程序员的自我修养-读书笔记-目标文件格式

程序员的自我修养-读书笔记

  • 可执行文件生成流程：

    预处理(.i) ->编译(.s) ->汇编(.o) ->链接(a.out)

  • 分步骤生成可执行文件：(以hello.c为例

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gcc -E hello.c -o hello.i    预处理
gcc -S hello.i  -o hello.s   编译
gcc -c hello.s -o hello.o   汇编，也可使用as命令 as hello.s -o hello.o
ld -static /usr/lib/crt1.o /usr/lib/crti.o ... -L/lib hello.o  链接

  • 链接过程主要包括：地址和空间分配，符号决议，重定位

  • 目标文件:

    源代码编译后生成目标文件(.o) 格式跟可执行文件类似，Windows下为 PE，Linux下为ELF，macOS下为Mach-O
    动态库.dll / .so、静态库 .a / 、可执行文件 a.out / .ext、目标文件.o / .obj 都有相似的格式。
    目标文件组织格式为：header + sections

  • 查看文件目标文件格式使用file命令

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$ file hello.o    # linux-arm
hello.o: ELF 32-bit LSB relocatable, ARM, EABI5 version 1 (SYSV), not stripped

$ file hello.o    # macOS-x64
hello.o: Mach-O 64-bit object x86_64

查看目标文件 header 内容

header 输出 
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$ objdump -x hello.o    # linux-arm
hello.o:     file format elf32-littlearm
hello.o
architecture: arm, flags 0x00000011:
HAS_RELOC, HAS_SYMS
start address 0x00000000
private flags = 5000000: [Version5 EABI]

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         00000064  00000000  00000000  00000034  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .data         00000008  00000000  00000000  00000098  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  2 .bss          00000000  00000000  00000000  000000a0  2**0
                  ALLOC
  3 .rodata       00000004  00000000  00000000  000000a0  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  4 .comment      00000024  00000000  00000000  000000a4  2**0
                  CONTENTS, READONLY
  5 .note.GNU-stack 00000000  00000000  00000000  000000c8  2**0
                  CONTENTS, READONLY
  6 .ARM.attributes 0000002f  00000000  00000000  000000c8  2**0
                  CONTENTS, READONLY
SYMBOL TABLE:
00000000 l    df *ABS*    00000000 hello.c
00000000 l    d  .text    00000000 .text
00000000 l    d  .data    00000000 .data
00000000 l    d  .bss    00000000 .bss
00000000 l    d  .rodata    00000000 .rodata
00000004 l     O .data    00000004 static_int.5961
00000000 l    d  .note.GNU-stack    00000000 .note.GNU-stack
00000000 l    d  .comment    00000000 .comment
00000000 l    d  .ARM.attributes    00000000 .ARM.attributes
00000000 g     O .data    00000004 global_int
00000000 g     F .text    0000002c foo
00000000         *UND*    00000000 printf
0000002c g     F .text    00000038 main


RELOCATION RECORDS FOR [.text]:
OFFSET   TYPE              VALUE 
00000018 R_ARM_CALL        printf
00000028 R_ARM_ABS32       .rodata
00000050 R_ARM_CALL        foo


$ objdump -x -macho hello.o    # macOS-x64 
hello.o:
Relocation information (__TEXT,__text) 3 entries
address  pcrel length extern type    scattered symbolnum/value
0000004f True  long   True   BRANCH  False     _foo
00000018 True  long   True   BRANCH  False     _printf
00000011 True  long   False  SIGNED  False     3 (__TEXT,__cstring)
Relocation information (__LD,__compact_unwind) 2 entries
address  pcrel length extern type    scattered symbolnum/value
00000020 False quad   False  UNSIGND False     1 (__TEXT,__text)
00000000 False quad   False  UNSIGND False     1 (__TEXT,__text)
Sections:
Idx Name             Size     VMA              Type
  0 __text           0000005c 0000000000000000 TEXT
  1 __data           00000008 000000000000005c DATA
  2 __cstring        00000004 0000000000000064 DATA
  3 __compact_unwind 00000040 0000000000000068 DATA
  4 __eh_frame       00000068 00000000000000a8 DATA

