Linux下ELF文件类型分为以下几种：
1、可重定位文件，例如SimpleSection.o；
2、可执行文件，例如/bin/bash；
3、共享目标文件，例如/lib/libc.so。
再接下来的文章中，我们会使用objdump，readelf，hexdump，nm等来分析一个Linux中可重定位文件SimpleSection.o。
首先附上SimpleSection.c源代码：
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 a;
}使用命令:
gcc -c SimpleSection.c
得到SimpleSection.o，下面我们首先附上SimpleSection.o的二进制内容以及整体轮廓。
使用命令：
hexdump -C SimpleSection.o，得到SimpleSection.o的二进制内容。
计算机科学中，二进制0 1可以代表代码，字母，数字（十进制数和十六进制数）。
000000007f 45 4c 46 02 01 01 0000 00 00 00 00 00 00 00|.ELF............|
0000001001 00 3e 00 01 00 00 0000 00 00 00 00 00 00 00|..>.............|
0000002000 00 00 00 00 00 00 0088 01 00 00 00 00 00 00|................|
0000003000 00 00 00 40 00 00 0000 00 40 00 0d 00 0a 00|....@.....@.....|
0000004055 48 89 e5 48 83 ec 1089 7d fc 8b 45 fc 89 c6|UH..H....}..E...|
00000050bf 00 00 00 00 b8 00 0000 00 e8 00 00 00 00 c9|................|
00000060c3 55 48 89 e5 48 83 ec10 c7 45 f8 01 00 00 00|.UH..H....E.....|
000000708b 15 00 00 00 00 8b 0500 00 00 00 01 d0 03 45|...............E|
00000080f8 03 45 fc 89 c7 e8 0000 00 00 8b 45 f8 c9 c3|..E.........E...|
0000009054 00 00 00 55 00 00 0025 64 0a 00 00 47 43 43|T...U...%d...GCC|
000000a03a 20 28 55 62 75 6e 7475 2f 4c 69 6e 61 72 6f|: (Ubuntu/Linaro|
000000b020 34 2e 36 2e 33 2d 3175 62 75 6e 74 75 35 29| 4.6.3-1ubuntu5)|
000000c020 34 2e 36 2e 33 00 0014 00 00 00 00 00 00 00| 4.6.3..........|
000000d001 7a 52 00 01 78 10 011b 0c 07 08 90 01 00 00|.zR..x..........|
000000e01c 00 00 00 1c 00 00 0000 00 00 00 21 00 00 00|............!...|
000000f000 41 0e 10 86 02 43 0d06 5c 0c 07 08 00 00 00|.A....C..\......|
000001001c 00 00 00 3c 00 00 0000 00 00 00 2f 00 00 00|....<......./...|
0000011000 41 0e 10 86 02 43 0d06 6a 0c 07 08 00 00 00|.A....C..j......|
0000012000 2e 73 79 6d 74 61 6200 2e 73 74 72 74 61 62|..symtab..strtab|
0000013000 2e 73 68 73 74 72 7461 62 00 2e 72 65 6c 61|..shstrtab..rela|
000001402e 74 65 78 74 00 2e 6461 74 61 00 2e 62 73 73|.text..data..bss|
0000015000 2e 72 6f 64 61 74 6100 2e 63 6f 6d 6d 65 6e|..rodata..commen|
0000016074 00 2e 6e 6f 74 65 2e47 4e 55 2d 73 74 61 63|t..note.GNU-stac|
000001706b 00 2e 72 65 6c 61 2e65 68 5f 66 72 61 6d 65|k..rela.eh_frame|
0000018000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
*
000001c000 00 00 00 00 00 00 0020 00 00 00 01 00 00 00|........ .......|
000001d006 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
000001e040 00 00 00 00 00 00 0050 00 00 00 00 00 00 00|@.......P.......|
000001f000 00 00 00 00 00 00 0004 00 00 00 00 00 00 00|................|
0000020000 00 00 00 00 00 00 001b 00 00 00 04 00 00 00|................|
0000021000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
00000220b0 06 00 00 00 00 00 0078 00 00 00 00 00 00 00|........x.......|
000002300b 00 00 00 01 00 00 0008 00 00 00 00 00 00 00|................|
0000024018 00 00 00 00 00 00 0026 00 00 00 01 00 00 00|........&.......|
