Bruce Dawson on compiler bugsPascal Cuoq - 21st Oct 2013
Bruce Dawson has written a superb blog post on a Visual C++ compiler bug (now fixed) covering every aspect an essay on compiler bugs should cover.
I really like one section that I am going to quote in full:
In these paranoid days of the NSA subverting every computer system available every bug is a possible security flaw. This bug seems curiously commonplace – why wasn’t it discovered when building Windows or other 64-bit products? The most likely explanation is chance and happenstance but compiler bugs can be used to make innocuous source code do surprising things. Maybe this bug is used as part of a back door to let code execution go in a direction that the source code says is impossible. This happened accidentally on the Xbox 360 where the low 32 bits of r0 were checked and then all 64 bits were used. This inconsistency allowed a hypervisor privilege escalation that led to arbitrary code execution.
This bug is probably not security related but security is another reason to ensure that compiler bugs get fixed.
Bruce's post also illustrates the “most compiler bugs aren't” problem and provides a step-by-step tutorial on manual bug reduction. Regarding the latter my reaction to Bruce's post was to bring up automatic testcase reduction with “delta debugging” but Gravatar user jrrr beat me to it.
Frama-C contributes to making automatic testcase reduction work in practice for C compiler bugs by providing a reference C implementation that detects the undefined behaviors that could make the (original or reduced) bug reports invalid. We also use delta debugging internally to simplify our own reports of bugs in Frama-C. The next post in this blog will illustrate this use of delta debugging on a concrete example.