{"id":48,"date":"2024-06-12T08:45:00","date_gmt":"2024-06-11T22:45:00","guid":{"rendered":"https:\/\/davidshoon.com\/?p=48"},"modified":"2026-10-20T09:50:50","modified_gmt":"2026-10-19T22:50:50","slug":"smallest-hello-world-in-linux-assembly","status":"publish","type":"post","link":"https:\/\/davidshoon.com\/?p=48","title":{"rendered":"Smallest hello world in linux assembly"},"content":{"rendered":"

Smallest hello world in linux assembly<\/h1>\n

From https:\/\/jameshfisher.com\/2018\/03\/10\/linux-assembly-hello-world\/<\/a><\/p>\n

global _start<\/p>\n

section .text<\/p>\n

_start:
  mov rax, 1        ; write(
  mov rdi, 1        ;   STDOUT_FILENO,
  mov rsi, msg      ;   “Hello, world!n”,
  mov rdx, msglen   ;   sizeof(“Hello, world!n”)
  syscall           ; );<\/p>\n

  mov rax, 60       ; exit(
  mov rdi, 0        ;   EXIT_SUCCESS
  syscall           ; );<\/p>\n

section .rodata
  msg: db “Hello, world!”, 10
  msglen: equ $ – msg<\/p>\n

<\/span>However, when we assemble and link it, it will actually be quite large.<\/p>\n

$ nasm -f elf64 -o hello.o hello.s
$ ld -o hello hello.o
$ .\/hello
Hello, world!<\/p>\n

$ ls -al hello
-rwxrwxr-x 1 d d 8872 Jun 12 08:22 hello
<\/span><\/p>\n

That’s because it uses a section .rodata, which requires a read-only memory page, on top of a code page (read-executable). So that requires 8KB (if 4KB pages are used) in your binary.<\/p>\n

Still, it’s smaller than a “puts(“Hello world”)” C example program.<\/p>\n


#include <stdio.h><\/p>\n

int main()
{
    puts(“Hello, world!”);
    return 0;
}
<\/span><\/p>\n

Size of C program:<\/p>\n


$ gcc test.c
$ ls -al a.out
-rwxrwxr-x 1 d d 15960 Jun 12 08:24 a.out<\/p>\n

$ strip a.out
$ ls -al a.out
-rwxrwxr-x 1 d d 14472 Jun 12 08:25 a.out<\/p>\n

<\/span><\/p>\n

Going back to the assembly program, to shrink it down even more so, the modified source code is then:<\/p>\n


global _start<\/p>\n

section .text<\/p>\n

_start:
  mov rax, 1        ; write(
  mov rdi, 1        ;   STDOUT_FILENO,
  mov rsi, msg      ;   “Hello, world!n”,
  mov rdx, msglen   ;   sizeof(“Hello, world!n”)
  syscall           ; );<\/p>\n

  mov rax, 60       ; exit(
  mov rdi, 0        ;   EXIT_SUCCESS
  syscall           ; );<\/p>\n

  msg: db “Hello, world!”, 10
  msglen: equ $ – msg
<\/span><\/p>\n

The same code, without the section .rodata. It just means the text string “Hello World” is actually in the code (.text section) page.<\/p>\n

Next step is to strip the binary — to make it even slightly more smaller.<\/p>\n

This is the best we can do, without hacking the binary any further.<\/p>\n


$ ls -al hello
-rwxrwxr-x 1 d d 4360 Jun 12 08:19 hello
<\/span><\/p>\n

So that’s 4KB, plus abit more, if you include the ELF headers.<\/p>\n

<\/p>\n

Creating our own ELF headers<\/h2>\n

Taking from the following articles:<\/p>\n

https:\/\/www.muppetlabs.com\/%7Ebreadbox\/software\/tiny\/teensy.html<\/a><\/p>\n

https:\/\/stackoverflow.com\/questions\/53382589\/smallest-executable-program-x86-64-linux<\/a><\/p>\n

We create our own ELF header and just build using `nasm -o test test.s`<\/p>\n


bits 64
            org 0x08048000<\/p>\n

ehdr:                                           ; Elf64_Ehdr
            db  0x7F, “ELF”, 2, 1, 1, 0         ;   e_ident
    times 8 db  0
            dw  2                               ;   e_type
            dw  62                              ;   e_machine
            dd  1                               ;   e_version
            dq  _start                          ;   e_entry
            dq  phdr – $$                       ;   e_phoff
            dq  0                               ;   e_shoff
            dd  0                               ;   e_flags
            dw  ehdrsize                        ;   e_ehsize
            dw  phdrsize                        ;   e_phentsize
            dw  1                               ;   e_phnum
            dw  0                               ;   e_shentsize
            dw  0                               ;   e_shnum
            dw  0                               ;   e_shstrndx<\/p>\n

ehdrsize    equ $ – ehdr<\/p>\n

phdr:                                           ; Elf64_Phdr
            dd  1                               ;   p_type
            dd  5                               ;   p_flags
            dq  0                               ;   p_offset
            dq  $$                              ;   p_vaddr
            dq  $$                              ;   p_paddr
            dq  filesize                        ;   p_filesz
            dq  filesize                        ;   p_memsz
            dq  0x1000                          ;   p_align<\/p>\n

phdrsize    equ     $ – phdr<\/p>\n

_start:
  mov rax, 1        ; write(
  mov rdi, 1        ;   STDOUT_FILENO,
  mov rsi, msg      ;   “Hello, world!n”,
  mov rdx, msglen   ;   sizeof(“Hello, world!n”)
  syscall           ; );<\/p>\n

  mov rax, 60       ; exit(
  mov rdi, 0        ;   EXIT_SUCCESS
  syscall           ; );<\/p>\n

  msg: db “Hello, world!”, 10
  msglen: equ $ – msg<\/p>\n

filesize      equ     $ – $$<\/span><\/p>\n

We build it with nasm:
<\/span><\/p>\n


$ nasm -o test test.s<\/p>\n

$ ls -al test
-rwxr-xr-x 1 d d 173 Jun 12 08:35 test
<\/span><\/p>\n

The output is 173 bytes! That’s small. But it can be improved, based off those articles above.<\/p>\n","protected":false},"excerpt":{"rendered":"

Smallest hello world in linux assembly From https:\/\/jameshfisher.com\/2018\/03\/10\/linux-assembly-hello-world\/ global _start section .text _start:  mov rax, 1        ; write(  mov rdi, 1        ;   STDOUT_FILENO,  mov rsi, msg      ;   “Hello, world!n”,  mov rdx, msglen   ;   sizeof(“Hello, world!n”)  syscall           ; );   mov rax, 60       ; exit(  mov rdi, 0        ;   EXIT_SUCCESS  syscall           ; ); section .rodata  msg: […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-48","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/posts\/48","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/davidshoon.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=48"}],"version-history":[{"count":1,"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/posts\/48\/revisions"}],"predecessor-version":[{"id":49,"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/posts\/48\/revisions\/49"}],"wp:attachment":[{"href":"https:\/\/davidshoon.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=48"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/davidshoon.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=48"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/davidshoon.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=48"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}