linux下的ELF对象格式
1.ELF概念
ELF(Executable and Linking Format)是unix-like系统下的一种文件格式，它是一种对象文件的格式，用于定义不同类型的对象文件(Object files)中都放了什么东西、以及都以什么样的格式去放这些东西。它自最早在 System V 系统上出现后，被unix-like世界所广泛接受，作为缺省的二进制文件格式来使用。可以说，ELF是构成众多unix-like系统的基础之一。
在unix-like平台下，二进制对象文件格式从a.out格式演变到COFF格式，再到现在的ELF格式。
备注，在windows平台上，动态链接库文件所使用的是称为 PE（Portable Executable）的文件格式，PE主要在 COFF 文件头部之上增加了一些专用头部信息演变而来的。
2.在unix-like平台上使用ELF格式的文件有如下类型
（1）可重定位的对象文件(Relocatable file)
其实就是由编译器编译生成目标对象文件（.o 文件）。链接器(link editor)将会拿一个或一些 Relocatable object files 作为输入，经链接处理后，生成一个可执行的对象文件 (Executable file) 或者一个可被共享的对象文件(Shared object file)。
（2）可执行的对象文件(Executable file)
就是二进制可执行文件，比如ping，vim等二进制文件。
（3）可被共享的对象文件(Shared object file)
就是unix-like平台下的动态库文件，比如.so文件。
（4）core dump核心转储文件
3.ELF文件的格式
ELF文件由几个重要的部分组成，分别是ELF头（ELF header）、程序头表（program header table）和节头表（Section header table）等。实际上，一个文件中不一定包含全部内容，而且他们的位置也没有固定顺序，只有ELF头的位置是固定的，其余各部分的位置、大小等信息都由ELF头中的各项值来决定。
（1）ELF头（ELF header）
使用readelf -h filename可以查看一个ELF文件的ELF头，如下：
ELF Header: Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 Class: ELF64 Data: 2's complement, little endian Version: 1 (current) OS/ABI: UNIX - System V ABI Version: 0 Type: EXEC (Executable file) Machine: Advanced Micro Devices X86-64 Version: 0x1 Entry point address: 0x42dfe0 Start of program headers: 64 (bytes into file) Start of section headers: 67460488 (bytes into file) Flags: 0x0 Size of this header: 64 (bytes) Size of program headers: 56 (bytes) Number of program headers: 8 Size of section headers: 64 (bytes) Number of section headers: 40 Section header string table index: 37
对应的数据结构是：
typedef struct
{
unsigned char e_ident[EI_NIDENT]; /* 魔数和相关信息 */
Elf32_Half e_type; /* 目标文件类型 */
Elf32_Half e_machine; /* 硬件体系 */
Elf32_Word e_version; /* 目标文件版本 */
Elf32_Addr e_entry; /* 程序进入点 */
Elf32_Off e_phoff; /* 程序头部偏移量 */
Elf32_Off e_shoff; /* 节头部偏移量 */
Elf32_Word e_flags; /* 处理器特定标志 */
Elf32_Half e_ehsize; /* ELF头部长度 */
Elf32_Half e_phentsize; /* 程序头部中一个条目的长度 */
Elf32_Half e_phnum; /* 程序头部条目个数 */
Elf32_Half e_shentsize; /* 节头部中一个条目的长度 */
Elf32_Half e_shnum; /* 节头部条目个数 */
Elf32_Half e_shstrndx; /* 节头部字符表索引 */
} Elf32_Ehdr;
（2） 节头表（Section header table），自称为“节表”
sections 是在ELF文件里头，用以装载内容数据的最小容器。在ELF文件里面，每一个 sections 内都装载了性质属性都不一样的内容，比方：
a).text section 里装载了可执行代码；
b).data section 里面装载了被初始化的数据；
c).bss section 里面装载了未被初始化的数据；
d).以.rec 打头的 sections 里面装载了重定位条目；
e).以.symtab 或者 .dynsym section 里面装载了符号信息；
f).以.strtab 或者 .dynstr section 里面装载了字符串信息；
g).其他还有为满足不同目的所设置的section，比方满足调试的目的、满足动态链接与加载的目的等等。
使用readelf -S filename可以查看一个ELF文件的ELF头，如下：
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .interp PROGBITS 0000000000400200 00000200
000000000000001c 0000000000000000 A 0 0 1
[ 2] .note.ABI-tag NOTE 000000000040021c 0000021c
0000000000000020 0000000000000000 A 0 0 4
[ 3] .hash HASH 0000000000400240 00000240
0000000000002db4 0000000000000004 A 4 0 8
