Presenter notes for Electric Fence, mudflap, and debugging talk
BarCampDenver, August 2006
16:00 MDT
C/C++ specific -- allows greater control. "Portable assembly language"
What are memory errors?
* Buffer overflow/underflow
* Memory leaks
* Accessing memory after it's been deleted
(What is the heap? Where can variables be allocated? (static, stack, heap))
The application:
$ g++ -g -Wall buffer.cxx -o buffer
ELECTRIC FENCE
Written by Bruce Perens for Pixar in 1987.
How it works: Uses mprotect() syscall to mark pages as invalid.
(Potential tangent: What are memory pages?)
Aligns dynamic memory (malloc()/free()) so that it adjoins an invalid page
Upon deallocation, marks deallocated page as invalid
Creates segmentation fault on overflow, or invalid use
Can be linked dynamically; use LD_PRELOAD to invoke at runtime:
$ LD_PRELOAD=libefence.so.0.0.0 ./eftest
Use gdb options to do the same thing:
(gdb) set environment LD_PRELOAD libefence.so.0.0
GDB takes you to the exact line that caused the buffer overflow
MUDFLAP
New for gcc 4 (right?)
Paper published in 2003 by Frank Eigler
Does more of what Electric Fence does; also protects against static and stack
Application must use gcc 4, must be recompiled
$ g++ -g -Wall buffer.cxx -o buffer -fmudflap -lmudflap
Behavior can be controlled at runtime
$ MUDFLAP_OPTIONS=-help ./mudflap
