5 min read

Building Go Programs for MIPS

I’ve got a Lenovo router last year, and I’ve been always using it as a room AP to make the WiFi better. Recently, it’s getting harder to bypass the Great Firewall so I flashed the router to OpenWrt and try to make it as a VPN client.

Device Info

After everything is done, the router now is a mini Linux server and I can SSH onto it. Here are some infomation of the device.

openwrt:# uname -a
Linux OpenWrt 4.14.151 #0 Tue Nov 5 14:12:18 2019 mips GNU/Linux

openwrt:# cat /proc/cpuinfo
system type	            : MediaTek MT7620A ver:2 eco:6
machine	                : Lenovo Y1
processor	            : 0
cpu model	            : MIPS 24KEc V5.0
BogoMIPS	            : 385.84
wait instruction	    : yes
microsecond timers	    : yes
tlb_entries	            : 32
extra interrupt vector	: yes
hardware watchpoint	    : yes, count: 4, address/irw mask: [0x0ffc, 0x0ffc, 0x0ffb, 0x0ffb]
isa	                    : mips1 mips2 mips32r1 mips32r2
ASEs implemented	    : mips16 dsp
shadow register sets	: 1
kscratch registers	    : 0
package	                : 0
core	                : 0
VCED exceptions	        : not available
VCEI exceptions	        : not available

openwrt:# df -h
Filesystem                Size      Used Available Use% Mounted on
/dev/root                 2.5M      2.5M         0 100% /rom
tmpfs                    61.2M      1.3M     60.0M   2% /tmp
/dev/mtdblock6           12.0M      1.5M     10.6M  12% /overlay
overlayfs:/overlay       12.0M      1.5M     10.6M  12% /
tmpfs                   512.0K         0    512.0K   0% /dev

It has a MIPS architected CPU MIPS 24KEc V5.0 and that’s new for me. Next step I want to install v2ray on it, a proxy client. But I found the official released package is too big for my device. It takes about 20MB of a zipped binary. The opkg package manager doesn’t provide v2ray either.

Build A Smaller v2ray

So, it’s time to do myself. I cloned the source code of v2ray on my macOS, it’s written in Go and must be easy to cross-compile.

macbook:$ mkdir -p $GOPATH/src/v2ray.com/core
macbook:$ git clone https://github.com/v2ray/v2ray-core $GOPATH/src/v2ray.com/core
macbook:$ cd $GOPATH/src/v2ray.com/core/main

(For go1.12+ with go mod, you can clone it to anywhere.)

According to the GoMips wiki, I ran the following command to compile.

macbook:$ GOOS=linux GOARCH=mips GOMIPS=softfloat go build -o v2ray

After a few minutes of retrieving packages, everything’s OK, I got the MIPS binary.

macbook:$ ls -alh v2ray
-rwxr-xr-x  1 ferdi  staff    21M Feb 10 21:34 v2ray
macbook:$ file v2ray
v2ray: ELF 32-bit MSB executable, MIPS, MIPS32 version 1 (SYSV), statically linked, stripped

It’s 21M and I guess this is the official way of building. To make the output smaller, I retry the build command with some parameters. (Although the router supports external USB stick, I don’t have spare one and it’s unnecessary.)

macbook:$ GOOS=linux GOARCH=mips GOMIPS=softfloat go build -trimpath -ldflags="-s -w" -o v2ray
macbook:$ ls -alh v2ray
-rwxr-xr-x  1 ferdi  staff    15M Feb 10 21:36 v2ray

It’s smaller now as 15M, but still can’t fit my device since the router has only 10.6M left. Then I found a tool named upx, which can compress a ELF file.

macbook:$ upx -9 v2ray
                       Ultimate Packer for eXecutables
                          Copyright (C) 1996 - 2020
UPX 3.96        Markus Oberhumer, Laszlo Molnar & John Reiser   Jan 23rd 2020

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
  15728640 ->   5461384   34.72%   linux/mips    v2ray

Packed 1 file.

macbook:$ ls -alh v2ray
-rwxr-xr-x   1 ferdi  staff   5.2M Feb 10 21:36 v2ray

It’s acceptable now so I copied it to my router.

macbook:$ scp v2ray root@

On the router, it’s there! The sad news is, I can’t execute it.

openwrt:$ cd /tmp && ls -alh v2ray
-rwxr-xr-x    1 root     root        5.2M Feb 10 13:44 v2ray
openwrt:$ ./v2ray
openwrt:$ ./v2ray: line 2: syntax error: unterminated quoted string

Anything wrong? Do the upx make it corrupted? Or any of the build parameter wrong? I checked the build steps and try to figure that out what’s going on.

Try With A Helloworld Program

Then I tried a hello-world program without any build parameter and no upx, it also failed.

package main

import (

func main() {
	fmt.Println("hello, mips")
macbook:$ GOOS=linux GOARCH=mips GOMIPS=softfloat go build -o hello
macbook:$ scp hello root@

# on the router
openwrt:/tmp# ./hello
./hello: line 1: syntax error: unexpected "("

I started googling the problem, and here is my go info.

macbook:$ go version
go version go1.13.4 darwin/amd64

macbook:$ go env
GOENV="/Users/ferdi/Library/Application Support/go/env"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/zs/fbr4t1hd1p52lw7vfz084rcm0000gn/T/go-build718094728=/tmp/go-build -gno-record-gcc-switches -fno-common"

The Solution

I even looked through the offical MIPS32® 24KE™ Core spec and it’s strange there’s no indication of Big-Endian or Little-Endian.

At last I solved the problem by setting GOARCH=mipsle, it works now.

macbook:$ GOOS=linux GOARCH=mipsle GOMIPS=softfloat go build -o hello
macbook:$ scp hello root@

openwrt:/tmp# ./hello
hello, mips

The v2ray binary also works now,

macbook:$ GOOS=linux GOARCH=mipsle GOMIPS=softfloat go build -trimpath -ldflags="-s -w" -o v2ray
macbook:$ scp v2ray root@

openwrt:/tmp# ./v2ray --help
Usage of ./v2ray:
  -config string
    	Config file for V2Ray.
  -format string
    	Format of input file. (default "json")
    	Test config file only, without launching V2Ray server.
    	Show current version of V2Ray.

But the upx compressed one hang on command line, let’s skip this.

root@OpenWrt:/tmp# ./v2ray --help
<hang, nothing prints>


If you also have trouble building programs for a different operating system and architecture, remember the first step is to determine the exact target device information. Some tools may be helpful.

# check big-endian/little-endian
$ lscpu | grep "Byte Order"

# check go supported OS/Arch
$ go tool dist list | grep mips

The router above is newifi mini Y1 and labeled Model R6830 on the backside. It has 16M ROM along with 128MB of memory, and it’s Okay to install and run OpenWrt. I searched the device on openwrt.org and luckily it’s fully supported. So I followed the instruction page and get the router flashed! If you’re insterested on how to flash the router, just leave a comment below and I may write another post on how to install and configure OpenWrt.


[1] StackOverflow: Writing and Compiling Program For OpenWrt. https://stackoverflow.com/questions/55878437/writing-and-compiling-program-for-openwrt/60161561#60161561

[2] Wikipedia: Endianness https://en.wikipedia.org/wiki/Endianness

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