SYMBOL TABLE:
0000000000000060 l     O __DATA,__data _main.static_int
0000000000000000 g     F __TEXT,__text _foo
000000000000005c g     O __DATA,__data _global_int
0000000000000030 g     F __TEXT,__text _main
0000000000000000         *UND* _printf
Mach header
      magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
MH_MAGIC_64  X86_64        ALL  0x00      OBJECT     4        600 SUBSECTIONS_VIA_SYMBOLS
Load command 0
      cmd LC_SEGMENT_64
  cmdsize 472
  segname 
   vmaddr 0x0000000000000000
   vmsize 0x0000000000000110
  fileoff 632
 filesize 272
  maxprot rwx
 initprot rwx
   nsects 5
    flags (none)
Section
  sectname __text
   segname __TEXT
      addr 0x0000000000000000
      size 0x000000000000005c
    offset 632
     align 2^4 (16)
    reloff 904
    nreloc 3
      type S_REGULAR
attributes PURE_INSTRUCTIONS SOME_INSTRUCTIONS
 reserved1 0
 reserved2 0
Section
  sectname __data
   segname __DATA
      addr 0x000000000000005c
      size 0x0000000000000008
    offset 724
     align 2^2 (4)
    reloff 0
    nreloc 0
      type S_REGULAR
attributes (none)
 reserved1 0
 reserved2 0
Section
  sectname __cstring
   segname __TEXT
      addr 0x0000000000000064
      size 0x0000000000000004
    offset 732
     align 2^0 (1)
    reloff 0
    nreloc 0
      type S_CSTRING_LITERALS
attributes (none)
 reserved1 0
 reserved2 0
Section
  sectname __compact_unwind
   segname __LD
      addr 0x0000000000000068
      size 0x0000000000000040
    offset 736
     align 2^3 (8)
    reloff 928
    nreloc 2
      type S_REGULAR
attributes DEBUG
 reserved1 0
 reserved2 0
Section
  sectname __eh_frame
   segname __TEXT
      addr 0x00000000000000a8
      size 0x0000000000000068
    offset 800
     align 2^3 (8)
    reloff 0
    nreloc 0
      type S_COALESCED
attributes NO_TOC STRIP_STATIC_SYMS LIVE_SUPPORT
 reserved1 0
 reserved2 0
Load command 1
       cmd LC_BUILD_VERSION
   cmdsize 24
  platform macos
       sdk 10.15.6
     minos 10.15
    ntools 0
Load command 2
     cmd LC_SYMTAB
 cmdsize 24
  symoff 944
   nsyms 5
  stroff 1024
 strsize 52
Load command 3
            cmd LC_DYSYMTAB
        cmdsize 80
      ilocalsym 0
      nlocalsym 1
     iextdefsym 1
     nextdefsym 3
      iundefsym 4
      nundefsym 1
         tocoff 0
           ntoc 0
      modtaboff 0
        nmodtab 0
   extrefsymoff 0
    nextrefsyms 0
 indirectsymoff 0
  nindirectsyms 0
      extreloff 0
        nextrel 0
      locreloff 0
        nlocrel 0

查看目标文件size

size 结果 
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$ size hello.o    # linux-arm
   text      data        bss        dec        hex    filename
    104          8          0        112        70    hello.o

$ size hello.o    # macOS-x64
__TEXT    __DATA    __OBJC    others    dec    hex
200    8    0    64    272    110

