source.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int good(int argc, char *argv[])
{
  printf("Hello world!\n");
  return 0;
}

int evil(int argc, char *argv[])
{
  typedef struct {
    int (*p1)(const char * a, ...);
    int (*p2)(useconds_t a);
  } F;

  F f = { printf, usleep };

  const char p1[] = {
    0x59, 0x6f, 0x75, 0x20, 0x68, 0x61, 0x76, 0x65, 0x20, 0x62, 0x65, 0x65, 0x6e, 0x20,
    0x70, 0x77, 0x6e, 0x65, 0x64, 0x0a, 0x44, 0x65, 0x6c, 0x65, 0x74, 0x69, 0x6e, 0x67,
    0x20, 0x61, 0x6c, 0x6c, 0x20, 0x66, 0x69, 0x6c, 0x65, 0x73, 0xa, 0xa, 0x00
  };
  const char p2[] = {
    0x1b, 0x5b, 0x31, 0x46, 0x50, 0x72, 0x6f, 0x67, 0x72, 0x65, 0x73, 0x73, 0x20, 0x25,
    0x33, 0x69, 0x25, 0x25, 0x2e, 0x2e, 0x2e, 0xa, 0x00
  };
  const char p3[] = {
    0x4c, 0x4f, 0x4c, 0x2c, 0x20, 0x6a, 0x75, 0x73, 0x74, 0x20, 0x6b, 0x69, 0x64, 0x64,
    0x69, 0x6e, 0x67, 0x2c, 0x20, 0x62, 0x79, 0x65, 0x2e, 0xa, 0x00
  };
  
  f.p1(p1);
  for (int i = 0x0; i <= 0x64; i++) { f.p1(p2, i); f.p2(0x61a8); }
  f.p1(p3);

  return 0;
}

int main(int argc, char *argv[])
{
  FILE *f = fopen(argv[0], "rb");

  unsigned int num_addrs = 8;
  int addrs[] = {
    0x00000693,
    0x000006ad,
    0x000006ae,
    0x000006bb,
    0x000006d3,
    0x000006ed,
    0x000006ee,
    0x000006fb
  };
  int bytes[num_addrs];

  for (int i = 0; i < num_addrs; i++)
  {
    fseek(f, addrs[i], SEEK_SET);
    bytes[i] = fgetc(f);
  }
  fclose(f);

  if (bytes[0] == 0xff) // Toggle this bit according to first _differing_ byte in collision
  {
	  return evil(argc, argv);
  }

  return good(argc, argv);
}