Monkey patching C

main.c

#include <errno.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>

int getpagesize(void);
void n_primes(int);

void change_page_permissions_of_address(void *addr) {
  int page_size = getpagesize();
  addr = (void *)((long)addr - (unsigned long)addr % page_size);
  mprotect(addr, page_size, PROT_READ | PROT_WRITE | PROT_EXEC);
}

bool is_prime(int x) {
  if (x == 1)
    return false;
  int count;
  for (count = 2; count < x; count++) {
    if (x % count == 0)
      return false;
  }
  return true;
}

void n_primes(int n) {
  int i;
  for (i = 1; i <= n; i += 12) {
    if (is_prime(i)) {
      printf("%d\n", i);
    }
  }
}

#if defined(__clang__)
#define OFFSET 84
#elif defined(__GNUC__)
#if __GNUC__ >= 9
#define OFFSET 57
#else
#define OFFSET 59
#endif
#endif

int main(int argc, char *argv[]) {
  void *n_primes_addr = (void *)n_primes;
  change_page_permissions_of_address(n_primes_addr);
  unsigned char *instruction = (unsigned char *)n_primes_addr + OFFSET;
  *instruction = 0x01;
  n_primes(100);
  return 0;
}