[ 4] .dynsym DYNSYM 0000000000402ff8 00002ff8
000000000000b160 0000000000000018 A 5 1 8
[ 5] .dynstr STRTAB 000000000040e158 0000e158
00000000000121e5 0000000000000000 A 0 0 1
[ 6] .gnu.version VERSYM 000000000042033e 0002033e
0000000000000ec8 0000000000000002 A 4 0 2
[ 7] .gnu.version_r VERNEED 0000000000421208 00021208
0000000000000120 0000000000000000 A 5 7 8
[ 8] .rela.dyn RELA 0000000000421328 00021328
0000000000008c58 0000000000000018 A 4 0 8
[ 9] .rela.plt RELA 0000000000429f80 00029f80
0000000000002688 0000000000000018 A 4 11 8
[10] .init PROGBITS 000000000042c608 0002c608
0000000000000018 0000000000000000 AX 0 0 4
[11] .plt PROGBITS 000000000042c620 0002c620
00000000000019c0 0000000000000010 AX 0 0 4
[12] .text PROGBITS 000000000042dfe0 0002dfe0
000000000043d958 0000000000000000 AX 0 0 16
[13] .fini PROGBITS 000000000086b938 0046b938
000000000000000e 0000000000000000 AX 0 0 4
[14] .rodata PROGBITS 000000000086b960 0046b960
000000000013abb5 0000000000000000 A 0 0 32
[15] .eh_frame_hdr PROGBITS 00000000009a6518 005a6518
0000000000027804 0000000000000000 A 0 0 4
[16] .eh_frame PROGBITS 00000000009cdd20 005cdd20
00000000000a5f84 0000000000000000 A 0 0 8
[17] .gcc_except_table PROGBITS 0000000000a73ca8 00673ca8
0000000000056639 0000000000000000 A 0 0 8
[18] .ctors PROGBITS 0000000000bca2e8 006ca2e8
00000000000004f8 0000000000000000 WA 0 0 8
[19] .dtors PROGBITS 0000000000bca7e0 006ca7e0
0000000000000010 0000000000000000 WA 0 0 8
[20] .jcr PROGBITS 0000000000bca7f0 006ca7f0
0000000000000008 0000000000000000 WA 0 0 8
[21] .data.rel.ro PROGBITS 0000000000bca800 006ca800
0000000000009260 0000000000000000 WA 0 0 32
[22] .dynamic DYNAMIC 0000000000bd3a60 006d3a60
0000000000000210 0000000000000010 WA 5 0 8
[23] .got PROGBITS 0000000000bd3c70 006d3c70
0000000000002e00 0000000000000008 WA 0 0 8
[24] .got.plt PROGBITS 0000000000bd6a70 006d6a70
0000000000000cf0 0000000000000008 WA 0 0 8
[25] .data PROGBITS 0000000000bd7760 006d7760
00000000000554f8 0000000000000000 WA 0 0 32
[26] .bss NOBITS 0000000000c2cc60 0072cc58
0000000000371358 0000000000000000 WA 0 0 32
[27] .comment PROGBITS 0000000000000000 0072cc58
0000000000003516 0000000000000000 0 0 1
[28] .debug_aranges PROGBITS 0000000000000000 0073016e
00000000000933e0 0000000000000000 0 0 1
[29] .debug_pubnames PROGBITS 0000000000000000 007c354e
00000000003eccfb 0000000000000000 0 0 1
[30] .debug_info PROGBITS 0000000000000000 00bb0249
00000000026fcafd 0000000000000000 0 0 1
[31] .debug_abbrev PROGBITS 0000000000000000 032acd46
000000000008ac3d 0000000000000000 0 0 1
[32] .debug_line PROGBITS 0000000000000000 03337983
00000000001e952d 0000000000000000 0 0 1
[33] .debug_frame PROGBITS 0000000000000000 03520eb0
0000000000181240 0000000000000000 0 0 8
[34] .debug_str PROGBITS 0000000000000000 036a20f0
000000000029c013 0000000000000000 0 0 1
[35] .debug_loc PROGBITS 0000000000000000 0393e103
