ELF (executable) generation example

gen_hello.c

// $ gcc gen_hello.c -o gen_hello
// $ ./gen_hello
// $ ./hello

#include <stdio.h>
#include <stdlib.h>
#include <elf.h>

#define SEGMENTS 1
#define SEG_EXEC 0

Elf64_Ehdr ehdr;
Elf64_Phdr phdr[SEGMENTS];

uint8_t text[] = {
  // hello
  0x48, 0x89, 0xe5,                         // mov %rsp, %rbp
  0xc6, 0x45, 0xf4, 0x48,                   // movb $72, -12(%rbp)
  0xc6, 0x45, 0xf5, 0x65,                   // movb $101, -11(%rbp)
  0xc6, 0x45, 0xf6, 0x6c,                   // movb $108, -10(%rbp)
  0xc6, 0x45, 0xf7, 0x6c,                   // movb $108, -9(%rbp)
  0xc6, 0x45, 0xf8, 0x6f,                   // movb $111, -8(%rbp)
  0xc6, 0x45, 0xf9, 0x20,                   // movb $32, -7(%rbp)
  0xc6, 0x45, 0xfa, 0x57,                   // movb $87, -6(%rbp)
  0xc6, 0x45, 0xfb, 0x6f,                   // movb $111, -5(%rbp)
  0xc6, 0x45, 0xfc, 0x72,                   // movb $114, -4(%rbp)
  0xc6, 0x45, 0xfd, 0x6c,                   // movb $108, -3(%rbp)
  0xc6, 0x45, 0xfe, 0x64,                   // movb $100, -2(%rbp)
  0xc6, 0x45, 0xff, 0x0a,                   // movb $10, -1(%rbp)
  0x48, 0xc7, 0xc2, 0x0c, 0x00, 0x00, 0x00, // mov $12, %rdx
  0x48, 0x8d, 0x75, 0xf4,                   // lea -12(%rbp), %rsi
  0x48, 0xc7, 0xc7, 0x01, 0x00, 0x00, 0x00, // mov $1, %rdi
  0x48, 0xc7, 0xc0, 0x01, 0x00, 0x00, 0x00, // mov $1, %rax
  0x0f, 0x05,                               // syscall
  0x48, 0xc7, 0xc7, 0x00, 0x00, 0x00, 0x00, // mov $0, %rdi
  0x48, 0xc7, 0xc0, 0x3c, 0x00, 0x00, 0x00, // mov $60, %rax
  0x0f, 0x05,                               // syscall
};

int main(void) {
  Elf64_Off offset_phdr = sizeof(ehdr);
  Elf64_Off offset_text = sizeof(ehdr) + sizeof(phdr);
  Elf64_Addr entry = 0x400000 + offset_text;
  uint64_t align = 0x200000;

  ehdr.e_ident[0] = ELFMAG0;
  ehdr.e_ident[1] = ELFMAG1;
  ehdr.e_ident[2] = ELFMAG2;
  ehdr.e_ident[3] = ELFMAG3;
  ehdr.e_ident[4] = ELFCLASS64;
  ehdr.e_ident[5] = ELFDATA2LSB;
  ehdr.e_ident[6] = EV_CURRENT;
  ehdr.e_ident[7] = ELFOSABI_NONE;
  ehdr.e_type = ET_EXEC;
  ehdr.e_machine = EM_X86_64;
  ehdr.e_version = EV_CURRENT;
  ehdr.e_entry = entry;
  ehdr.e_phoff = offset_phdr;
  ehdr.e_ehsize = sizeof(Elf64_Ehdr);
  ehdr.e_phentsize = sizeof(Elf64_Phdr);
  ehdr.e_phnum = SEGMENTS;

  phdr[SEG_EXEC].p_type = PT_LOAD;
  phdr[SEG_EXEC].p_flags = PF_R | PF_X;
  phdr[SEG_EXEC].p_offset = offset_text;
  phdr[SEG_EXEC].p_vaddr = entry;
  phdr[SEG_EXEC].p_paddr = entry;
  phdr[SEG_EXEC].p_filesz = sizeof(text);
  phdr[SEG_EXEC].p_memsz = sizeof(text);
  phdr[SEG_EXEC].p_align = align;

  FILE *out = fopen("hello", "wb");
  fwrite(&ehdr, sizeof(ehdr), 1, out);
  fwrite(&phdr, sizeof(phdr), 1, out);
  fwrite(&text, sizeof(text), 1, out);
  fclose(out);

  return 0;
}