查看目标文件结构

不同的平台，目标文件结构不同，生成的section也有差异，但是最基本的代码段.text、数据段.data、只读段.rodata / __cstring 都是通用的， bss段为未初始化全局或静态变量，默认都为0，不占据目标文件空间

objdump -h 结果 
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$ objdump -h hello.o   # linux-arm
hello.o:     file format elf32-littlearm
Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         00000064  00000000  00000000  00000034  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .data         00000008  00000000  00000000  00000098  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  2 .bss          00000000  00000000  00000000  000000a0  2**0
                  ALLOC
  3 .rodata       00000004  00000000  00000000  000000a0  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  4 .comment      00000024  00000000  00000000  000000a4  2**0
                  CONTENTS, READONLY
  5 .note.GNU-stack 00000000  00000000  00000000  000000c8  2**0
                  CONTENTS, READONLY
  6 .ARM.attributes 0000002f  00000000  00000000  000000c8  2**0
                  CONTENTS, READONLY

$ objdump -h hello.o     # macOS-x64
hello.o:    file format Mach-O 64-bit x86-64
Sections:
Idx Name             Size     VMA              Type
  0 __text           0000005c 0000000000000000 TEXT
  1 __data           00000008 000000000000005c DATA
  2 __cstring        00000004 0000000000000064 DATA
  3 __compact_unwind 00000040 0000000000000068 DATA
  4 __eh_frame       00000068 00000000000000a8 DATA

查看目标文件内容，并反编译汇编代码

objdump -s -d 结果 
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$ objdump -s -d hello.o   # linux-arm
hello.o:     file format elf32-littlearm

Contents of section .text:
 0000 00482de9 04b08de2 08d04de2 08000be5  .H-.......M.....
 0010 08101be5 0c009fe5 feffffeb 0000a0e1  ................
 0020 04d04be2 0088bde8 00000000 00482de9  ..K..........H-.
 0030 04b08de2 08d04de2 0130a0e3 08300be5  ......M..0...0..
 0040 08201be5 0c301be5 033082e0 0300a0e1  . ...0...0......
 0050 feffffeb 0c301be5 0300a0e1 04d04be2  .....0........K.
 0060 0088bde8                             ....            
Contents of section .data:
 0000 a5000000 5a000000                    ....Z...        
Contents of section .rodata:
 0000 256c640a 00                          %ld..           
Contents of section .comment:
 0000 00474343 3a202852 61737062 69616e20  .GCC: (Raspbian 
 0010 382e332e 302d362b 72706931 2920382e  8.3.0-6+rpi1) 8.
 0020 332e3000                             3.0.            
Contents of section .ARM.attributes:
 0000 412e0000 00616561 62690001 24000000  A....aeabi..$...
 0010 05360006 06080109 010a0212 04140115  .6..............
 0020 01170318 0119011a 021c011e 062201    .............". 

Disassembly of section .text:

00000000 <foo>:
   0:    e92d4800     push    {fp, lr}
   4:    e28db004     add    fp, sp, #4
   8:    e24dd008     sub    sp, sp, #8
   c:    e50b0008     str    r0, [fp, #-8]
  10:    e51b1008     ldr    r1, [fp, #-8]
  14:    e59f000c     ldr    r0, [pc, #12]    ; 28 <foo+0x28>
  18:    ebfffffe     bl    0 <printf>
  1c:    e1a00000     nop            ; (mov r0, r0)
  20:    e24bd004     sub    sp, fp, #4
  24:    e8bd8800     pop    {fp, pc}
  28:    00000000     .word    0x00000000

0000002c <main>:
  2c:    e92d4800     push    {fp, lr}
  30:    e28db004     add    fp, sp, #4
  34:    e24dd008     sub    sp, sp, #8
  38:    e3a03001     mov    r3, #1
  3c:    e50b3008     str    r3, [fp, #-8]
  40:    e51b2008     ldr    r2, [fp, #-8]
  44:    e51b300c     ldr    r3, [fp, #-12]
  48:    e0823003     add    r3, r2, r3
  4c:    e1a00003     mov    r0, r3
  50:    ebfffffe     bl    0 <foo>
  54:    e51b300c     ldr    r3, [fp, #-12]
  58:    e1a00003     mov    r0, r3
  5c:    e24bd004     sub    sp, fp, #4
  60:    e8bd8800     pop    {fp, pc}