0000000000634fc5 0000000000000000 0 0 1
[36] .debug_ranges PROGBITS 0000000000000000 03f730c8
00000000000e2b40 0000000000000000 0 0 1
[37] .shstrtab STRTAB 0000000000000000 04055c08
000000000000017c 0000000000000000 0 0 1
[38] .symtab SYMTAB 0000000000000000 04056788
0000000000100890 0000000000000018 39 21532 8
[39] .strtab STRTAB 0000000000000000 04157018
000000000019008a 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
对应的数据结构是：
（3）程序头表（program header table），自称为“段表”
section是作为输入，给链接器那去做链接用的，所以这些section也经常被称做输入section。链接器在链接可执行文件或动态库的过程中，它会把来自不同对象文件中的相同名称的 section 合并起来构成同名的 section。接着，它又会把带有相同属性(比方都是只读并可加载的)的 section 都合并成所谓 segment。segment作为链接器的输出，常被称为输出segment。
一个单独的 segment 通常会包含几个不同的 section，比方一个可被加载的、只读的segment 通常就会包括可执行代码.text section、只读的数据.rodata section 以及给动态链接器使用的符号.dymsym section等等。section是被链接器使用的，但是segment是被加载器所使用的。加载器会将所需要的segment加载到内存空间中运行。和用sections header table来指定一个ELF对象文件中到底有哪些section一样。也需要有一种信息结构来指出包含ELF对象文件有哪些segments，这种信息结构就是program header table。
使用readelf -l filename可以查看一个ELF文件的程序头表，如下：
Elf file type is EXEC (Executable file)
Entry point 0x42dfe0
There are 8 program headers, starting at offset 64
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
PHDR 0x0000000000000040 0x0000000000400040 0x0000000000400040
0x00000000000001c0 0x00000000000001c0 R E 8
INTERP 0x0000000000000200 0x0000000000400200 0x0000000000400200
0x000000000000001c 0x000000000000001c R 1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000
0x00000000006ca2e1 0x00000000006ca2e1 R E 100000
LOAD 0x00000000006ca2e8 0x0000000000bca2e8 0x0000000000bca2e8
0x0000000000062970 0x00000000003d3cd0 RW 100000
DYNAMIC 0x00000000006d3a60 0x0000000000bd3a60 0x0000000000bd3a60
0x0000000000000210 0x0000000000000210 RW 8
NOTE 0x000000000000021c 0x000000000040021c 0x000000000040021c
0x0000000000000020 0x0000000000000020 R 4
GNU_EH_FRAME 0x00000000005a6518 0x00000000009a6518 0x00000000009a6518
0x0000000000027804 0x0000000000027804 R 4
GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 RW 8
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .note.ABI-tag .hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame .gcc_except_table
03 .ctors .dtors .jcr .data.rel.ro .dynamic .got .got.plt .data .bss
04 .dynamic
05 .note.ABI-tag
06 .eh_frame_hdr
07
备注，上面的Section to Segmet mapping，就表示的段segment和节section的映射关系。
对应的数据结构是：
typedef struct {
Elf32_Word p_type; /* 段类型 */
Elf32_Off p_offset; /* 段位置相对于文件开始处的偏移量 */
Elf32_Addr p_vaddr; /* 段在内存中的地址 */
Elf32_Addr p_paddr; /* 段的物理地址 */
Elf32_Word p_filesz; /* 段在文件中的长度 */
Elf32_Word p_memsz; /* 段在内存中的长度 */
Elf32_Word p_flags; /* 段的标记 */
Elf32_Word p_align; /* 段在内存中对齐标记 */
} Elf32_Phdr;
4.一个readelf完整输出的例子
$readelf -all a.out
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: Intel 80386
Version: 0x1
Entry point address: 0x8048330
Start of program headers: 52 (bytes into file)