0000025003 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
0000026090 00 00 00 00 00 00 0008 00 00 00 00 00 00 00|................|
0000027000 00 00 00 00 00 00 0004 00 00 00 00 00 00 00|................|
0000028000 00 00 00 00 00 00 002c 00 00 00 08 00 00 00|........,.......|
0000029003 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
000002a098 00 00 00 00 00 00 0004 00 00 00 00 00 00 00|................|
000002b000 00 00 00 00 00 00 0004 00 00 00 00 00 00 00|................|
000002c000 00 00 00 00 00 00 0031 00 00 00 01 00 00 00|........1.......|
000002d002 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
000002e098 00 00 00 00 00 00 0004 00 00 00 00 00 00 00|................|
000002f000 00 00 00 00 00 00 0001 00 00 00 00 00 00 00|................|
0000030000 00 00 00 00 00 00 0039 00 00 00 01 00 00 00|........9.......|
0000031030 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|0...............|
000003209c 00 00 00 00 00 00 002b 00 00 00 00 00 00 00|........+.......|
0000033000 00 00 00 00 00 00 0001 00 00 00 00 00 00 00|................|
0000034001 00 00 00 00 00 00 0042 00 00 00 01 00 00 00|........B.......|
0000035000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
00000360c7 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
0000037000 00 00 00 00 00 00 0001 00 00 00 00 00 00 00|................|
0000038000 00 00 00 00 00 00 0057 00 00 00 01 00 00 00|........W.......|
0000039002 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
000003a0c8 00 00 00 00 00 00 0058 00 00 00 00 00 00 00|........X.......|
000003b000 00 00 00 00 00 00 0008 00 00 00 00 00 00 00|................|
000003c000 00 00 00 00 00 00 0052 00 00 00 04 00 00 00|........R.......|
000003d000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
000003e028 07 00 00 00 00 00 0030 00 00 00 00 00 00 00|(.......0.......|
000003f00b 00 00 00 08 00 00 0008 00 00 00 00 00 00 00|................|
0000040018 00 00 00 00 00 00 0011 00 00 00 03 00 00 00|................|
0000041000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
0000042020 01 00 00 00 00 00 0061 00 00 00 00 00 00 00| .......a.......|
0000043000 00 00 00 00 00 00 0001 00 00 00 00 00 00 00|................|
0000044000 00 00 00 00 00 00 0001 00 00 00 02 00 00 00|................|
0000045000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
00000460c8 04 00 00 00 00 00 0080 01 00 00 00 00 00 00|................|
000004700c 00 00 00 0b 00 00 0008 00 00 00 00 00 00 00|................|
0000048018 00 00 00 00 00 00 0009 00 00 00 03 00 00 00|................|
0000049000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
000004a048 06 00 00 00 00 00 0066 00 00 00 00 00 00 00|H.......f.......|
000004b000 00 00 00 00 00 00 0001 00 00 00 00 00 00 00|................|
000004c000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
*
000004e001 00 00 00 04 00 f1 ff00 00 00 00 00 00 00 00|................|
