引言

本文是对程序员的自我修养：链接、装载与库中第3章的实践总结(和结构相关的示意图都是用Gliffy Diagrams画的🤓)，通过使用工具readelf、objdump对目标文件进行解析，学习目标文件的结构。

1. 目标文件

1.1 目标文件的定义

编译器编译源代码后生成的文件叫做目标文件。在Linux下，使用gcc -c xxxx.c编译生成.o文件。

gcc -c xxxx.c编译生成目标文件

1.2 编译过程回顾

编译过程

目标文件的文件类型为ELF，在Linux下对应文件后缀为.o的文件，Window下对应文件后缀为.obj的文件。使用file命令可以查看到.o和.obj文件均为ELF类型。

ckt@ubuntu:~/work/elf$ file simple.o
simple.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

ckt@ubuntu:~/work/elf$ file mp4_player.obj 
mp4_player.obj: ELF 32-bit LSB relocatable, ARM, version 1 (SYSV), not stripped

目标文件只是ELF文件的可重定位文件(Relocatable file)，ELF文件一共有4种类型：Relocatable file、Executable file、Shared object file和Core Dump file

ELF文件类型

示例

在Linux下，使用命令 gcc -c xxxx.c就可以编译生成.o文件

ckt@ubuntu:~/work/elf$ gcc -c simple.c 
ckt@ubuntu:~/work/elf$ ls
simple.c  simple.o

在 simple.c中，我们只加入了下面这一个函数fun，函数内容为空

void fun()
{

}

使用UltraEdit将simple.o打开，里面的内容有机器指令代码、数据等，我们的程序就是由这些字节组成的。对于程序员来说，使用高级语言(C/C++，Java等)实现的代码是最容易阅读和理解的，但是对于计算机来说，它只懂得机器语言，它更喜欢二进制，将0转换为低电平，1转换成高电平，这样一个程序就可以跑起来了。

我们可以使用工具readelf 和objdump对目标文件simple.o进行分析。为了加深对目标文件的理解，在使用readelf & objdump进行前，需要先要了解ELF文件的结构。

