ELFIO/examples/write_obj/write_obj.cpp

/*
Copyright (C) 2001-present by Serge Lamikhov-Center

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
*/

/*
 * This example shows how to create ELF object file for Linux on x86
 *
 * Instructions:
 * 1. Compile and link this file with ELFIO library
 *    g++ write_obj.cpp -o writer_obj
 * 2. Execute result file write_obj
 *    ./write_obj
 * 3. Link output file hello.o:
 *    ld -o hello hello.o
 * 4. Run the result file:
 *    ./hello
 */

#include <elfio/elfio.hpp>

using namespace ELFIO;

int main( void )
{
    elfio writer;

    // You can't proceed before this function call!
    writer.create( ELFCLASS64, ELFDATA2LSB );

    writer.set_os_abi( ELFOSABI_LINUX );
    writer.set_type( ET_REL );
    writer.set_machine( EM_X86_64 );

    // This is our code
    char text[] = {
        '\xB8', '\x04', '\x00', '\x00', '\x00', // mov eax, 4
        '\xBB', '\x01', '\x00', '\x00', '\x00', // mov ebx, 1
        '\xB9', '\x00', '\x00', '\x00', '\x00', // mov ecx, msg
        '\xBA', '\x0E', '\x00', '\x00', '\x00', // mov edx, 14
        '\xCD', '\x80',                         // int 0x80
        '\xB8', '\x01', '\x00', '\x00', '\x00', // mov eax, 1
        '\xCD', '\x80',                         // int 0x80
        '\x48', '\x65', '\x6C', '\x6C', '\x6F', // msg: db   'Hello, World!', 10
        '\x2C', '\x20', '\x57', '\x6F', '\x72',
        '\x6C', '\x64', '\x21', '\x0A' };
    Elf64_Addr place_to_adjust = 11;

    // Create code section
    section* text_sec = writer.sections.add( ".text" );
    text_sec->set_type( SHT_PROGBITS );
    text_sec->set_flags( SHF_ALLOC | SHF_EXECINSTR );
    text_sec->set_addr_align( 0x10 );
    text_sec->set_data( text, sizeof( text ) );

    // Create string table section
    section* str_sec = writer.sections.add( ".strtab" );
    str_sec->set_type( SHT_STRTAB );

    // Create string table writer
    string_section_accessor stra( str_sec );
    // Add label name
    Elf32_Word str_index = stra.add_string( "msg" );

    // Create symbol table section
    section* sym_sec = writer.sections.add( ".symtab" );
    sym_sec->set_type( SHT_SYMTAB );
    sym_sec->set_info( 1 );
    sym_sec->set_addr_align( 0x4 );
    sym_sec->set_entry_size( writer.get_default_entry_size( SHT_SYMTAB ) );
    sym_sec->set_link( str_sec->get_index() );

    // Create symbol table writer
    symbol_section_accessor syma( writer, sym_sec );
    // Add symbol entry (msg has offset == 29)
    Elf_Word sym_to_adjust = syma.add_symbol(
        str_index, 29, 0, STB_GLOBAL, STT_OBJECT, 0, text_sec->get_index() );
    // Another way to add symbol
    syma.add_symbol( stra, "_start", 0x00000000, 0, STB_WEAK, STT_FUNC, 0,
                     text_sec->get_index() );

    // Create relocation table section
    section* rel_sec = writer.sections.add( ".rel.text" );
    rel_sec->set_type( SHT_REL );
    rel_sec->set_info( text_sec->get_index() );
    rel_sec->set_addr_align( 0x4 );
    rel_sec->set_entry_size( writer.get_default_entry_size( SHT_REL ) );
    rel_sec->set_link( sym_sec->get_index() );

    // Create relocation table writer
    relocation_section_accessor rela( writer, rel_sec );
    // Add relocation entry (adjust address at offset 11)
    rela.add_entry( place_to_adjust, sym_to_adjust,
                    (unsigned char)R_386_RELATIVE );

    // Another method to add the same relocation entry at one step is:
    // rela.add_entry( stra, "msg",
    //                 syma, 29, 0,
    //                 ELF_ST_INFO( STB_GLOBAL, STT_OBJECT ), 0,
    //                 text_sec->get_index(),
    //                 place_to_adjust, (unsigned char)R_386_RELATIVE );

    // Create note section
    section* note_sec = writer.sections.add( ".note" );
    note_sec->set_type( SHT_NOTE );

    note_section_accessor note_writer( writer, note_sec );
    note_writer.add_note( 0x01, "Created by ELFIO", 0, 0 );
    char descr[6] = { 0x31, 0x32, 0x33, 0x34, 0x35, 0x36 };
    note_writer.add_note( 0x01, "Never easier!", descr, sizeof( descr ) );

    // We don't use local symbols here. There is no need to rearrange them.
    // But, for the completeness, we do this just prior 'save'
    syma.arrange_local_symbols( [&]( Elf_Xword first, Elf_Xword second ) {
        rela.swap_symbols( first, second );
    } );

    // Create ELF object file
    writer.save( "hello.o" );

    return 0;
}