simply polymorphic C program that changes it's own binary

poly.c

// this program has been tested on macOS and Alpine Linux, but not Windows. Both
// GCC Clang and GNU GCC have also been tested to work. Ideally, this program
// should work on any x86-64 machine.
//
// the program assumes it's binary is called "poly", so you must change the
// source code if you want to rename the executable to anything else.

#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

// can be any arbitary value, just needs to be unique
#define MARKER 0x99999999

// Prints the starting state of the program.
void print_state() {
  volatile int __attribute__((unused)) MARK1 = MARKER;
  int a = 0x99;
  printf("Starting value (hardcoded into the binary): 0x%x\n", a);
}

// Increments a value based in the binary
void inc_val(unsigned long val_pos, FILE *file) {
  // read the value at val_pos
  int curr;
  fseek(file, val_pos, SEEK_SET);
  if (!fread(&curr, sizeof(int), 1, file)) {
    perror("fread()");
    exit(1);
  }
  printf("Found value 0x%x at %lu.\n", curr, val_pos);

  // increment the value and write it to the binary
  curr++;
  fseek(file, val_pos, SEEK_SET);
  if (!fwrite(&curr, sizeof(int), 1, file)) {
    perror("fwrite()");
    exit(1);
  }
  printf("Set %lu to 0x%x\n", val_pos, curr);
}

int main() {
  print_state();
  puts(""); // empty line

  // Copy binary to prevent "Resource busy" error
  if (system("cp poly poly.tmp") != 0) {
    fputs("Copy failed.\n", stderr);
    return 1;
  }

  // open binary
  FILE *file = fopen("poly.tmp", "r+b");
  if (file == NULL) {
    perror("fopen()");
    return 1;
  }

  // get size of binary file
  fseek(file, 0, SEEK_END);
  unsigned long len = ftell(file);
  rewind(file);

  // read the binary into memory
  uint8_t *buf = malloc(len);
  if (fread(buf, 1, len, file) < len) {
    perror("fread()");
    return 1;
  }

  // search for the marker, then increment the value. value is at a set offset
  // from the marker
  for (unsigned long i = 0; i < len-8; i++) {
    uint32_t frame = htonl(*((uint32_t *)(buf + i)));
    if (frame == MARKER) {
      printf("Found marker 0x%x at: %lu\n", MARKER, i);
      inc_val(i + sizeof(int) + 3, file);
      break;
    }
  }
  puts("File patched successfully.");

  // Replace poly with poly.tmp
  if (rename("poly.tmp", "poly") < 0) {
    perror("rename()");
    return 1;
  }

  // cleanup
  free(buf);
  fclose(file);
  return 0;
}