How could reading numbers using sscanf crash?

11
votes

Cppcheck has detected a potential problem in a code like this:

float a, b, c;
int count = sscanf(data, "%f,%f,%f", &a, &b, &c);

It says that: "scanf without field width limits can crash with huge data". How is that possible? Is that a known bug in some sscanf implementations? I understand that the numbers may overflow (numerically), but how could the program crash? Is that a false positive in cppcheck?

I have found a similar question: scanf Cppcheck warning, but the answer is not completely satisfying. The answer mentions type safety, but that should not be an issue here.

3
c++cppcheck
Try sscanf_s instead. As normal scanf, sscanf is not overflow safe. - guitarflow
@guitarflow: The problem is that I don't see where it may overflow. - Juraj Blaho
@guitarflow Or don’t. sscanf_s isn’t portable and also not actually safe, despite what the name suggests and Microsoft claims. - Konrad Rudolph
en.wikipedia.org/wiki/Format_string_attack is also important to pay attention to. Buffer overflows alone aren't the only vulnerabilities in scans. If you allow the user to input the format string they can use %x to print arbitrary memory locations and %n to write them. Among other things. - synthesizerpatel
@synthesizerpatel: As you can see, format is a string literal here, so that is not a problem. - Juraj Blaho

3 Answers

7
votes

I am a Cppcheck developer.

Yes this is a weird crash. With "huge data" it means millions of digits.

If you use the --verbose flag then cppcheck will actually write a little example code that usually crashes on linux computers.

Here is an example code that crashes with a segmentation fault on my Ubuntu 11.10 computer:

#include <stdio.h>

#define HUGE_SIZE 100000000

int main()
{
    int i;
    char *data = new char[HUGE_SIZE];
    for (int i = 0; i < HUGE_SIZE; ++i)
        data[i] = '1';
    data[HUGE_SIZE-1] = 0;
    sscanf(data, "%i", &i);
    delete [] data;
    return 0;
}

For your info I don't get a crash when I try this example code on visual studio.

I used g++ version 4.6.1 to compile.

4
votes

The segmentation fault seems to be a bug in glibc.

I've just tested this with a similar program, which crashes in ubuntu 10.04, but works in ubuntu 12.04.

As Daniel Marjamäki said, his program crashes in 11.10, I believe the bug is fixed in between.

1
votes

OK, consider this code:

int main(int argc, char *argv[]) {
    const char* data = "9999999999999999999999999.9999999999999999999999//i put alot more 9's there, this just to get the point through
    float a;
    int count = sscanf(data, "%f", &a);
    printf("%f",a);
}

the output of this program is "inf" - no crash. And I put a huge amounts of 9's there. So I suspect Cppcheck is just plain wrong about this.