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?

Possible duplicate of Embedding resources in executable using GCC. In the past I seem to recall using objcpy to do it. In your case you should use fwrite and not fprintf. The start of data is binary_lss_start and the length of data is binary_lss_end - binary_lss_start + 1. — jww
– jww, Commented Aug 30, 2018 at 17:46

cleblanc · Accepted Answer · 2018-08-31 13:05:38Z

2

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;
}

edited Aug 31, 2018 at 13:05

answered Aug 30, 2018 at 17:59

cleblanc

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2 Comments

user10296487 Over a year ago

i've tried but i wasnt able to make it work (compil error, seg fault, exec error..) As i alrady my c knoledge is very low, can you provide a basic exemple ?

cleblanc Over a year ago

@V1n5 I've added an example, it's untested but should work.

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