Question

How can you determine the stack usage of a function call from an API that has been provided to you? You do not know what the function looks like and you have access only to the API. For example,

    int main() {
      // call some API function  
      some_func_called();

      // rest of your main
    }

The only thing that I can think of is by populating the stack memory with a known pattern before calling the function and later checking the stack to see how many bytes of the the known pattern was changed after the function returns. Any other ideas? (This was an interview question)

Answer 1

Something like this, not fully tested, may have limit problems. Only works if stack pointer decreases by PUSHING, so this is not portable across all hardware…

#include <stdlib.h>
#include <string.h>

#define STK_CNT 65536

size_t STK_fill()
{
    volatile size_t i, cnt = 0;

    volatile u_int32_t fill[STK_CNT];

    for(i = 0; i < STK_CNT; i++)
    {
        fill[i] = 0xDEADBEEFUL;
    }

    return cnt;
}

size_t STK_find()
{
    volatile size_t i, cnt = 0;

    volatile u_int32_t fill[STK_CNT];

    for(i = 0; i < STK_CNT; i++)
    {
        if(fill[i] == 0xDEADBEEFUL)
        {
            cnt++;
        }else{
            break;
        }
    }

    return ((STK_CNT - cnt) * 4);
}

void victim(char *po_str, const char *pi_str)
{
    char str[1024];

    strcpy(str, "stk_TEST_");
    strcat(str, pi_str);
    strcat(str, "_stk_TEST");

    strcpy(po_str, str);
}

int main()
{
    int  temp;
    int  used;
    char str[4096] = {0};

    temp = (int)STK_fill();
    victim(str, "STK_tst_STK_tst_STK_tst_STK_tst_STK_tst_STK_tst_STK_tst_STK");
    used = (int)STK_find();

    printf("VICTIM function used %d bytes of stack\n", used);

    return 0;
}