The return of the Stanford Checker

We first looked at the "Stanford Checker" back in March, 2001. The Checker is a system built on top of gcc which analyzes large amounts of source code and looks for obscure errors. In the past, it has been responsible for many kernel bug fixes. The Checker team has been quiet for a while; now, perhaps with the end of the academic year, the group has returned with a new set of error reports.

So what has the checker group found this time?

• Missing unlocks. Here, the Checker looked for situations where kernel code could either take out a lock or disable interrupts, then fail to undo the action before returning. 18 possible errors were found.

• Memory leaks. The Checker looked for failure paths which failed to return allocated memory. "while we only include 24 errors, there were lots in general."

• Failure to check return codes. Numerous places were found where kernel code does not look at the return status from a function which can fail.

• Missing null pointer checks (54 errors). Most of the errors seem to be with calls to kmalloc.

• Large stack variables (37). Allocating a variable of size greater than 1KB may not be, strictly, an error, but it can lead to problems quickly when the stack runs out of space.

The Checker code itself remains unreleased, unfortunately. The Checker group does the kernel a great service by performing this testing and passing on the problems for fixing. But there are no end of other development projects out there that could benefit from this code. One can only hope that, someday, the Checker code will be more widely available.

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