00000000h: 7F 45 4C 46 02 01 01 00 00 00 00 00 00 00 00 00 ; �ELF............
00000010h: 01 00 3E 00 01 00 00 00 00 00 00 00 00 00 00 00 ; ..>.............
00000020h: 00 00 00 00 00 00 00 00 08 01 00 00 00 00 00 00 ; ................
00000030h: 00 00 00 00 40 00 00 00 00 00 40 00 0B 00 08 00 ; ....@.....@.....
00000040h: 55 48 89 E5 5D C3 00 00 00 47 43 43 3A 20 28 55 ; UH夊]?..GCC: (U
00000050h: 62 75 6E 74 75 2F 4C 69 6E 61 72 6F 20 34 2E 36 ; buntu/Linaro 4.6
00000060h: 2E 33 2D 31 75 62 75 6E 74 75 35 29 20 34 2E 36 ; .3-1ubuntu5) 4.6
00000070h: 2E 33 00 00 00 00 00 00 14 00 00 00 00 00 00 00 ; .3..............
00000080h: 01 7A 52 00 01 78 10 01 1B 0C 07 08 90 01 00 00 ; .zR..x......?..
00000090h: 1C 00 00 00 1C 00 00 00 00 00 00 00 06 00 00 00 ; ................
000000a0h: 00 41 0E 10 86 02 43 0D 06 41 0C 07 08 00 00 00 ; .A..?C..A......
000000b0h: 00 2E 73 79 6D 74 61 62 00 2E 73 74 72 74 61 62 ; ..symtab..strtab
000000c0h: 00 2E 73 68 73 74 72 74 61 62 00 2E 74 65 78 74 ; ..shstrtab..text
000000d0h: 00 2E 64 61 74 61 00 2E 62 73 73 00 2E 63 6F 6D ; ..data..bss..com
000000e0h: 6D 65 6E 74 00 2E 6E 6F 74 65 2E 47 4E 55 2D 73 ; ment..note.GNU-s
000000f0h: 74 61 63 6B 00 2E 72 65 6C 61 2E 65 68 5F 66 72 ; tack..rela.eh_fr
00000100h: 61 6D 65 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ame.............
00000110h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
00000120h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
00000130h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
00000140h: 00 00 00 00 00 00 00 00 1B 00 00 00 01 00 00 00 ; ................
00000150h: 06 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
00000160h: 40 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 ; @...............
00000170h: 00 00 00 00 00 00 00 00 04 00 00 00 00 00 00 00 ; ................
00000180h: 00 00 00 00 00 00 00 00 21 00 00 00 01 00 00 00 ; ........!.......
00000190h: 03 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
000001a0h: 48 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; H...............
000001b0h: 00 00 00 00 00 00 00 00 04 00 00 00 00 00 00 00 ; ................
000001c0h: 00 00 00 00 00 00 00 00 27 00 00 00 08 00 00 00 ; ........'.......
000001d0h: 03 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
000001e0h: 48 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; H...............
000001f0h: 00 00 00 00 00 00 00 00 04 00 00 00 00 00 00 00 ; ................
00000200h: 00 00 00 00 00 00 00 00 2C 00 00 00 01 00 00 00 ; ........,.......
00000210h: 30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; 0...............
00000220h: 48 00 00 00 00 00 00 00 2B 00 00 00 00 00 00 00 ; H.......+.......
00000230h: 00 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 ; ................
00000240h: 01 00 00 00 00 00 00 00 35 00 00 00 01 00 00 00 ; ........5.......
00000250h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
00000260h: 73 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; s...............
00000270h: 00 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 ; ................
00000280h: 00 00 00 00 00 00 00 00 4A 00 00 00 01 00 00 00 ; ........J.......
00000290h: 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
000002a0h: 78 00 00 00 00 00 00 00 38 00 00 00 00 00 00 00 ; x.......8.......
000002b0h: 00 00 00 00 00 00 00 00 08 00 00 00 00 00 00 00 ; ................
000002c0h: 00 00 00 00 00 00 00 00 45 00 00 00 04 00 00 00 ; ........E.......
000002d0h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
000002e0h: B0 04 00 00 00 00 00 00 18 00 00 00 00 00 00 00 ; ?..............
000002f0h: 09 00 00 00 06 00 00 00 08 00 00 00 00 00 00 00 ; ................
00000300h: 18 00 00 00 00 00 00 00 11 00 00 00 03 00 00 00 ; ................
00000310h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
00000320h: B0 00 00 00 00 00 00 00 54 00 00 00 00 00 00 00 ; ?......T.......
00000330h: 00 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 ; ................
00000340h: 00 00 00 00 00 00 00 00 01 00 00 00 02 00 00 00 ; ................
00000350h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
00000360h: C8 03 00 00 00 00 00 00 D8 00 00 00 00 00 00 00 ; ?......?......
00000370h: 0A 00 00 00 08 00 00 00 08 00 00 00 00 00 00 00 ; ................
00000380h: 18 00 00 00 00 00 00 00 09 00 00 00 03 00 00 00 ; ................
00000390h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
000003a0h: A0 04 00 00 00 00 00 00 0E 00 00 00 00 00 00 00 ; ?..............
000003b0h: 00 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 ; ................
000003c0h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
000003d0h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
000003e0h: 01 00 00 00 04 00 F1 FF 00 00 00 00 00 00 00 00 ; ......?........
000003f0h: 00 00 00 00 00 00 00 00 00 00 00 00 03 00 01 00 ; ................
00000400h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
00000410h: 00 00 00 00 03 00 02 00 00 00 00 00 00 00 00 00 ; ................
00000420h: 00 00 00 00 00 00 00 00 00 00 00 00 03 00 03 00 ; ................
00000430h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
00000440h: 00 00 00 00 03 00 05 00 00 00 00 00 00 00 00 00 ; ................
00000450h: 00 00 00 00 00 00 00 00 00 00 00 00 03 00 06 00 ; ................
00000460h: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; ................
00000470h: 00 00 00 00 03 00 04 00 00 00 00 00 00 00 00 00 ; ................
00000480h: 00 00 00 00 00 00 00 00 0A 00 00 00 12 00 01 00 ; ................
00000490h: 00 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 ; ................
000004a0h: 00 73 69 6D 70 6C 65 2E 63 00 66 75 6E 00 00 00 ; .simple.c.fun...
000004b0h: 20 00 00 00 00 00 00 00 02 00 00 00 02 00 00 00 ;  ...............
000004c0h: 00 00 00 00 00 00 00 00                         ; ........

