Linux 静态链接
基本概念
预编译—>编译—>汇编—>链接
宏定义代替，预编译命令，文本去注释—>.S—>.O—>ELF
文本变成中间文件, 也就是编译+汇编的过程:
词法分析，语法分析，语义分析，中间过程优化
ELF 文件
ELF 是一种二进制可执行文件.
目标文件(*.obj)和最终可执行文件(ELF 格式)统称为 ELF 文件.
另外*.a *.so coredump文件也都是 ELF 文件的不同类型.
最重要的是，数据是分段存储，不同段的有不同的功能.段的类型:
为了后面装载的方便，段在链接期间，是可以合并，优化空间的,
例子
以 a.c, b.c 为例子:
//in a.c
#include <stdio.h>
extern int shared;
int main(int argc, char* argv[])
{
int a = 100;
swap(&a, &shared);
printf("shared %d\n", shared);
return 0;
}
//in b.c
int shared = 99;
void swap(int* a, int* b)
{
int tmp = *a;
*a = *b;
*b = tmp;
}
编译(包含中间文件):
gcc a.c b.c -c -o ab
查看所有 elf 信息:
➜ compile-link-lib readelf -a ab
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: DYN (Shared object file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x5a0
Start of program headers: 64 (bytes into file)
Start of section headers: 6584 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 9
Size of section headers: 64 (bytes)
Number of section headers: 29
Section header string table index: 28
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 0000000000000238 00000238
000000000000001c 0000000000000000 A 0 0 1
[ 2] .note.ABI-tag NOTE 0000000000000254 00000254
0000000000000020 0000000000000000 A 0 0 4
[ 3] .note.gnu.build-i NOTE 0000000000000274 00000274
0000000000000024 0000000000000000 A 0 0 4
[ 4] .gnu.hash GNU_HASH 0000000000000298 00000298
000000000000001c 0000000000000000 A 5 0 8
[ 5] .dynsym DYNSYM 00000000000002b8 000002b8
00000000000000c0 0000000000000018 A 6 1 8
[ 6] .dynstr STRTAB 0000000000000378 00000378
000000000000009f 0000000000000000 A 0 0 1
[ 7] .gnu.version VERSYM 0000000000000418 00000418
0000000000000010 0000000000000002 A 5 0 2
[ 8] .gnu.version_r VERNEED 0000000000000428 00000428
0000000000000030 0000000000000000 A 6 1 8
[ 9] .rela.dyn RELA 0000000000000458 00000458
00000000000000c0 0000000000000018 A 5 0 8
[10] .rela.plt RELA 0000000000000518 00000518
0000000000000030 0000000000000018 AI 5 22 8
[11] .init PROGBITS 0000000000000548 00000548
0000000000000017 0000000000000000 AX 0 0 4
[12] .plt PROGBITS 0000000000000560 00000560
0000000000000030 0000000000000010 AX 0 0 16
[13] .plt.got PROGBITS 0000000000000590 00000590
0000000000000008 0000000000000008 AX 0 0 8
[14] .text PROGBITS 00000000000005a0 000005a0
0000000000000222 0000000000000000 AX 0 0 16
[15] .fini PROGBITS 00000000000007c4 000007c4
0000000000000009 0000000000000000 AX 0 0 4
[16] .rodata PROGBITS 00000000000007d0 000007d0
000000000000000f 0000000000000000 A 0 0 4
[17] .eh_frame_hdr PROGBITS 00000000000007e0 000007e0
0000000000000044 0000000000000000 A 0 0 4
[18] .eh_frame PROGBITS 0000000000000828 00000828
0000000000000128 0000000000000000 A 0 0 8
[19] .init_array INIT_ARRAY 0000000000200db0 00000db0
0000000000000008 0000000000000008 WA 0 0 8
[20] .fini_array FINI_ARRAY 0000000000200db8 00000db8
0000000000000008 0000000000000008 WA 0 0 8
[21] .dynamic DYNAMIC 0000000000200dc0 00000dc0
00000000000001f0 0000000000000010 WA 6 0 8
[22] .got PROGBITS 0000000000200fb0 00000fb0
0000000000000050 0000000000000008 WA 0 0 8
[23] .data PROGBITS 0000000000201000 00001000
0000000000000014 0000000000000000 WA 0 0 8
[24] .bss NOBITS 0000000000201014 00001014
0000000000000004 0000000000000000 WA 0 0 1
[25] .comment PROGBITS 0000000000000000 00001014
000000000000002a 0000000000000001 MS 0 0 1
[26] .symtab SYMTAB 0000000000000000 00001040
0000000000000648 0000000000000018 27 44 8
[27] .strtab STRTAB 0000000000000000 00001688
000000000000022d 0000000000000000 0 0 1
[28] .shstrtab STRTAB 0000000000000000 000018b5
00000000000000fe 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
l (large), p (processor specific)
There are no section groups in this file.
