Uninitialized Variable in C

Author

Affiliation

Mingze Gao, PhD

Macquarie University

Uninitialized variable in C can be anything (most of the time). I find, in some cases, we can know the value of an uninitialized variable and thus maybe exploit it.

The example code below compiled with gcc, without optimization, exits successfully. Very interesting!

#include <assert.h>
#include <limits.h>
#include <stdlib.h>

void f(int n) {
  // Declare and init `a` with the value of n
  // This would push n on the stack memory
  int a = n;
  return;
  // f() returns,
  // but `a` leaves on the stack a garbage value n
}

void g(int n) {
  // Declare but do no initialize `x` so
  // `x` may be of anything...?
  int x;
  assert(x == n);  // Should fail here if `x` is not n
}

int main() {
  for (int i = INT_MIN; i < INT_MAX; i++) {
    f(i);  // However, if we call f() and g() sequentially...
    g(i);  // The local variable `x` in g() will always be i,
           // which is the garbage value left by f() on the stack.
  }
  // This program will end peacefully
  return 0;
}

We can also try to “contaminate” the stack by filling it with a value, e.g.,

#include <assert.h>
#include <memory.h>
#include <stdint.h>
#include <stdio.h>

void f(uint8_t n) {
  // Try to "contaminate" the stack with value n
  uint8_t arr[BUFSIZ];
  memset(arr, n, BUFSIZ * sizeof(uint8_t));
}

void g(uint8_t n) {
  uint8_t x;
  assert(x == n);
  printf("uninitialized x is %d\n", x);
  uint8_t y;
  assert(y == n);  // uninitialized y is also n
}

int main() {
  for (uint8_t i = 0; i < UINT8_MAX; i++) {
    f(i);
    g(i);
  }
  // This program will end peacefully!
  return 0;
}

As a result, the uninitialized local variables x and y both have the same value of n because f(n) writes many n on the stack.

Studying C is real fun!