Write embedded executable binary from a C program to a file

Question

I'm trying to embed an executable binary inside an other one and then write the first binary to a file.

I successfully achieved this with a plain text file but when it comes to writing an executable I cant make it work.

So for example I want to copy /usr/bin/ls to ls2, so here what I tried

• First: objcopy --input binary --output elf64-x86-64 --binary-architecture i386 /usr/bin/ls lss.o

• the C code:

  #include <stdio.h>
  
  FILE *f;
  
  extern char _binary_lss_start;
  extern char _binary_lss_end;
  
  main()
      {
      f = fopen("lss", "w");
      fprintf(f, &_binary_lss_start);
      fclose(f);
      return 0;
  }
  

• Compilation: gcc main.c lss.o

The Code successfully compiled but when i'm trying ./a.out nothing is written to lss.

(I'm using Arch Linux 4.18.5 and gcc 8.2.0.)

Can I achieve this?

Answer 1

As @jww mentioned you should use fwrite. fprintf is looking for a 0 terminated string at _binary_lss_start which it might encounter in the very first byte and not write anything.

#include <stdio.h>

FILE *f;

extern char _binary_lss_start;
extern char _binary_lss_end;
int main(int c, char *v[])
{
    size_t size_to_write = &_binary_lss_end - &_binary_lss_start;

    f = fopen("lscpu2", "w");
    if (f != NULL) {
        printf("Writing %ld bytes\n", size_to_write);
        fwrite(&_binary_lss_start, 1, size_to_write, f);
        fclose(f);
    } else {
        printf("File not found [%s]\n", "lss");
    }
    return 0;
}