c/c++: embed binary data into elf

It's great idea when you store program data somewhere outside the binary.
It can be modified for changing program's behaivior or for rebranding.

But sometimes you want to keep some data immutable, hidden into executable binary.

This can be help sections. If you don't want to have smth like

void
usage (status)
     int status;
{
  fprintf (status ? stderr : stdout, "\
Usage: %s [-nV] [--quiet] [--silent] [--version] [-e script]\n\
        [-f script-file] [--expression=script] [--file=script-file] [file...]\n",
       myname);
  exit (status);
}

and don't want this help section be stored in the separate file.You can simply embed binary data into your executable.

Consider you have data.txt:

$cat data.txt 
data file

You have to convert it to elf.
I know two ways:

• use linker:
  
  

  ld -r -b binary -o data.o data.txt

• use objcopy:
  
  

  objcopy -I binary -O elf32-i386 --binary-architecture i386 data.txt data.o

Both of these commands produce elf:

$readelf -a data.o 
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              REL (Relocatable file)
  Machine:                           Intel 80386
  Version:                           0x1
  Entry point address:               0x0
  Start of program headers:          0 (bytes into file)
  Start of section headers:          96 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           0 (bytes)
  Number of program headers:         0
  Size of section headers:           40 (bytes)
  Number of section headers:         5
  Section header string table index: 2

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .data             PROGBITS        00000000 000034 00000a 00  WA  0   0  1
  [ 2] .shstrtab         STRTAB          00000000 00003e 000021 00      0   0  1
  [ 3] .symtab           SYMTAB          00000000 000128 000050 10      4   2  4
  [ 4] .strtab           STRTAB          00000000 000178 000043 00      0   0  1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings)
  I (info), L (link order), G (group), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

Symbol table '.symtab' contains 5 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 00000000     0 SECTION LOCAL  DEFAULT    1 
     2: 00000000     0 NOTYPE  GLOBAL DEFAULT    1 _binary_data_txt_start
     3: 0000000a     0 NOTYPE  GLOBAL DEFAULT    1 _binary_data_txt_end
     4: 0000000a     0 NOTYPE  GLOBAL DEFAULT  ABS _binary_data_txt_size

_binary_data_txt_size and _binary_data_txt_end contain 

Ok, you have data.o with your data in .data section and three symbols: _binary_data_txt_start, _binary_data_txt_end, _binary_data_txt_size

_binary_data_txt_end and _binary_data_txt_size have the same value here. So I'll use _binary_data_txt_size only.
Let's make a simple c program to use data from the object. It's a bit tricky.

#include <stdio.h>

extern int _binary_data_txt_start;
extern int _binary_data_txt_size;

int
main(int argc, char **argv)
{
    int size = (int)&_binary_data_txt_size;
    char *data = (char *)&_binary_data_txt_start;
    
    printf("%d", size);
    printf("%s", data);

    return 0;
}

_binary_data_txt_start and _binary_data_txt_size contain values in their addresses. So &_binary_data_txt_size contains not an address of the symbol but actually value of the symbol that holds the size of the data and &_binary_data_txt_start contains address of the data.

To compile

gcc test.c data.o