000004f000 00 00 00 00 00 00 0000 00 00 00 03 00 01 00|................|
0000050000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
0000051000 00 00 00 03 00 03 0000 00 00 00 00 00 00 00|................|
0000052000 00 00 00 00 00 00 0000 00 00 00 03 00 04 00|................|
0000053000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
0000054000 00 00 00 03 00 05 0000 00 00 00 00 00 00 00|................|
0000055000 00 00 00 00 00 00 0011 00 00 00 01 00 03 00|................|
0000056004 00 00 00 00 00 00 0004 00 00 00 00 00 00 00|................|
0000057021 00 00 00 01 00 04 0000 00 00 00 00 00 00 00|!...............|
0000058004 00 00 00 00 00 00 0000 00 00 00 03 00 07 00|................|
0000059000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
000005a000 00 00 00 03 00 08 0000 00 00 00 00 00 00 00|................|
000005b000 00 00 00 00 00 00 0000 00 00 00 03 00 06 00|................|
000005c000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
000005d032 00 00 00 11 00 03 0000 00 00 00 00 00 00 00|2...............|
000005e004 00 00 00 00 00 00 0042 00 00 00 11 00 f2 ff|........B.......|
000005f004 00 00 00 00 00 00 0004 00 00 00 00 00 00 00|................|
0000060054 00 00 00 12 00 01 0000 00 00 00 00 00 00 00|T...............|
0000061021 00 00 00 00 00 00 005a 00 00 00 10 00 00 00|!.......Z.......|
0000062000 00 00 00 00 00 00 0000 00 00 00 00 00 00 00|................|
0000063061 00 00 00 12 00 01 0021 00 00 00 00 00 00 00|a.......!.......|
000006402f 00 00 00 00 00 00 0000 53 69 6d 70 6c 65 53|/........SimpleS|
0000065065 63 74 69 6f 6e 2e 6300 73 74 61 74 69 63 5f|ection.c.static_|
0000066076 61 72 2e 31 35 39 3400 73 74 61 74 69 63 5f|var.1594.static_|
0000067076 61 72 32 2e 31 35 3935 00 67 6c 6f 62 61 6c|var2.1595.global|
000006805f 69 6e 69 74 5f 76 6172 00 67 6c 6f 62 61 6c|_init_var.global|
000006905f 75 6e 69 6e 69 74 5f76 61 72 00 66 75 6e 63|_uninit_var.func|
000006a031 00 70 72 69 6e 74 6600 6d 61 69 6e 00 00 00|1.printf.main...|
000006b011 00 00 00 00 00 00 000a 00 00 00 05 00 00 00|................|
000006c000 00 00 00 00 00 00 001b 00 00 00 00 00 00 00|................|
000006d002 00 00 00 0e 00 00 00fc ff ff ff ff ff ff ff|................|
000006e032 00 00 00 00 00 00 0002 00 00 00 03 00 00 00|2...............|
000006f000 00 00 00 00 00 00 0038 00 00 00 00 00 00 00|........8.......|
0000070002 00 00 00 04 00 00 00fc ff ff ff ff ff ff ff|................|
0000071047 00 00 00 00 00 00 0002 00 00 00 0d 00 00 00|G...............|
00000720fc ff ff ff ff ff ff ff20 00 00 00 00 00 00 00|........ .......|
0000073002 00 00 00 02 00 00 0000 00 00 00 00 00 00 00|................|
0000074040 00 00 00 00 00 00 0002 00 00 00 02 00 00 00|@...............|
0000075021 00 00 00 00 00 00 00|!.......|图1
使用命令ls -l SimpleSection.o，可以得到文件大小为1880字节，上面二进制内容正好也是1880个字节（0x758转换为十机制为1880）。
SimpleSection.o的整体轮廓图如下，可能读者会想为什么会得到这样一张图，随着我们深入分析每个段的内容，答案自然会揭晓。
文章图片
图 2