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
PHDR 0x0000000000000040 0x0000000000000040 0x0000000000000040
0x00000000000001f8 0x00000000000001f8 R 0x8
INTERP 0x0000000000000238 0x0000000000000238 0x0000000000000238
0x000000000000001c 0x000000000000001c R 0x1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
LOAD 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000950 0x0000000000000950 R E 0x200000
LOAD 0x0000000000000db0 0x0000000000200db0 0x0000000000200db0
0x0000000000000264 0x0000000000000268 RW 0x200000
DYNAMIC 0x0000000000000dc0 0x0000000000200dc0 0x0000000000200dc0
0x00000000000001f0 0x00000000000001f0 RW 0x8
NOTE 0x0000000000000254 0x0000000000000254 0x0000000000000254
0x0000000000000044 0x0000000000000044 R 0x4
GNU_EH_FRAME 0x00000000000007e0 0x00000000000007e0 0x00000000000007e0
0x0000000000000044 0x0000000000000044 R 0x4
GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 RW 0x10
GNU_RELRO 0x0000000000000db0 0x0000000000200db0 0x0000000000200db0
0x0000000000000250 0x0000000000000250 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 .rela.dyn .rela.plt .init .plt .plt.got .text .fini .rodata .eh_frame_hdr .eh_frame
03 .init_array .fini_array .dynamic .got .data .bss
04 .dynamic
05 .note.ABI-tag .note.gnu.build-id
06 .eh_frame_hdr
07
08 .init_array .fini_array .dynamic .got
Dynamic section at offset 0xdc0 contains 27 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x000000000000000c (INIT) 0x548
0x000000000000000d (FINI) 0x7c4
0x0000000000000019 (INIT_ARRAY) 0x200db0
0x000000000000001b (INIT_ARRAYSZ) 8 (bytes)
0x000000000000001a (FINI_ARRAY) 0x200db8
0x000000000000001c (FINI_ARRAYSZ) 8 (bytes)
0x000000006ffffef5 (GNU_HASH) 0x298
0x0000000000000005 (STRTAB) 0x378
0x0000000000000006 (SYMTAB) 0x2b8
0x000000000000000a (STRSZ) 159 (bytes)
0x000000000000000b (SYMENT) 24 (bytes)
0x0000000000000015 (DEBUG) 0x0
0x0000000000000003 (PLTGOT) 0x200fb0
0x0000000000000002 (PLTRELSZ) 48 (bytes)
0x0000000000000014 (PLTREL) RELA
0x0000000000000017 (JMPREL) 0x518
0x0000000000000007 (RELA) 0x458
0x0000000000000008 (RELASZ) 192 (bytes)
0x0000000000000009 (RELAENT) 24 (bytes)
0x000000000000001e (FLAGS) BIND_NOW
0x000000006ffffffb (FLAGS_1) Flags: NOW PIE
0x000000006ffffffe (VERNEED) 0x428
0x000000006fffffff (VERNEEDNUM) 1
0x000000006ffffff0 (VERSYM) 0x418
0x000000006ffffff9 (RELACOUNT) 3
0x0000000000000000 (NULL) 0x0
Relocation section '.rela.dyn' at offset 0x458 contains 8 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000200db0 000000000008 R_X86_64_RELATIVE 6a0
000000200db8 000000000008 R_X86_64_RELATIVE 660
000000201008 000000000008 R_X86_64_RELATIVE 201008
000000200fd8 000100000006 R_X86_64_GLOB_DAT 0000000000000000 _ITM_deregisterTMClone + 0
000000200fe0 000400000006 R_X86_64_GLOB_DAT 0000000000000000 __libc_start_main@GLIBC_2.2.5 + 0
000000200fe8 000500000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0
