Development/Tutorials/PolicyKit/Helper HowTo

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Using the caller-helper model to perform actions as root
Tutorial Series   PolicyKit Tutorial
Previous   Development/Tutorials/PolicyKit/Introduction
What's Next   None


Further Reading   n/a

Before you start

Applications running under root privileges has always been a major problem in Linux, and PolicyKit was created exactly to make the whole process easier and more secure. Though, running applications as root, even if small and controlled, can be still a major issue. So there are a few things you should take in account that will help you minimize possible issues:

  • Include in the helper just the strictly needed code. The fact itself that is a helper delegated to run just small parts of code implies that only the few lines of code you need to run as root should go in
  • Link against and use the minor number of libraries possible. The helper itself requires only QtCore and QtDBus. In particular, try not to use KDELibs as possible, as they were not designed to be used as root.
  • DO NOT USE SETUID IN HELPER. We'll see in this tutorial how to get root privileges without messing with the setuid bit.

If you are aware of this, and you're also sure that your application actually needs root privileges, you can go on reading.

What we need to do

What is cool about PolicyKit and this approach is that we need to write a minimum portion of code, don't need hacks or executon bits, and we actually get root privileges for a minimum portion of code. Suppose we still have our foo application we saw in the precedent Tutorial. From our .policy file, we know action2 actually does something that requires authentication as root. In fact, the following lines of code in action2 definitely require root privileges:

eraseHardDrive(); killUser(); detonatePC(); runAsFastAsYouCan();

But foo is a huge program that runs as standard user apart from those lines. So the option is to make it run as root as a whole, or split those 4 lines in a different program running as root. That is the approach that we will take in this tutorial.

Creating the Helper

Our helper will be a standard QCoreApplication. We need to create the following stuff:

  • A .policy file defining the various actions, that we should have already done
  • A DBus interface for our Helper
  • A DBus policy file
  • A file for DBus Activation
  • The main class

We already know how create Policy files, let's get deeper on the other points and let's try to understand what we need to do, how and why.

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