Detecting Overflows in Java Arithmetic Operators

posted at
January 3, 2014
with tags java

A couple of months ago I came across The CERT Oracle Secure Coding Standard for Java. It is suprising how come I did not find this fascinating resource before. I first skimmed through the chapters and figured out that the recommended conventions are so practical that I can even directly use some of them in my existing code base. Right at that time I was working on a routing engine, which performs some arithmetic operators over link costs. It was a good place to apply the conventions metioned in the chapter NUM00-J. Detect or prevent integer overflow. Hence, I directly copied safe versions of arithmetic operators into my `util`

package. (Everybody does have a `util`

, right?)

```
public static int safeAdd(int l, int r) throws OverflowException {
if (r > 0
? l > Integer.MAX_VALUE - r
: l < Integer.MIN_VALUE - r)
throw new ArithmeticException(String.format(
"Integer overflow: %d + %d", l, r));
return l + r;
}
public static int safeMultiply(int l, int r)
throws ArithmeticException {
if (r > 0
? l > Integer.MAX_VALUE/r || l < Integer.MIN_VALUE/r
: (r < -1
? l > Integer.MIN_VALUE/r || l < Integer.MAX_VALUE/r
: r == -1 && l == Integer.MIN_VALUE))
throw new ArithmeticException(String.format(
"Integer overflow: %d x %d", l, r));
return l * r;
}
```

To my suprise, I observed integer overflow error messages in the server logs today. Yay! Further examination of the problem revealed that I cause an integer overflow while taking the square of link costs for a particular routing algorithm. Some might argue about the overhead of replacing operators with methods, but let’s think about this: How would I ever figure this problem out if I would not be using these safe arithmetic operators. No need to mention about the problem’s consequences.