000000200ff0 000600000006 R_X86_64_GLOB_DAT 0000000000000000 _ITM_registerTMCloneTa + 0
000000200ff8 000700000006 R_X86_64_GLOB_DAT 0000000000000000 __cxa_finalize@GLIBC_2.2.5 + 0
Relocation section '.rela.plt' at offset 0x518 contains 2 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000200fc8 000200000007 R_X86_64_JUMP_SLO 0000000000000000 __stack_chk_fail@GLIBC_2.4 + 0
000000200fd0 000300000007 R_X86_64_JUMP_SLO 0000000000000000 printf@GLIBC_2.2.5 + 0
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
Symbol table '.dynsym' contains 8 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterTMCloneTab
2: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __stack_chk_fail@GLIBC_2.4 (2)
3: 0000000000000000 0 FUNC GLOBAL DEFAULT UND printf@GLIBC_2.2.5 (3)
4: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.2.5 (3)
5: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
6: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMCloneTable
7: 0000000000000000 0 FUNC WEAK DEFAULT UND __cxa_finalize@GLIBC_2.2.5 (3)
Symbol table '.symtab' contains 67 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000238 0 SECTION LOCAL DEFAULT 1
2: 0000000000000254 0 SECTION LOCAL DEFAULT 2
3: 0000000000000274 0 SECTION LOCAL DEFAULT 3
4: 0000000000000298 0 SECTION LOCAL DEFAULT 4
5: 00000000000002b8 0 SECTION LOCAL DEFAULT 5
6: 0000000000000378 0 SECTION LOCAL DEFAULT 6
7: 0000000000000418 0 SECTION LOCAL DEFAULT 7
8: 0000000000000428 0 SECTION LOCAL DEFAULT 8
9: 0000000000000458 0 SECTION LOCAL DEFAULT 9
10: 0000000000000518 0 SECTION LOCAL DEFAULT 10
11: 0000000000000548 0 SECTION LOCAL DEFAULT 11
12: 0000000000000560 0 SECTION LOCAL DEFAULT 12
13: 0000000000000590 0 SECTION LOCAL DEFAULT 13
14: 00000000000005a0 0 SECTION LOCAL DEFAULT 14
15: 00000000000007c4 0 SECTION LOCAL DEFAULT 15
16: 00000000000007d0 0 SECTION LOCAL DEFAULT 16
17: 00000000000007e0 0 SECTION LOCAL DEFAULT 17
18: 0000000000000828 0 SECTION LOCAL DEFAULT 18
19: 0000000000200db0 0 SECTION LOCAL DEFAULT 19
20: 0000000000200db8 0 SECTION LOCAL DEFAULT 20
21: 0000000000200dc0 0 SECTION LOCAL DEFAULT 21
22: 0000000000200fb0 0 SECTION LOCAL DEFAULT 22
23: 0000000000201000 0 SECTION LOCAL DEFAULT 23
24: 0000000000201014 0 SECTION LOCAL DEFAULT 24
25: 0000000000000000 0 SECTION LOCAL DEFAULT 25
26: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
27: 00000000000005d0 0 FUNC LOCAL DEFAULT 14 deregister_tm_clones
28: 0000000000000610 0 FUNC LOCAL DEFAULT 14 register_tm_clones
29: 0000000000000660 0 FUNC LOCAL DEFAULT 14 __do_global_dtors_aux
30: 0000000000201014 1 OBJECT LOCAL DEFAULT 24 completed.7696
31: 0000000000200db8 0 OBJECT LOCAL DEFAULT 20 __do_global_dtors_aux_fin
32: 00000000000006a0 0 FUNC LOCAL DEFAULT 14 frame_dummy
33: 0000000000200db0 0 OBJECT LOCAL DEFAULT 19 __frame_dummy_init_array_
34: 0000000000000000 0 FILE LOCAL DEFAULT ABS a.c
