#define _GNU_SOURCE
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <elf.h>

#define PATTERN "\xeb\x1f\x4f\x53\x46\x20\x20\x20\x20\x20\x20\x20\x20\x20" \
                "\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20" \
                "\x20\x20\x20\x20\x20\xb8"

int main (int argc, char **argv) {
   int fd;
   char *ptr, *ptr2, *ptr3;
   struct stat st;
   Elf32_Ehdr *elf;
   
   if (argc != 2) {
      printf("Usage: %s <file>\n\n", argv[0]);
      exit(EXIT_FAILURE);
   }
   
   printf("%s: ", argv[1]);
   fd = open(argv[1], O_RDWR);
   if (fd < 0) {
      printf("open: %s\n", strerror(errno));
      exit(EXIT_FAILURE);
   }
   fstat(fd, &st);
   
   ptr = mmap(NULL, st.st_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
   if (ptr == MAP_FAILED) {
      printf("mmap: %s\n", strerror(errno));
      exit(EXIT_FAILURE);
   }
   
   if (memcmp(ptr, ELFMAG, SELFMAG)) {
      printf("Not an ELF file.\n");
      exit(EXIT_FAILURE);
   }
   elf = (Elf32_Ehdr *) ptr;
   
   ptr2 = memmem(ptr, st.st_size, PATTERN, strlen(PATTERN));
   if (!ptr2) {
      printf("Pattern not found!\n");
      exit(EXIT_SUCCESS);
   }
   
   ptr3 = memmem(ptr2+1, st.st_size - (ptr2-ptr), PATTERN, strlen(PATTERN));
   if (ptr3) {
      printf("Pattern found twice! Repair manually!\n");
      exit(EXIT_FAILURE);
   }
   printf("Found Pattern - ");
   memset(ptr2, 0, strlen(PATTERN));
   ptr2 += strlen(PATTERN);
   printf("Entry point: m: 0x%08x; o: 0x%p - done.\n", elf->e_entry,
	  *((void **)(ptr2)));
   memcpy(ptr+24, ptr2, 4);
   munmap(ptr, st.st_size);
   close(fd);
   exit(EXIT_SUCCESS);
}