Start of section headers: 4412 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 9
Size of section headers: 40 (bytes)
Number of section headers: 30
Section header string table index: 27
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .interp PROGBITS 08048154 000154 000013 00 A 0 0 1
[ 2] .note.ABI-tag NOTE 08048168 000168 000020 00 A 0 0 4
[ 3] .note.gnu.build-i NOTE 08048188 000188 000024 00 A 0 0 4
[ 4] .gnu.hash GNU_HASH 080481ac 0001ac 000020 04 A 5 0 4
[ 5] .dynsym DYNSYM 080481cc 0001cc 000050 10 A 6 1 4
[ 6] .dynstr STRTAB 0804821c 00021c 00004c 00 A 0 0 1
[ 7] .gnu.version VERSYM 08048268 000268 00000a 02 A 5 0 2
[ 8] .gnu.version_r VERNEED 08048274 000274 000020 00 A 6 1 4
[ 9] .rel.dyn REL 08048294 000294 000008 08 A 5 0 4
[10] .rel.plt REL 0804829c 00029c 000018 08 A 5 12 4
[11] .init PROGBITS 080482b4 0002b4 00002e 00 AX 0 0 4
[12] .plt PROGBITS 080482f0 0002f0 000040 04 AX 0 0 16
[13] .text PROGBITS 08048330 000330 00018c 00 AX 0 0 16
[14] .fini PROGBITS 080484bc 0004bc 00001a 00 AX 0 0 4
[15] .rodata PROGBITS 080484d8 0004d8 000011 00 A 0 0 4
[16] .eh_frame_hdr PROGBITS 080484ec 0004ec 000034 00 A 0 0 4
[17] .eh_frame PROGBITS 08048520 000520 0000c4 00 A 0 0 4
[18] .ctors PROGBITS 08049f14 000f14 000008 00 WA 0 0 4
[19] .dtors PROGBITS 08049f1c 000f1c 000008 00 WA 0 0 4
[20] .jcr PROGBITS 08049f24 000f24 000004 00 WA 0 0 4
[21] .dynamic DYNAMIC 08049f28 000f28 0000c8 08 WA 6 0 4
[22] .got PROGBITS 08049ff0 000ff0 000004 04 WA 0 0 4
[23] .got.plt PROGBITS 08049ff4 000ff4 000018 04 WA 0 0 4
[24] .data PROGBITS 0804a00c 00100c 000008 00 WA 0 0 4
[25] .bss NOBITS 0804a014 001014 000008 00 WA 0 0 4
[26] .comment PROGBITS 00000000 001014 00002a 01 MS 0 0 1
[27] .shstrtab STRTAB 00000000 00103e 0000fc 00 0 0 1
[28] .symtab SYMTAB 00000000 0015ec 000410 10 29 45 4
[29] .strtab STRTAB 00000000 0019fc 0001f9 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
There are no section groups in this file.
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
PHDR 0x000034 0x08048034 0x08048034 0x00120 0x00120 R E 0x4
INTERP 0x000154 0x08048154 0x08048154 0x00013 0x00013 R 0x1
[Requesting program interpreter: /lib/ld-linux.so.2]
LOAD 0x000000 0x08048000 0x08048000 0x005e4 0x005e4 R E 0x1000
LOAD 0x000f14 0x08049f14 0x08049f14 0x00100 0x00108 RW 0x1000
DYNAMIC 0x000f28 0x08049f28 0x08049f28 0x000c8 0x000c8 RW 0x4
NOTE 0x000168 0x08048168 0x08048168 0x00044 0x00044 R 0x4
GNU_EH_FRAME 0x0004ec 0x080484ec 0x080484ec 0x00034 0x00034 R 0x4
GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RW 0x4
GNU_RELRO 0x000f14 0x08049f14 0x08049f14 0x000ec 0x000ec R 0x1
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .note.ABI-tag .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rel.dyn .rel.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame
03 .ctors .dtors .jcr .dynamic .got .got.plt .data .bss
