32-bit support in WSL

32-bit support in WSL.txt

https://github.com/microsoft/wsl/issues/2468#issuecomment-374904520
sudo service binfmt-support start
https://github.com/WhitewaterFoundry/Pengwin/issues/273#issuecomment-451274200

Based on some tinkering I was doing with qemu for some ARM dev, I think I may have found a technique to allow general 32-bit support in WSL. Hat-tip to @therealkenc for the concept 😁

Edit: requires "Fall Creators Update", 1709, build 16299 or newer (I think)

Presuming a fresh Ubuntu WSL instance, you'll need to install the qemu-user-static package, add the i386 binfmt, enable the i386 architecture, update your package lists, and install some i386 packages:
Install qemu and binfmt

sudo apt update
sudo apt install qemu-user-static
sudo update-binfmts --install i386 /usr/bin/qemu-i386-static --magic '\x7fELF\x01\x01\x01\x03\x00\x00\x00\x00\x00\x00\x00\x00\x03\x00\x03\x00\x01\x00\x00\x00' --mask '\xff\xff\xff\xff\xff\xff\xff\xfc\xff\xff\xff\xff\xff\xff\xff\xff\xf8\xff\xff\xff\xff\xff\xff\xff'

[Edit: whoops, need to update package lists, added sudo apt update]

This will activate i386 support by causing them to be executed through qemu-i386-static, and drop a config file into /var/lib/binfmts/ for future reactivation.
You will need to reactivate this every time you restart WSL and want i386 support:

sudo service binfmt-support start
Enable i386 architecture and packages

sudo dpkg --add-architecture i386
sudo apt update
sudo apt install gcc:i386

Try it out

$ file /usr/bin/gcc-5
/usr/bin/gcc-5: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=2637bb7cb85f8f12b40f03cd015d404930c3c790, stripped

$ /usr/bin/gcc-5 --version
gcc-5 (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc helloworld.c -o helloworld

$ ./helloworld
Hello, world!

$ file helloworld
helloworld: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=3a0c7be5c6a8d45613e4ef2b7b3474df6224a5da, not stripped

Proof

And to prove it really was working, disable i386 support and try again:

$ sudo service binfmt-support stop
 * Disabling additional executable binary formats binfmt-support [ OK ]

$ ./helloworld
-bash: ./helloworld: cannot execute binary file: Exec format error


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Just a small update. I was able to get the above workaround to work in wlinux by doing the following.

git clone git://git.qemu-project.org/qemu.git
mkdir qemu/build && cd qemu/build
sudo apt install libglib2.0-dev libfdt-dev libpixman-1-dev zlib1g-dev libaio-dev libbluetooth-dev libbrlapi-dev libbz2-dev  libcap-dev libcap-ng-dev libcurl4-gnutls-dev libgtk-3-dev libibverbs-dev libjpeg-dev libncurses5-dev libnuma-dev librbd-dev librdmacm-dev libsasl2-dev libsdl2-dev libseccomp-dev libsnappy-dev libssh2-1-dev libvde-dev libvdeplug-dev libvte-dev libxen-dev liblzo2-dev valgrind xfslibs-dev
../configure --target-list=i386-linux-user,i386-softmmu
make -j$(nproc)
sudo make -j$(nproc) install

If it was a success you can now
cd ../.. && rm -rf qemu

then run

sudo apt install binfmt-support
sudo update-binfmts --install i386 /usr/local/bin/qemu-i386 --magic '\x7fELF\x01\x01\x01\x03\x00\x00\x00\x00\x00\x00\x00\x00\x03\x00\x03\x00\x01\x00\x00\x00' --mask '\xff\xff\xff\xff\xff\xff\xff\xfc\xff\xff\xff\xff\xff\xff\xff\xff\xf8\xff\xff\xff\xff\xff\xff\xff'
sudo service binfmt-support start
sudo dpkg --add-architecture i386
sudo apt update
sudo apt install gcc-multilib g++-multilib

You should now be able to execute both 32-bit and 64 bit code. The best part is it appears to stay running. I only had to run the binfmt-support start command once.