shellAdder1.c

#include <stdio.h>
#include <fcntl.h>
#include "elf.h"
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char** argv){
  char target_section[] = ".mytext";
  char *shstr = NULL;
  char *content = NULL;
  Elf32_Ehdr ehdr;
  Elf32_Shdr shdr;
  int i;
  unsigned int base, length;
  unsigned short nblock;
  unsigned short nsize;
  unsigned char block_size = 16;
  
  int fd;
  
  if(argc < 2){
    puts("Input .so file");
    return -1;
  }
  
  fd = open(argv[1], O_RDWR);
  if(fd < 0){
    printf("open %s failed\n", argv[1]);
    goto _error;
  }
  
  if(read(fd, &ehdr, sizeof(Elf32_Ehdr)) != sizeof(Elf32_Ehdr)){
    puts("Read ELF header error");
    goto _error;
  }
  
  lseek(fd, ehdr.e_shoff + sizeof(Elf32_Shdr) * ehdr.e_shstrndx, SEEK_SET);
  
  if(read(fd, &shdr, sizeof(Elf32_Shdr)) != sizeof(Elf32_Shdr)){
    puts("Read ELF section string table error");
    goto _error;
  }
  
  if((shstr = (char *) malloc(shdr.sh_size)) == NULL){
    puts("Malloc space for section string table failed");
    goto _error;
  }
  
  lseek(fd, shdr.sh_offset, SEEK_SET);
  if(read(fd, shstr, shdr.sh_size) != shdr.sh_size){
    puts("Read string table failed");
    goto _error;
  }
  
  lseek(fd, ehdr.e_shoff, SEEK_SET);
  for(i = 0; i < ehdr.e_shnum; i++){
    if(read(fd, &shdr, sizeof(Elf32_Shdr)) != sizeof(Elf32_Shdr)){
      puts("Find section .text procedure failed");
      goto _error;
    }
    if(strcmp(shstr + shdr.sh_name, target_section) == 0){
      base = shdr.sh_offset;
      length = shdr.sh_size;
      printf("Find section %s\n", target_section);
      break;
    }
  }
  
  lseek(fd, base, SEEK_SET);
  content = (char*) malloc(length);
  if(content == NULL){
    puts("Malloc space for content failed");
    goto _error;
  }
  if(read(fd, content, length) != length){
    puts("Read section .text failed");
    goto _error;
  }
  
  nblock = length / block_size;
  nsize = base / 4096 + (base % 4096 == 0 ? 0 : 1);
  printf("base = %d, length = %d\n", base, length);
  printf("nblock = %d, nsize = %d\n", nblock, nsize);
  
//  ehdr.e_entry = (length << 16) + nsize;
//  ehdr.e_shoff = base;
  
  
  
  for(i=0;i<length;i++){
    content[i] = ~content[i];
  }
  
  
  
//  lseek(fd, 0, SEEK_SET);
//  if(write(fd, &ehdr, sizeof(Elf32_Ehdr)) != sizeof(Elf32_Ehdr)){
//    puts("Write ELFhead to .so failed");
//    goto _error;
//  }
  
  
  lseek(fd, base, SEEK_SET);
  if(write(fd, content, length) != length){
    puts("Write modified content to .so failed");
    goto _error;
  }
  
  
  puts("Completed");
_error:
  free(content);
  free(shstr);
  close(fd);
  return 0;
}