open source contribution

Open-source contribution: chdir for BusyBox

Coming soon to the Linux box near you:

Hopefully this doesn’t age like milk

So yeah, I managed to get a commit into one of the open-source projects that I use on a daily basis: BusyBox. I guess many others use it too, either knowingly or unknowingly. BusyBox is a software suite providing plenty of Unix utilities in a minimized single executable. For example, when you’re using dmesg command you don’t necessarily know if the implementation comes from util-linux or BusyBox. But if you’re using OpenWRT, Alpine or Yocto you’re most likely using the BusyBox version.

The Problem

Because the BusyBox binary is minimized, the utilities it provides are often missing lesser-used features. As mentioned in the previous Aioli devblog, start-stop-daemon is for example missing -d/--chdir option present in the full Debian counterpart. As mentioned in that text, I wrote a patch to add that feature. What I didn’t really mention is that I submitted the patch to the BusyBox mailing list. I was hoping that it would get applied, and eventually it did!

start-stop-daemon is a program that’s commonly used in the SysVinit scripts to control the lifecycle of the system services. It doesn’t only start and stop daemons, it can also reload them, check their status and… well that’s primarily that. What --chdir option does is that it changes the working directory of the start-stop-daemon process before it launches the program it’s been assigned to start. This effectively changes the working directory of the process that will actually be started.

The Solution

The patch for this feature was quite straightforward. Mostly it consisted of adding a variable to hold the new working directory, inserting the new -d option to the opt list for the option parser, and editing the usage message. Then, if the new option flag was set, it was just a matter of calling the xchdir() in libbb (BusyBox’s library) to change the directory to the given directory (or die).

The less popular sequel to “Skate or Die” and “Ski or Die”.

In addition to this, I looked at how the tests for BusyBox work and wrote tests for the new flag. And cleaned up the TODO. In the end, the commit delta ended up being less than 60 lines. From what I’ve understood of the commit stats, the start-stop-daemon got bloated by about 79 bytes as a result. So the next time you’re updating BusyBox and curse the fact that it doesn’t fit into your root file system that has 67 bytes of free space remaining, you know who to blame.

All in all, getting the patch merged was an interesting process. I could definitely contribute more to BusyBox if there are suitable issues. Something perhaps a bit less simple the next time. But whether there will be more commits or not, it’s wild to think that my code could be running in Linux boxes around the world. Although, I guess that would require the device vendors to update their devices to run the new (still unreleased) version of BusyBox, so I guess it’s not happening too soon.