Trying to find out what is going on in your machine at the lowest levels
can be a fun and gratifying experience. Knowing how exactly application
code is loaded and prepared for execution is one such aspect.
Besides gaining a sense of control over the inner working in a computer
we can also start to reason about security topics and play some golf using
Lecture at CCC Ulm:
In December 2010 I gave a ChaosSeminar about this topic at the local chapter of the Chaos
Computer Club in Ulm. The Recordings are available in their archive
Lecture for Cocoaheads Munich:
In november 2010 I held a short lecture about having fun with mach-o at
the cocoaheads meeting in munich. You can download the
(slighty edited to fix typos)
and the files I used:
- a minimal C application
- bare bones assembly version, crashes due to missing stack
- minimal assembly version (164 bytes)
- slightly better HFS+ compressible version of tiny_mfeiri.asm
I used the tiny mach-o executables by Amit Singh
as a reference.
His smallest possible executable weights 165 bytes but that runs on mac os x 10.5 only. His smallest valid mach-o file weights 242 bytes. The executable
produced by tiny_mfeiri.asm is believed to be the smallest possible valid mach-o application. It should be noted though that Mac OS X natively
supports execution of shellscripts, which means that scripts can be launched that are even shorter that this minimal mach-o executable.