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Using Redundancies to Find Errors
gsbarnes writes "Two Stanford researchers (Dawson Engler and Yichen Xie) have written a paper (pdf) showing that seemingly harmless redundant code is frequently a sign of not so harmless errors. Examples of redundant code: assigning a variable to itself, or dead code (code that is never reached). Some of their examples are obvious errors, some of them subtle. All are taken from a version of the Linux kernel (presumably they have already reported the bugs they found). Two interesting lessons: Apparently harmless mistakes often indicate serious troubles, so run lint and pay attention to its output. Also, in addition to its obvious practical uses, Linux provides a huge open codebase useful for researchers investigating questions about software engineering."
I saw Dawson's talk at FSE (Foundations of Software Engineering). He uses static flow analysis to find problems in the code (like an advanced form of pclint). The most interesting part of his tool is in the ranking of the problem reports. He has developed a couple of heuristics that sort the problems by order of importance and they supposedly do a very good job. Static analysis tools find most of their problems in rarely run code, such as error handlers. Such problems are problematic and sometimes lead to non-deterministic problems, which are extremely hard to find with standard testing and debugging. (This is especially true, when the program under consideration is a kernel.) Dawson also verifies configurations of the kernel that no one would compile, because he tries to get as many possible drivers at the same time as he can. The more code, the better the consistency checks do at finding problems.
By making assumptions about the program and checking the consistency of the program, his tool finds lots of problems. For instance, assume there is a function named foo that takes a pointer argument. His tool will notice how many of the callers of foo treat the parameter as freed versus how many treat the parameter as unfreed. The bigger the ratio, the more likely the 'bad' callers are to represent a bug. It doesn't really matter which view is correct. If the programmer is treating the parameter inconsistently, it is very likely a bug.
He also mentioned that counter to his expectations, the most useful part of his tool was to find 'local' bugs. By local, I mean bugs that are local to a single procedure. They are both easier for the tool to find, more likely to actually be bugs, and much easier for the programmer to verify if they are in fact bugs.
He analyzed a couple of the 2.2.x and 2.4.x versions of the kernel and found hundreds of bugs. Some of them were fixed promptly. Others were fixed slowly. Some were fixed by removing the code (almost always a device driver) from the kernel. Others he couldn't find anyone that cared about the bug enough to fix it. He was surprised at the amount of abandonware in the Linux kernel.
It is extremely frustrating that Dawson won't release his tool to other researchers (or even better to the open source community at large). Without letting other people run his tool (or even better modify it), his research ultimately does little good other than finding bugs in linux device drivers. *heavy sigh* Oh well, eventually someone WILL reimplement this stuff and release it to the world.
On a snide comment, if he was a company he would no doubt have been bought by Microsoft already. Intrinsa was doing some interesting stuff with static analysis and now after they were bought a couple of years ago, their tool is only available inside of Microsoft. *sigh*