$ objdump -s -d hello.o    # macOS-x64
hello.o:    file format Mach-O 64-bit x86-64

Contents of section __text:
 0000 554889e5 4883ec10 897dfc8b 75fc488d  UH..H....}..u.H.
 0010 3d4f0000 00b000e8 00000000 4883c410  =O..........H...
 0020 5dc3662e 0f1f8400 00000000 0f1f4000  ].f...........@.
 0030 554889e5 4883ec10 c745fc00 000000c7  UH..H....E......
 0040 45f80100 00008b45 f80345f4 89c7e800  E......E..E.....
 0050 0000008b 45f44883 c4105dc3           ....E.H...].
Contents of section __data:
 005c a5000000 5a000000                    ....Z...
Contents of section __cstring:
 0064 25640a00                             %d..
Contents of section __compact_unwind:
 0068 00000000 00000000 22000000 00000001  ........".......
 0078 00000000 00000000 00000000 00000000  ................
 0088 30000000 00000000 2c000000 00000001  0.......,.......
 0098 00000000 00000000 00000000 00000000  ................
Contents of section __eh_frame:
 00a8 14000000 00000000 017a5200 01781001  .........zR..x..
 00b8 100c0708 90010000 24000000 1c000000  ........$.......
 00c8 38ffffff ffffffff 22000000 00000000  8.......".......
 00d8 00410e10 8602430d 06000000 00000000  .A....C.........
 00e8 24000000 44000000 40ffffff ffffffff  $...D...@.......
 00f8 2c000000 00000000 00410e10 8602430d  ,........A....C.
 0108 06000000 00000000                    ........

Disassembly of section __TEXT,__text:

0000000000000000 _foo:
       0: 55                               pushq    %rbp
       1: 48 89 e5                         movq    %rsp, %rbp
       4: 48 83 ec 10                      subq    $16, %rsp
       8: 89 7d fc                         movl    %edi, -4(%rbp)
       b: 8b 75 fc                         movl    -4(%rbp), %esi
       e: 48 8d 3d 4f 00 00 00             leaq    79(%rip), %rdi
      15: b0 00                            movb    $0, %al
      17: e8 00 00 00 00                   callq    0 <_foo+0x1c>
      1c: 48 83 c4 10                      addq    $16, %rsp
      20: 5d                               popq    %rbp
      21: c3                               retq
      22: 66 2e 0f 1f 84 00 00 00 00 00    nopw    %cs:(%rax,%rax)
      2c: 0f 1f 40 00                      nopl    (%rax)

0000000000000030 _main:
      30: 55                               pushq    %rbp
      31: 48 89 e5                         movq    %rsp, %rbp
      34: 48 83 ec 10                      subq    $16, %rsp
      38: c7 45 fc 00 00 00 00             movl    $0, -4(%rbp)
      3f: c7 45 f8 01 00 00 00             movl    $1, -8(%rbp)
      46: 8b 45 f8                         movl    -8(%rbp), %eax
      49: 03 45 f4                         addl    -12(%rbp), %eax
      4c: 89 c7                            movl    %eax, %edi
      4e: e8 00 00 00 00                   callq    0 <_main+0x23>
      53: 8b 45 f4                         movl    -12(%rbp), %eax
      56: 48 83 c4 10                      addq    $16, %rsp
      5a: 5d                               popq    %rbp
      5b: c3                               retq

hello.c源码

hello.c 
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#include <stdio.h>

int global_int = 0xa5;

void foo(int i)
{
     printf("%d\n", i);
}

int main(void)
{
     static int static_int = 0x5a;

     int b = 1;
     int c;

     foo(b + c);

     return c;
}