target

Luckily rust brings a lot of modern tooling to the table, including the ability to cross-compile quite easily by adding targets:

$ rustup target list | grep mips | grep musl
$ rustup target add mips-unknown-linux-musl

We can create a trivial project and cross-compile it:

$ cargo new trivial-project
$ cd trivial-project; cargo build --target mips-unknown-linux-musl
root@OpenWrt:~# ./trivial-project
Hello, world!
root@OpenWrt:~# ldd trivial-project
        /lib/ (0x77ee6000) => /lib/ (0x77e5a000) => /lib/ (0x77ee6000)

This works! and rust knows which dynamic linker to use directly.

Getting side-tracked: binary size

There is one initial 'problem' with the binary size: it's 3.2MB and our entire rootfs is 10.9MB.

A trivial change to cargo's build can help us quite a bit with this

opt-level = 'z' # opt for size
lto = true
panic = "abort"

This drops the binary size from 3.2MB to 1.3MB. That's substantial. If you are interested in reducing this further, there's a very handy guide on github.

Stripping the binary after that (mips-linux-gnu-strip binary) drops it to a respectable 285KB.

Compiling the application for the target

In my case, what I want to build uses the MQTT protocol, for which the crate I picked uses the libmosquitto C-bindings.

When trying to build, this points towards another problem:

$ cargo build --target mips-unknown-linux-musl
mips-openwrt-linux-musl/bin/ld: cannot find -lmosquitto

Our linker cannot find libmosquitto because it needs a shared library compiled for the target architecture. Before linking to it, I know we should target the same ABI as what will be installed in the device, so I checked what's available in the package manager

root@OpenWrt:~# opkg list | grep libmosq
libmosquitto-nossl - 1.6.12-1

Cool, we need to get and cross-compile[1] libmosquitto version 1.6.12

$ git clone
$ git checkout 1.6.12
$ export CROSS_COMPILE=~/openwrt-toolchain/bin/mips-openwrt-linux-musl-
$ export CC=gcc
$ file lib/
lib/ ELF 32-bit MSB shared object, MIPS, MIPS32 rel2 version 1 (SYSV), dynamically linked, with debug_info, not stripped

Excellent! We have a library! At this point I spent a long while trying to get cargo to tell the linker where to find this library and failed -- I am not sure how to do that, so I did the next best thing: I put the library in the cross-compiler's path:

$ cp lib/ ~/openwrt-toolchain/lib/

Let's give that a try..

$ cargo build --target mips-unknown-linux-musl
Finished dev [unoptimized + debuginfo] target(s) in 0.44s

Nice! Let's check what's in the binary

root@OpenWrt:~# ldd ./wifi-presence
        /lib/ (0x77f22000) => /usr/lib/ (0x77ea8000) => /lib/ (0x77f22000) => /lib/ (0x77e84000) => /usr/lib/ (0x77e04000) => /usr/lib/ (0x77c2a000)

And let's run it:

root@OpenWrt:~# ./wifi-presence
Hello, world!
thread 'main' panicked at 'Failed!: Error { text: "connect: Invalid function arguments provided.", errcode: 3, connect:
false }', src/

Ha! Turns out this crate is a bit broken. There's an open PR for this issue, but seems to be ignored.

I guess I'll take a look at other crates or see if I can build my own crate with the patch.

[1] maybe I could've just scp'd it from the router?