19.12.2018

The Advent of Void: Day 19: upx

Code golf is trying to solve a problem in the fewest lines of code. I like to think there’s a similar thing for trying to build a complete working system in the fewest number of bytes. You can get pretty far using projects like busybox to replace large parts of the system with a single binary that provides shared functionality, but to get the last bit of the way, its often necessary to shrink the remaining binaries as well. That’s where upx comes in.

The Ultimate Packer for eXecutables does exactly what its name implies. It packs executables to make them smaller. This is done with compression which is undone on the fly when a binary is called.

Lets look at a quick example:

$ ls -lh total 128K -rwxr-xr-x 1 maldridge maldridge 127K Dec 19 00:03 ls

This is the standard ls that ships with Void and is already pretty small. Lets make it smaller!

$ upx ls Ultimate Packer for eXecutables Copyright (C) 1996 - 2018 UPX 3.95 Markus Oberhumer, Laszlo Molnar & John Reiser Aug 26th 2018 File size Ratio Format Name -------------------- ------ ----------- ----------- 129760 -> 60624 46.72% linux/amd64 ls Packed 1 file.

As you can see, the file is now only 60k in size. A pretty good start!

Lets take a look now at a Go executable. Go is known for generating large binaries and upx can help shrink them down to size.

For this example we’ll use the netauthd binary which forms part of Void’s server authentication system.

$ ls -lh total 17M -rwxr-xr-x 1 maldridge maldridge 17M Dec 19 00:08 netauthd

We can get part of the way there by stripping symbols out of the binary, since Go builds those in by default:

$ strip netauthd $ ls -lh total 13M -rwxr-xr-x 1 maldridge maldridge 13M Dec 19 00:10 netauthd

Now to see how much further upx can get us:

$ upx netauthd Ultimate Packer for eXecutables Copyright (C) 1996 - 2018 UPX 3.95 Markus Oberhumer, Laszlo Molnar & John Reiser Aug 26th 2018 File size Ratio Format Name -------------------- ------ ----------- ----------- 12724112 -> 4714692 37.05% linux/amd64 netauthd Packed 1 file.

The netauthd file is now just 4.5M, quite a bit smaller than the 17M starting size.

UPX can be a great tool to keep in your embedded toolbox, just remember not to run it on files owned by the package manager!