ELF文件结构

和class文件类似，ELF文件存放数据的格式也是固定的，计算机在解析目标文件时，就是按照它每个字段的数据结构进行逐字解析的。ELF文件结构信息定义在/usr/include/elf.h中，整个ELF文件的结构如下图：

ELF文件的结构

ELF Header

ELF Header是ELF文件的第一部分，64 bit的ELF文件头的结构体如下：

typedef struct
{
  unsigned char e_ident[EI_NIDENT]; /* Magic number and other info */
  Elf64_Half    e_type;         /* Object file type */
  Elf64_Half    e_machine;      /* Architecture */
  Elf64_Word    e_version;      /* Object file version */
  Elf64_Addr    e_entry;        /* Entry point virtual address */
  Elf64_Off e_phoff;        /* Program header table file offset */
  Elf64_Off e_shoff;        /* Section header table file offset */
  Elf64_Word    e_flags;        /* Processor-specific flags */
  Elf64_Half    e_ehsize;       /* ELF header size in bytes */
  Elf64_Half    e_phentsize;        /* Program header table entry size */
  Elf64_Half    e_phnum;        /* Program header table entry count */
  Elf64_Half    e_shentsize;        /* Section header table entry size */
  Elf64_Half    e_shnum;        /* Section header table entry count */
  Elf64_Half    e_shstrndx;     /* Section header string table index */
} Elf64_Ehdr;

接下来我们会使用到第一个分析目标文件的工具readelf，通过man readelf命令，我们可以查到readelf的作用就是用来显示ELF文件的信息

DESCRIPTION
   readelf displays information about one or more ELF format object files.

使用readelf -h simple.o来进行对Header的解析，通过man readelf命令同样可以查询到对-h参数的说明，
-h用来显示ELF header的相关信息。

OPTIONS
   -h
   --file-header
       Displays the information contained in the ELF header at the start of the file.

Header中主要存放的是一些基本信息，通过Header中的信息，我们可以确定后面其他字段的大小和起始地址，通常比较关心的部分是：ELF文件类型、是32bit还是64bit、Header部分大小、Section部分大小和拥有Section的个数等。

结合Elf64_Ehdr来看，对应解析结果如下：

readelf -h simple.o

Section

完成了对Header的解析，再接着分析Section部分，Section对应结构体如下：

typedef struct
{
  Elf64_Word  sh_name;    /* Section name (string tbl index) */
  Elf64_Word  sh_type;    /* Section type */
  Elf64_Xword sh_flags;   /* Section flags */
  Elf64_Addr  sh_addr;    /* Section virtual addr at execution */
  Elf64_Off sh_offset;    /* Section file offset */
  Elf64_Xword sh_size;    /* Section size in bytes */
  Elf64_Word  sh_link;    /* Link to another section */
  Elf64_Word  sh_info;    /* Additional section information */
  Elf64_Xword sh_addralign;   /* Section alignment */
  Elf64_Xword sh_entsize;   /* Entry size if section holds table */
} Elf64_Shdr;

Section部分主要存放的是机器指令代码和数据，执行命令readelf -S -W simple.o对Section部分的解析，解析结果和Elf64_Shdr也是一一对应的。

ckt@ubuntu:~/work/elf$ readelf -S -W simple.o
There are 11 section headers, starting at offset 0x108:

Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
  [ 1] .text             PROGBITS        0000000000000000 000040 000006 00  AX  0   0  4
  [ 2] .data             PROGBITS        0000000000000000 000048 000000 00  WA  0   0  4
  [ 3] .bss              NOBITS          0000000000000000 000048 000000 00  WA  0   0  4
  [ 4] .comment          PROGBITS        0000000000000000 000048 00002b 01  MS  0   0  1
  [ 5] .note.GNU-stack   PROGBITS        0000000000000000 000073 000000 00      0   0  1
  [ 6] .eh_frame         PROGBITS        0000000000000000 000078 000038 00   A  0   0  8
  [ 7] .rela.eh_frame    RELA            0000000000000000 0004b0 000018 18      9   6  8
  [ 8] .shstrtab         STRTAB          0000000000000000 0000b0 000054 00      0   0  1
  [ 9] .symtab           SYMTAB          0000000000000000 0003c8 0000d8 18     10   8  8
  [10] .strtab           STRTAB          0000000000000000 0004a0 00000e 00      0   0  1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
  I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

