Pointless Linux Hacks

I nearly always find it interesting to muck about in someone else’s code, often to add simple features or to make it do something silly, and the Linux kernel is no exception to that. What follows is my own first adventure into patching Linux to do my evil bidding.

Aside from mucking about in code for fun, digging through public source code such as that provided by Linux can be very useful when developing something new.

A short story

I was doing nothing of particular importance yesterday afternoon when I was booting up my previously mentioned netbook. The machine usually runs on a straight framebuffer powered by KMS on i915 hardware, and my kernel is configured to show the famous Tux logo while booting.

Readers familiar with the logo behaviour might already see where I’m going with this, but the kernel typically displays one copy of the logo for each processor in the system (so a uniprocessor machine shows one tux, a quad-core shows four, etc..). As a bit of a joke, then, suggested a friend, why not patch my kernel to make it look like a much more powerful machine than it really is? Of course, that’s exactly what I did, and here’s the patch for Linux 2.6.38.

--- drivers/video/fbmem.c.orig	2011-04-14 07:26:34.865849376 -0400
+++ drivers/video/fbmem.c	2011-04-13 13:06:28.706011678 -0400
@@ -635,7 +635,7 @@
 	int y;

 	y = fb_show_logo_line(info, rotate, fb_logo.logo, 0,
-			      num_online_cpus());
+			      4 * num_online_cpus());
 	y = fb_show_extra_logos(info, y, rotate);

 	return y;

Quite simply, my netbook now pretends to have an eight-core processor (the Atom with SMT reports two logical cores) as far as the visual indications go while booting up.


Thus we come to source-diving, a term I’ve borrowed from the community of Nethack players to describe the process of searching for the location of a particular piece of code in some larger project.

Diving in someone else’s source is frequently useful, although I don’t have any specific examples of it in my own work at the moment. For an outside example, have a look at musca, which is a tiling window manager for X which was written from scratch but used ratpoison and dwm (two other X window managers) as models:

Musca’s code is actually written from scratch, but a lot of useful stuff was gleaned from reading the source code of those two excellent projects.

A personal recommendation for anyone seeking to go source-diving: become good friends with grep. In the case of my patch above, the process went something like this:

  • grep -R LOGO_LINUX linux-2.6.38/ to find all references to LOGO_LINUX in the source tree.
  • Examine the related files, find drivers/video/fbmem.c, which contains the logo display code.
  • Find the part which controls the number of logos to display by searching that file for ‘cpu’, assuming (correctly) that it must call some outside function to get the number of CPUs active in the system.
  • Patch line 638 (for great justice).

Next up in my source-diving adventures will be finding the code which controls what happens when the user presses control+alt+delete, in anticipation of sometime rewriting fb-hitler into a standalone kernel rather than a program running on top of Linux..