我们看到0x758是所有段结束的位置，换算成十进制就是1880个字节。和我们刚才获取的文件大小一样。
下面我们来利用命令来分析ELF文件结构的每个部分：
1、ELF Header
使用命令readelf -h SimpleSection.o，得到下图。
文章图片
图 3
ELF文件头结构及相关参数被定义在“/usr/include/elf.h”中，如下：
typedef struct
{
unsigned char e_ident[EI_NIDENT];
/* Magic number and other info */
Elf32_Halfe_type;
/* Object file type */
Elf32_Halfe_machine;
/* Architecture */
Elf32_Worde_version;
/* Object file version */
Elf32_Addre_entry;
/* Entry point virtual address */
Elf32_Offe_phoff;
/* Program header table file offset */
Elf32_Offe_shoff;
/* Section header table file offset */
Elf32_Worde_flags;
/* Processor-specific flags */
Elf32_Halfe_ehsize;
/* ELF header size in bytes */
Elf32_Halfe_phentsize;
/* Program header table entry size */
Elf32_Halfe_phnum;
/* Program header table entry count */
Elf32_Halfe_shentsize;
/* Section header table entry size */
Elf32_Halfe_shnum;
/* Section header table entry count */
Elf32_Halfe_shstrndx;
/* Section header string table index */
} Elf32_Ehdr;
Type：ELF文件类型，本例中为REL（Relocatable File），可重定位文件。
Start of section headers，段表在文件中偏移，就是图2中Section Table的位置为392（0x188）。
Size of section headers，ELF文件头的大小为64个字节。
Number of section headers，ELF拥有多少个断，本例为13个段。见图 7。
Section header string table index，段表字符串表所在的段在段表中的下标。本例中等于10，见图 7。
2、.text
使用命令：
objdump -d SimpleSection.o，得到了下图，由于是代码段，所以二进制代表汇编代码。
文章图片
图 4
3、.data
使用命令objdump -s SimpleSection.o，得到数据段，如下图：
文章图片
图 5
本例中存入数据段的是
int global_init_var = 84;
static int static_var = 85;
共8个字节，一个是0x00000054，十进制是84；一个是0x0000000056，十进制是85。
4、.bss
使用命令objdump -h SimpleSection.o，得到下图：
文章图片
图 6
本例中存入数据段的是
static int static_var2;
大家会注意到int global_uninit_var; 既没有在.data段中，也没有在.bss段中。如果在前面加上static，那么则存在.bss段中。
5、.rodata
.rodata存放的只读数据。25640a00，查看ASCII表代表的就是%d\n。
6、.shstrtab（段表字符串表）
如图1，存放的是
..symtab..strtab..shstrtab..rela.text..data..bs..rodata..comment..note.GNU-stak..rela.eh_frame
7、.strtab（字符串表）
如图1，存放的是
SimpleSection.c.static_var.1594.static_var2.1595.global_init_var.global_uninit_var.func1.printf.main
8、Section Table
使用命令，readelf -S SimpleSection.o，得到下图：
文章图片
图 7
这就解释了图1，为什么要那么画。
Offset表示段偏移，Size表示段大小。
Type，PROGBITS表示段，NOBITS表示不占空间，.RELA表示重定位段，STRTAB表示字符串表，SYMTAB表示符号表。
EntSize表示如果段中有重复的内容，则表示重复内容大小。比如下面要介绍的符号表就是重复内容组成的。
在TYPE为RELA时，Link表示该段所使用的相应符号表在段表中，本例中为11。Info表示该重定位表所作用的段在段表中的下标。.rela.text为1，.rela.eh_frame为8。
9、.symtab（符号表）
使用命令，readelf -s SimpleSection.o，得到下图：
文章图片
图 8
Name，表示字符串在字符串表中的下标；
Ndx，SimpleSection.c为ABS，global_uninit_var为COM，表示这个变量是强引用或者弱引用，目前即不在.data段中，也不在.bss段中，等待链接时会确定。
printf为UND，表示没有定义，即引用了外部的函数。
global_init_var，NDX为3，表示在.data中，其余类似。表示符号所在的段在段表中的下标；参考图 7。
Bind GLOBAL表示可以被外部引用或者引用外部的函数和变量。
TYPE为OBJECT表示对象，FUNC表示函数，SECTION表示段，FILE表示文件，printf为NOTYPE，表示没有定义，是引用外部函数。
SIZE表示大小。
Value表示在本段中的偏移，比如static_var.1594表示在.data段中的偏移为4。 main在.data段中的偏移为21。
最后介绍个命令，nm SimpleSection.o，结果如下：
文章图片
可以看出示所有可以被外部引用或者引用外部的函数和变量。
T表示text；D和d表示data；b表示bss，C表示Common，U表示Undef。
程序中的非静态局部变量即不在数据段也不在代码段，可能在堆栈段。
【Linux 可重定位文件 ELF结构】至此，所有段都分析完了。本文参考程序员的自我修养。
