Hell Labs

Following the development of the investigations on the recently reported Mesa problem triggered by Neverball and Metisse, Ademar reports that “the workaround is not effective with at least ATI 9250 video cards, where we now have a crash at a different place”. We set up a system with an ATI Radeon 9250, and, indeed, it still crashes:

Mesa: Mesa 7.0.1 DEBUG build Oct  1 2007 18:52:02
Mesa warning: couldn't open libtxc_dxtn.so, software DXTn compression/
decompression unavailable
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1218062640 (LWP 17221)]
_generic_read_RGBA_span_RGB565_MMX () at x86/read_rgba_span_x86.S:590
590             pushl   MASK_565_H

A segfault in a push instruction sounds very odd. Read on to see what the differential diagnosis session with Boto and Salem lead us into. And unlike the previous patch, this one resulted in a real fix for a real problem:

• read_RGBA_span_RGB565_MMX patch for Mesa 7.0.1

Don’t read the patch if you want to find the bug yourself based on the scenario description below.

(more…)

Or a case study on how the knowledge of the basics of the Ancient Art of Assembly Programming can still save the day. Remember the last time you had to binary patch a program in Linux? Neither did I. But when we came across a binary-only executable doing very stupid things, with no source code in sight, no viable replacement at hand and no time or patience to code our own, binary patching didn’t sound too bad.

(more…)