From c706eb91161ed2077f85ad2a771d0f4c69ef09c1 Mon Sep 17 00:00:00 2001
From: Arturs Artamonovs <dos21h@gmail.com>
Date: Sat, 27 Sep 2025 09:23:22 +0100
Subject: add post on arm64 tiny binaries

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+title: ARM64 make tiny binaries
+keywords:raspi5,asm,c,kernel,arm64
+
+# ARM64 make tiny binaries
+
+
+## Intro
+
+This notes based on [Making C executable smaller](/writeup/making_c_executables_smaller.md).
+Now time came to create version for ARM. One example using C and one example using assembler.
+Examples printout hello world message and exit, nothing else. 
+
+## Minimal C program
+
+Two minimal C programs one that using libc other that doesn't
+
+### Source code without libc
+
+___mainc.c___
+```c
+char str[] = "Hello world\n";
+unsigned int str_len = 12;
+void _start() {
+	//write
+	asm(\
+		"mov x0, #1\n"
+		"mov x1, %0\n"
+		"mov x2, %1\n"
+		"mov w8, #64\n"
+		"svc #0\n"
+	:
+	:"r"(str),"r"(str_len)
+	);
+	//exit
+	asm(\
+		"mov x0, #0\n"                /* status := 0 */
+		"mov w8, #93\n"               /* exit is syscall #1 */
+		"svc #0\n"
+	);
+}
+
+```
+
+Compiles with
+```bash
+gcc -Os -c mainc.c  mainc.o
+ld mainc.o -o mainc
+```
+
+### Source code with libc 
+
+__mainc2.c__
+```c
+#include <stdlib.h>
+void _start() {
+	write(1,"Hello world\n",12);
+	exit(0);
+}
+```
+
+Compiles with
+```bash
+gcc -Os -c mainc2.c mainc2.o
+ld mainc2.o -lc -o mainc2
+```
+
+
+## Minimal Assembler program
+
+Both examples will work with raspberry pi 4 as such. One example for ARMv7 
+and other for ARMv8
+
+### Source code for ARMv7
+
+__main7.S__
+
+```c
+.text            
+.global _start
+_start:
+    mov r0, #1
+    ldr r1, =message
+    ldr r2, =len
+    mov r7, #4
+    swi 0
+
+    mov r7, #1
+    swi 0
+
+.data
+message:
+    .asciz "hello world\n"
+len = .-message 
+```
+
+
+```bash
+arm-linux-gnueabi-gcc -Os -c main7.S main7.o
+arm-linux-gnueabi-ld -s main7.o -o main7
+```
+
+### Source code for ARMv8
+
+
+__main.S__
+```c
+.data
+
+/* Data segment: define our message string and calculate its length. */
+helloworld:
+    .ascii        "Hello, ARM64!\n"
+helloworld_len = . - helloworld
+
+.text
+
+/* Our application's entry point. */
+.globl _start
+_start:
+    /* syscall write(int fd, const void *buf, size_t count) */
+    mov     x0, #1              /* fd := STDOUT_FILENO */
+    ldr     x1, =helloworld     /* buf := msg */
+    ldr     x2, =helloworld_len /* count := len */
+    mov     w8, #64             /* write is syscall #64 */
+    svc     #0                  /* invoke syscall */
+
+    /* syscall exit(int status) */
+    mov     x0, #0               /* status := 0 */
+    mov     w8, #93              /* exit is syscall #1 */
+    svc     #0
+
+```
+
+
+```bash
+gcc -Os -c main.S -o main.o
+ld main.o -o main
+```
+
+## Binary size before compression
+
+After all examples compiled lets compare sizes
+
+| Binary | Size, bytes |
+|---|---|
+| C, libc| 67976 |
+| C, no libc | 66680 |
+| Asm, ARMv7 | 432 |
+| Asm, ARMv8 | 1152 |
+
+## Packing the binaries into self extracting archive
+
+Binaries are stripped with 
+
+```bash
+strip
+```
+
+Then compressed with 
+```bash
+7z a -tGZip -mx=9 main.gz main > /dev/null
+```
+
+Then self unpacking header is added
+
+__unpack.header__
+```bash
+a=/tmp/I;tail -n+2 $0|zcat>$a;chmod +x $a;$a;rm $a;exit
+```
+
+```bash
+chmod a+x main
+```
+
+Now the binaries can run as regular executable.
+
+## Binary sizes of compression
+
+Lets compare sizes after packing up the binaries
+
+| Binary | Compressed size, bytes | Original size, bytes | Size of original |
+|---|---|---|---|
+| C, libc| 1049 | 67976 | 1.5% |
+| C, no libc | 517 | 66680 | 0.7% |
+| Asm, ARMv7 | 291 | 432 | 67% |
+| Asm, ARMv8 | 250 | 1152 | 21% |
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