04 .dynamic
05 .note.ABI-tag .note.gnu.build-id
06 .eh_frame_hdr
07
08 .ctors .dtors .jcr .dynamic .got
Dynamic section at offset 0xf28 contains 20 entries:
Tag Type Name/Value
0x00000001 (NEEDED) Shared library: [libc.so.6]
0x0000000c (INIT) 0x80482b4
0x0000000d (FINI) 0x80484bc
0x6ffffef5 (GNU_HASH) 0x80481ac
0x00000005 (STRTAB) 0x804821c
0x00000006 (SYMTAB) 0x80481cc
0x0000000a (STRSZ) 76 (bytes)
0x0000000b (SYMENT) 16 (bytes)
0x00000015 (DEBUG) 0x0
0x00000003 (PLTGOT) 0x8049ff4
0x00000002 (PLTRELSZ) 24 (bytes)
0x00000014 (PLTREL) REL
0x00000017 (JMPREL) 0x804829c
0x00000011 (REL) 0x8048294
0x00000012 (RELSZ) 8 (bytes)
0x00000013 (RELENT) 8 (bytes)
0x6ffffffe (VERNEED) 0x8048274
0x6fffffff (VERNEEDNUM) 1
0x6ffffff0 (VERSYM) 0x8048268
0x00000000 (NULL) 0x0
Relocation section '.rel.dyn' at offset 0x294 contains 1 entries:
Offset Info Type Sym.Value Sym. Name
08049ff0 00000206 R_386_GLOB_DAT 00000000 __gmon_start__
Relocation section '.rel.plt' at offset 0x29c contains 3 entries:
Offset Info Type Sym.Value Sym. Name
0804a000 00000107 R_386_JUMP_SLOT 00000000 printf
0804a004 00000207 R_386_JUMP_SLOT 00000000 __gmon_start__
0804a008 00000307 R_386_JUMP_SLOT 00000000 __libc_start_main
There are no unwind sections in this file.
Symbol table '.dynsym' contains 5 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00000000 0 FUNC GLOBAL DEFAULT UND printf@GLIBC_2.0 (2)
2: 00000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
3: 00000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.0 (2)
4: 080484dc 4 OBJECT GLOBAL DEFAULT 15 _IO_stdin_used
Symbol table '.symtab' contains 65 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 08048154 0 SECTION LOCAL DEFAULT 1
2: 08048168 0 SECTION LOCAL DEFAULT 2
3: 08048188 0 SECTION LOCAL DEFAULT 3
4: 080481ac 0 SECTION LOCAL DEFAULT 4
5: 080481cc 0 SECTION LOCAL DEFAULT 5
6: 0804821c 0 SECTION LOCAL DEFAULT 6
7: 08048268 0 SECTION LOCAL DEFAULT 7
8: 08048274 0 SECTION LOCAL DEFAULT 8
9: 08048294 0 SECTION LOCAL DEFAULT 9
10: 0804829c 0 SECTION LOCAL DEFAULT 10
11: 080482b4 0 SECTION LOCAL DEFAULT 11
12: 080482f0 0 SECTION LOCAL DEFAULT 12
13: 08048330 0 SECTION LOCAL DEFAULT 13
14: 080484bc 0 SECTION LOCAL DEFAULT 14
15: 080484d8 0 SECTION LOCAL DEFAULT 15
16: 080484ec 0 SECTION LOCAL DEFAULT 16
17: 08048520 0 SECTION LOCAL DEFAULT 17
18: 08049f14 0 SECTION LOCAL DEFAULT 18
19: 08049f1c 0 SECTION LOCAL DEFAULT 19
20: 08049f24 0 SECTION LOCAL DEFAULT 20
21: 08049f28 0 SECTION LOCAL DEFAULT 21
22: 08049ff0 0 SECTION LOCAL DEFAULT 22
23: 08049ff4 0 SECTION LOCAL DEFAULT 23
24: 0804a00c 0 SECTION LOCAL DEFAULT 24
25: 0804a014 0 SECTION LOCAL DEFAULT 25
26: 00000000 0 SECTION LOCAL DEFAULT 26
27: 00000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
28: 08049f14 0 OBJECT LOCAL DEFAULT 18 __CTOR_LIST__
29: 08049f1c 0 OBJECT LOCAL DEFAULT 19 __DTOR_LIST__
