PoC to checksum a given portion of the code

self_checksumming.c

#define  _GNU_SOURCE
#include <unistd.h>
#include <dlfcn.h>
#include <sys/mman.h>
#include <link.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>

void *begin;
void *end;

int callback(struct dl_phdr_info *info, size_t size, void *data) {
  if (info->dlpi_name && info->dlpi_name[0] != '\0')
    return 0;
  size_t page = sysconf(_SC_PAGESIZE);
  int i;
  for (i = 0; i < (size_t)info->dlpi_phnum; i++)
    if ((info->dlpi_phdr[i].p_flags & PF_X)) {
        size_t ptr = (size_t)info->dlpi_phdr[i].p_vaddr;
        size_t len = (size_t)info->dlpi_phdr[i].p_memsz;
        if (ptr % page) {
            len += ptr % page;
            ptr -= ptr % page;
        }
        if (len % page)
            len += page - (len % page);
        if (mprotect((void *)ptr, len, PROT_READ | PROT_WRITE | PROT_EXEC))
          if (errno != ENOMEM)
            return errno;
    }    
  return 0;
}

int makeTextWritable() {
  return dl_iterate_phdr(callback, NULL);
}

int test(int x, int nb, int dummy) {
  if(dummy) {
    begin = &&begin;
    end = &&end;
    return 0;
  }

  int r=1;
  int i;
  begin:
    for (i=0;i<nb;i++)
      r = r * x;
  end:
  return r;
}


int show_code() {
  char* p=begin;
  printf("Code:");
  while (p<(char*) end) {
    printf("%02hhx ",(char)*p);
    p++;
  }
  printf("\n");
}

int hash_fun() {
  const int HASH = 3801;
  int sum = 0;
  char* p=begin;
  while (p<(char*) end) {
    sum += (unsigned char)*p++;
  }
  printf("Sum:%d\n", sum);
  return sum == HASH;
}

void cipher(char key) {
    char* p=begin;
    while (p<(char*) end) {
      *p++ ^= key;
    }
}



void main() {

  //Demo Tamperproofing
  test(0,0,1);
  if(hash_fun())
    printf("Hash OK !\n");
  else
    printf("Fail hash\n");
  //-------------------
  

  //Demo self-modification
  makeTextWritable();
  show_code();
  cipher(99);
  show_code();
  if(hash_fun())
    printf("Hash OK !\n");
  else
    printf("Fail hash\n");
  test(0,0,0);
  //----------------------
}