CODEWITHSUNDEEP
csecuritydebugging
Mayank Verma
Mayank Verma

Reputation: 643

Detecting arithmetic overflow in C

  1. As the title suggests, is there a way in C to detect such errors? For example if I try a complex computation like int a=b*c+d*e/f; can I detect if the result has gone beyond the range of int?

  2. Suppose I decide that handling such errors comes at a cost of performance hit, what are the security implications and what at worst could happen? Can a malware author exploit this bug?

Upvotes: 2

Views: 309

Answers (2)

jlucier
jlucier

Reputation: 1572

Realistically, the answer to your first question is no. You can do checks for simple operations, but for larger ones the checks quickly become arduous. Datatypes and primitives like an int are only defined to be able to handle a finite range of values. If you're worried about overflow, the simple answer is use a bigger datatype (i.e. int64_t, etc.).

To answer your latter question, it depends on context. If a space shuttle's navigation software encountered an overflow in the right part of it's calculations, it could change course erratically and crash. What I'm getting at is that technically anything can happen and it's very hard to be "completely" safe. Hedge your bets and make sure don't have overflow by using properly sized datatypes. It helps to find bounds on possible input values and use those as a guide, but if bounds aren't really applicable you can just use the largest types available. uint64_t can handle positive integers up to 1.8446744e+19 ... which is huge.

Upvotes: 1

Weather Vane
Weather Vane

Reputation: 34585

In the simpler case of unsigned int you can test for overflow before the multiplication like this

#include <stdio.h>
#include <limits.h>

void multiply(unsigned a, unsigned b)
{
    if(UINT_MAX / a >= b)
        printf("%u * %u = %u\n", a, b, a * b);
    else
        printf("%u * %u is out of range\n", a, b);
}

int main(void)
{
    multiply(70000, 60000);
    multiply(70000, 80000);

    return 0;
}

Program output:

70000 * 60000 = 4200000000
70000 * 80000 is out of range

And you can employ more extensive tests for int

Upvotes: 1

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