35: 0000000000000000 0 FILE LOCAL DEFAULT ABS b.c
36: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
37: 000000000000094c 0 OBJECT LOCAL DEFAULT 18 __FRAME_END__
38: 0000000000000000 0 FILE LOCAL DEFAULT ABS
39: 0000000000200db8 0 NOTYPE LOCAL DEFAULT 19 __init_array_end
40: 0000000000200dc0 0 OBJECT LOCAL DEFAULT 21 _DYNAMIC
41: 0000000000200db0 0 NOTYPE LOCAL DEFAULT 19 __init_array_start
42: 00000000000007e0 0 NOTYPE LOCAL DEFAULT 17 __GNU_EH_FRAME_HDR
43: 0000000000200fb0 0 OBJECT LOCAL DEFAULT 22 _GLOBAL_OFFSET_TABLE_
44: 00000000000007c0 2 FUNC GLOBAL DEFAULT 14 __libc_csu_fini
45: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterTMCloneTab
46: 0000000000201000 0 NOTYPE WEAK DEFAULT 23 data_start
47: 0000000000201014 0 NOTYPE GLOBAL DEFAULT 23 _edata
48: 00000000000007c4 0 FUNC GLOBAL DEFAULT 15 _fini
49: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __stack_chk_fail@@GLIBC_2
50: 0000000000000000 0 FUNC GLOBAL DEFAULT UND printf@@GLIBC_2.2.5
51: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@@GLIBC_
52: 0000000000201000 0 NOTYPE GLOBAL DEFAULT 23 __data_start
53: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
54: 0000000000201008 0 OBJECT GLOBAL HIDDEN 23 __dso_handle
55: 00000000000007d0 4 OBJECT GLOBAL DEFAULT 16 _IO_stdin_used
56: 0000000000000750 101 FUNC GLOBAL DEFAULT 14 __libc_csu_init
57: 0000000000201018 0 NOTYPE GLOBAL DEFAULT 24 _end
58: 00000000000005a0 43 FUNC GLOBAL DEFAULT 14 _start
59: 0000000000201014 0 NOTYPE GLOBAL DEFAULT 24 __bss_start
60: 00000000000006aa 113 FUNC GLOBAL DEFAULT 14 main
61: 0000000000201018 0 OBJECT GLOBAL HIDDEN 23 __TMC_END__
62: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMCloneTable
63: 000000000000071b 45 FUNC GLOBAL DEFAULT 14 swap
64: 0000000000000000 0 FUNC WEAK DEFAULT UND __cxa_finalize@@GLIBC_2.2
65: 0000000000000548 0 FUNC GLOBAL DEFAULT 11 _init
66: 0000000000201010 4 OBJECT GLOBAL DEFAULT 23 shared
Version symbols section '.gnu.version' contains 8 entries:
Addr: 0000000000000418 Offset: 0x000418 Link: 5 (.dynsym)
000: 0 (*local*) 0 (*local*) 2 (GLIBC_2.4) 3 (GLIBC_2.2.5)
004: 3 (GLIBC_2.2.5) 0 (*local*) 0 (*local*) 3 (GLIBC_2.2.5)
Version needs section '.gnu.version_r' contains 1 entry:
Addr: 0x0000000000000428 Offset: 0x000428 Link: 6 (.dynstr)
000000: Version: 1 File: libc.so.6 Cnt: 2
0x0010: Name: GLIBC_2.2.5 Flags: none Version: 3
0x0020: Name: GLIBC_2.4 Flags: none Version: 2
Displaying notes found in: .note.ABI-tag
Owner Data size Description
GNU 0x00000010 NT_GNU_ABI_TAG (ABI version tag)
OS: Linux, ABI: 3.2.0
Displaying notes found in: .note.gnu.build-id
Owner Data size Description
GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
Build ID: 1f04fda3dce8f1ed6bd62b23f5c98a9188ea3c3e
看 a.o 目标文件的段:
➜ compile-link-lib readelf -S a.o
There are 13 section headers, starting at offset 0x3e0:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000000000000 00000040
