Writing a Linux Debugger Part 1: Setup

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2021-06-20 21:30:06

Debuggers are one of the most valuable tools in any developer’s kit. However, although these tools are in such widespread use, there aren’t a lot of resources which tell you how they work and how to write one1, especially when compared to other toolchain technologies like compilers. In this post series we’ll learn what makes debuggers tick and write one for debugging Linux programs.

I’ll be focusing on C and C++ for this project, but it should work just as well with any language which compiles down to machine code and outputs standard DWARF debug information (if you don’t know what that is yet, don’t worry, this will be covered soon). Additionally, my focus will be on getting something up and running which works most of the time, so things like robust error handling will be eschewed in favour of simplicity.

Before we jump into things, let’s get our environment set up. I’ll be using two dependencies in this tutorial: Linenoise for handling our command line input, and libelfin for parsing the debug information. You could use the more traditional libdwarf instead of libelfin, but the interface is nowhere near as nice, and libelfin also provides a mostly complete DWARF expression evaluator, which will save you a lot of time if you want to read variables. Make sure that you use the fbreg branch of my fork of libelfin, as it hacks on some extra support for reading variables on x86.