[cfe-dev] New division by zero check

Jordan Rose jordan_rose at apple.com
Thu Jan 23 10:04:17 PST 2014

Hi, Anders. RecursiveASTVisitor isn't going to handle what you want here: you need something at least flow-sensitive (to know that the if-check actually follows the division) if not path-sensitive (to know that the value hasn't changed since you divided). The thing is, the analyzer's not actually good at answering questions of the form "does X occur on all paths through a function?". There are two reasons for this: first, the analyzer's model of C/C++ is known to be incomplete (e.g. we don't model catching exceptions), and second, we have a performance optimization that says if a function is called and we decide to inline it, we won't analyze it again as top level, which means we're usually not seeing the fully general case.

The next problem is that this can contribute to state explosion: where before we might have been able to merge two paths where a division only happens on one, we'd now be forced to treat them as separate because the denominator is marked along one path.

Ted, Anna, and I have actually talked about this kind of check, not so much for divide-by-zero as for null dereferences. (I forget what we were calling this; something like an "inverted null check", because the check and the dereference are backwards.) This is actually a real security issue a lot of the time; see John Regehr's blog post at http://blog.regehr.org/archives/970.

(It's fun to see Xi Wang cited; he was an LLVM contributor for a while several years ago.)

One idea we came up with is that while doing this as a fully general check is hard, it's probably worth checking this in the very simple case of straight-line code, where the division or the dereference happens in the same CFG block as the test. I still don't know if you want to do this as a CFG walk (with lots of care taken for the variable changing in between the use and the check) or as a full analyzer checker, and we haven't really worked out any of the details, but it does seem to be more feasible than the general check while still being immediately useful.

Let me know if you have any more questions!

On Jan 22, 2014, at 7:18 , Anders Rönnholm <Anders.Ronnholm at evidente.se> wrote:

> Hi,
> There is a division by zero check where the denominator value is already known. I would like some feedback on a new static check for division by zero I intend to write where the value isn’t known yet.
> It shall catch scenarios where the division is made before checking it against a value. To start with it will have to be directly after the division.
> e.g
> x = 100 / y;
> if (y == 0)
> I plan to do it by:
> 1. When a division is reached where the denominator is a variable with unknown value, “mark” the denominator variable.
> 2. When reaching a condition that checks if a variable is 0, check if the variable in the condition is “marked”.
> 3. Report bug
> I guess RecursiveASTVisitor is best for this?
> Is there perhaps a better way to do this that would allow me to follow the variable from division to check so that  it doesn't have to be directly after?
> //Anders
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