对于这部分内容，通常我们比较的Section是.text（存放代码）、.data（存放全局静态变量和局部静态变量）和.bss（存未初始化的全局变量和局部静态变量） ，在后面会对这几个段分别分进行解析。

根据readelf -S -W simple.o的输出结果，我们可以算出整个simple.o的组成部分和起始地址，使用ls -l 命令查看simple.o的大小，和simple.o结束地址0x0000048c是吻合的。

ckt@ubuntu:~/work/elf$ ls -l simple.o
-rw-rw-r-- 1 ckt ckt 1224 Apr 12 18:42 simple.o

simple.o组成

解析目标文件

分析完ELF文件结构，接着来解析一个目标文件。首先，准备好源码SimpleSection.c，执行命令gcc -c SimpleSection.c生成目标文件SimpleSection.o。

int printf(const char* format, ...);

int global_init_var = 84;
int global_uninit_var;

void func1(int i)
{
    printf("%d\n", i);
}

int main(void)
{
    static int static_var = 85;
    static int static_var2;

    int a = 1;
    int b;
    func1(static_var + static_var2 + a + b);
    return 0;
}

在这部分，我们会使用另外一个命令objdump，使用man objdump查看该命令，objdump是用来显示目标文件相关信息的。

DESCRIPTION
   objdump displays information about one or more object files.

查看目标文件的Section

执行命令objdump -h SimpleSection.o对Section部分进行解析，我们可以得到每个段的大小

ckt@ubuntu:~/work/elf$ objdump -h SimpleSection.o

SimpleSection.o:     file format elf64-x86-64

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         00000052  0000000000000000  0000000000000000  00000040  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .data         00000008  0000000000000000  0000000000000000  00000094  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  2 .bss          00000004  0000000000000000  0000000000000000  0000009c  2**2
                  ALLOC
  3 .rodata       00000004  0000000000000000  0000000000000000  0000009c  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  4 .comment      0000002b  0000000000000000  0000000000000000  000000a0  2**0
                  CONTENTS, READONLY
  5 .note.GNU-stack 00000000  0000000000000000  0000000000000000  000000cb  2**0
                  CONTENTS, READONLY
  6 .eh_frame     00000058  0000000000000000  0000000000000000  000000d0  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA

我们的代码是存放到.text中，已初始化全局变量和局部静态变量存放在.data中，未初始化全局变量和局部静态变量存放在.bss中

程序与目标文件对应关系

代码段

执行命令objdump -s -d SimpleSection.o对代码段(.text)的解析结果如下：

.text

数据段和只读数据段

执行命令objdump -s -d SimpleSection.o对数据段和只读数据段解析结果如下：

.data & .rodata

BSS段

执行命令objdump -x -s -d SimpleSection.o打印出目标文件的符号表，通过符号表我们可以知道各个变量的存放位置，只有未初始化的局部静态变量static_var2被放到了.bss段，而global_uninit_var被放入了comment段

.bss

另外，被初始化为0的静态变量也会被放入.bss段，因为未初始变量的值也是0，经过优化后被放入.bss段，这样可以节省磁盘空间，因为.bss不占磁盘空间

例如，下面的代码中x1会被放入.bss段，而x2被放入.data段

static int x1 = 0;
static int x2 = 12;

初始值为0的静态变量会被放进.bss

参考

posted on 2018-05-27 13:16 江召伟阅读(877) 评论(0) 编辑收藏举报

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qiuri2008

公告

引言

1. 目标文件

1.1 目标文件的定义

1.2 编译过程回顾

示例

ELF文件结构

ELF Header

Section

解析目标文件

查看目标文件的Section

代码段

数据段和只读数据段

BSS段

参考