30: 08049f24 0 OBJECT LOCAL DEFAULT 20 __JCR_LIST__
31: 08048360 0 FUNC LOCAL DEFAULT 13 __do_global_dtors_aux
32: 0804a014 1 OBJECT LOCAL DEFAULT 25 completed.6086
33: 0804a018 4 OBJECT LOCAL DEFAULT 25 dtor_idx.6088
34: 080483c0 0 FUNC LOCAL DEFAULT 13 frame_dummy
35: 00000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
36: 08049f18 0 OBJECT LOCAL DEFAULT 18 __CTOR_END__
37: 080485e0 0 OBJECT LOCAL DEFAULT 17 __FRAME_END__
38: 08049f24 0 OBJECT LOCAL DEFAULT 20 __JCR_END__
39: 08048490 0 FUNC LOCAL DEFAULT 13 __do_global_ctors_aux
40: 00000000 0 FILE LOCAL DEFAULT ABS a.c
41: 08049f14 0 NOTYPE LOCAL DEFAULT 18 __init_array_end
42: 08049f28 0 OBJECT LOCAL DEFAULT 21 _DYNAMIC
43: 08049f14 0 NOTYPE LOCAL DEFAULT 18 __init_array_start
44: 08049ff4 0 OBJECT LOCAL DEFAULT 23 _GLOBAL_OFFSET_TABLE_
45: 08048480 2 FUNC GLOBAL DEFAULT 13 __libc_csu_fini
46: 08048482 0 FUNC GLOBAL HIDDEN 13 __i686.get_pc_thunk.bx
47: 0804a00c 0 NOTYPE WEAK DEFAULT 24 data_start
48: 00000000 0 FUNC GLOBAL DEFAULT UND printf@@GLIBC_2.0
49: 0804a014 0 NOTYPE GLOBAL DEFAULT ABS _edata
50: 080484bc 0 FUNC GLOBAL DEFAULT 14 _fini
51: 08049f20 0 OBJECT GLOBAL HIDDEN 19 __DTOR_END__
52: 0804a00c 0 NOTYPE GLOBAL DEFAULT 24 __data_start
53: 00000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
54: 0804a010 0 OBJECT GLOBAL HIDDEN 24 __dso_handle
55: 080484dc 4 OBJECT GLOBAL DEFAULT 15 _IO_stdin_used
56: 00000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@@GLIBC_
57: 08048410 97 FUNC GLOBAL DEFAULT 13 __libc_csu_init
58: 0804a01c 0 NOTYPE GLOBAL DEFAULT ABS _end
59: 08048330 0 FUNC GLOBAL DEFAULT 13 _start
60: 080484d8 4 OBJECT GLOBAL DEFAULT 15 _fp_hw
61: 0804a014 0 NOTYPE GLOBAL DEFAULT ABS __bss_start
62: 080483e4 40 FUNC GLOBAL DEFAULT 13 main
63: 00000000 0 NOTYPE WEAK DEFAULT UND _Jv_RegisterClasses
64: 080482b4 0 FUNC GLOBAL DEFAULT 11 _init
Histogram for `.gnu.hash' bucket list length (total of 2 buckets):
Length Number % of total Coverage
0 1 ( 50.0%)
1 1 ( 50.0%) 100.0%
Version symbols section '.gnu.version' contains 5 entries:
Addr: 0000000008048268 Offset: 0x000268 Link: 5 (.dynsym)
000: 0 (*local*) 2 (GLIBC_2.0) 0 (*local*) 2 (GLIBC_2.0)
004: 1 (*global*)
Version needs section '.gnu.version_r' contains 1 entries:
Addr: 0x0000000008048274 Offset: 0x000274 Link: 6 (.dynstr)
000000: Version: 1 File: libc.so.6 Cnt: 1
0x0010: Name: GLIBC_2.0 Flags: none Version: 2
Notes at offset 0x00000168 with length 0x00000020:
Owner Data size Description
GNU 0x00000010 NT_GNU_ABI_TAG (ABI version tag)
OS: Linux, ABI: 2.6.15
Notes at offset 0x00000188 with length 0x00000024:
Owner Data size Description
GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
Build ID: 17fb9651029b6a8543bfafec9eea23bd16454e65
学习资料来源于：
http://www.cnblogs.com/xmphoenix/archive/2011/10/23/2221879.html
https://www.ibm.com/developerworks/cn/linux/l-excutff/#resources