Hello World - 讓code size小於512 byts

在上一篇中，code size已經縮小到536 bytes。其反組譯的內容為...

$ objdump -S hello2

hello2:     file format elf32-i386


Disassembly of section .text:

08048094 <.text>:
 8048094: 55                    push   %ebp
 8048095: 89 e5                 mov    %esp,%ebp
 8048097: ba 0e 00 00 00        mov    $0xe,%edx
 804809c: 8b 0d c8 90 04 08     mov    0x80490c8,%ecx
 80480a2: bb 01 00 00 00        mov    $0x1,%ebx
 80480a7: b8 04 00 00 00        mov    $0x4,%eax
 80480ac: cd 80                 int    $0x80
 80480ae: 31 db                 xor    %ebx,%ebx
 80480b0: b8 01 00 00 00        mov    $0x1,%eax
 80480b5: cd 80                 int    $0x80
 80480b7: 5d                    pop    %ebp
 80480b8: c3                    ret  
$

這裡有一點需注意...由於編譯、執行時沒有將正確的stack建立，所以_start() function雖然有做stack的動作，但一不小心還是會造成stack錯誤，所以_start中只有push後並沒有做額外的stack存取。
另外...因為int 0x80(eax<1)為程式的結束點，所以可以不需要做return動作。至此...

$ gcc -nostartfiles -nostdlib -static hello2.c -o hello2
$ strip hello2
$ ./hello2
Hello World
$ ls -l hello2
-rwxr-xr-x 1 lungswu lungswu 528 2010-03-01 10:08 hello2
$ $

縮小為528 byte。

---

再過來...我們觀察編譯後的執行檔結構...

$ objdump -h hello2

hello2:     file format elf32-i386

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         00000025  08048094  08048094  00000094  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .rodata       0000000d  080480b9  080480b9  000000b9  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  2 .data         00000004  080490c8  080490c8  000000c8  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  3 .comment      0000002a  00000000  00000000  000000cc  2**0
                  CONTENTS, READONLY

裡面有一個.comment 的section，其內容為...

$ objdump -s hello2

hello2:     file format elf32-i386

Contents of section .text:
 8048094 5589e5ba 0e000000 8b0dc890 0408bb01  U...............
 80480a4 000000b8 04000000 cd8031db b8010000  ..........1.....
 80480b4 00cd805d c3                          ...].         
Contents of section .rodata:
 80480b9 48656c6c 6f20576f 726c640a 00        Hello World.. 
Contents of section .data:
 80490c8 b9800408                             ....          
Contents of section .comment:
 0000 00474343 3a202847 4e552920 332e342e  .GCC: (GNU) 3.4.
 0010 36202855 62756e74 7520332e 342e362d  6 (Ubuntu 3.4.6-
 0020 38756275 6e747532 2900               8ubuntu2).    

它是gcc編譯後的額外資料，是否可以將它移除？？？

答案是肯定的，GNU Binary utility中有一個命令...objcopy...它可以做一些object file(ELF)檔案的處理...

$ man objcopy
OBJCOPY(1)                           GNU Development Tools                          OBJCOPY(1)

NAME
       objcopy - copy and translate object files

SYNOPSIS
.... 省略 ....
       --remove-section=sectionname
           Remove  any  section  named  sectionname  from the output file.  This option may be
           given more than once.  Note that using this option  inappropriately  may  make  the
           output file unusable.
"--remove-section"這個參數正可以用上...

$ ls -l hello2
-rwxr-xr-x 1 lungswu lungswu 528 2010-03-01 10:08 hello2
$
$ objcopy --remove-section=.comment hello2 hello2.1
$
$ ./hello2.1
Hello World
$ ls -l hello2.1
-rwxr-xr-x 1 lungswu lungswu 436 2010-03-01 10:15 hello2.1
$

所以已經達到初步的目標，Hello world的code size已經小於512 byte，達到436 bytes.