0000000000000071 0000000000000000 AX 0 0 1
[ 2] .rela.text RELA 0000000000000000 000002d0
0000000000000090 0000000000000018 I 10 1 8
[ 3] .data PROGBITS 0000000000000000 000000b1
0000000000000000 0000000000000000 WA 0 0 1
[ 4] .bss NOBITS 0000000000000000 000000b1
0000000000000000 0000000000000000 WA 0 0 1
[ 5] .rodata PROGBITS 0000000000000000 000000b1
000000000000000b 0000000000000000 A 0 0 1
[ 6] .comment PROGBITS 0000000000000000 000000bc
000000000000002b 0000000000000001 MS 0 0 1
[ 7] .note.GNU-stack PROGBITS 0000000000000000 000000e7
0000000000000000 0000000000000000 0 0 1
[ 8] .eh_frame PROGBITS 0000000000000000 000000e8
0000000000000038 0000000000000000 A 0 0 8
[ 9] .rela.eh_frame RELA 0000000000000000 00000360
0000000000000018 0000000000000018 I 10 8 8
[10] .symtab SYMTAB 0000000000000000 00000120
0000000000000168 0000000000000018 11 9 8
[11] .strtab STRTAB 0000000000000000 00000288
0000000000000044 0000000000000000 0 0 1
[12] .shstrtab STRTAB 0000000000000000 00000378
0000000000000061 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
l (large), p (processor specific)
符号表
symtab 是代码中的用到的符号(函数，变量,elf 文件本身用到的字符串),组成的表.
obj 和最终 elf 都有自己的 symtab.
- elf文件中用到的符号作为单独的1个段来管理.
- 最终的symtab包含了目标文件里的符号表: 符号名，大小，类型(绑定信息)
符号内容包含 value,类型和绑定信息:
符号的值：
- obj文件中，可能是符号/函数的fake地址，段偏移，或其他的属性；
- elf执行文件中，就是符号的虚拟地址,是链接后被修正过的.
来看 a.o 的符号表情况:
➜ compile-link-lib readelf -s a.o
Symbol table '.symtab' contains 15 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS a.c
2: 0000000000000000 0 SECTION LOCAL DEFAULT 1
3: 0000000000000000 0 SECTION LOCAL DEFAULT 3
4: 0000000000000000 0 SECTION LOCAL DEFAULT 4
5: 0000000000000000 0 SECTION LOCAL DEFAULT 5
6: 0000000000000000 0 SECTION LOCAL DEFAULT 7
7: 0000000000000000 0 SECTION LOCAL DEFAULT 8
8: 0000000000000000 0 SECTION LOCAL DEFAULT 6
9: 0000000000000000 113 FUNC GLOBAL DEFAULT 1 main
10: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND shared
11: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _GLOBAL_OFFSET_TABLE_
12: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND swap
13: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND printf
14: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND __stack_chk_fail
符号修饰和函数签名:
- 不同文件可能定义同1符号，需要保证符号的唯一性，最终的符号导出时，要进行修饰 'func-->?func@@HMT@HZ'之类的
- 不同编译器搞法不一样，这是不同编译器编译后的文件不能相互link的原因
强符号和弱符号:
in file 1:
int a = 0;
int file 2:
int a = 0; //两个强符号，compile error
int a; //未初始化的全局变量为弱符号, ok 跟随file 1的强符号的定义
__attribute(weak) int a = 0; //手动指定弱符号 ，ok,同上
强引用和弱引用:
__attribute(weakref) void func(); //弱引用链接时没找到定义也不报错，(运行时则直接报错) 强引用则链接时就报错，
使用场景：某些库的模块需要扩展模块，扩展模块用弱引用, 没有扩展也能用(扩展不存在时，func 的地址为 0,可以判断出来。)
静态编译的链接过程
拼接分散的模块(*.obj),再修正跨模块引用符号地址,那些符号称为”重定位入口”,在链接阶段被”重定位”,
缺点：空间浪费,不方便修改
优点: 不依赖他人,方便使用,快
地址和空间分配(Address and Space Allocation)
- 相似段(a .text and b.text)叠加合并，重新调整段的大小，位置；
- 所有符号表也合并成1个global symbol table
- 给各个段(.text .data等)分配进程空间的虚拟地址
了解个更深的概念，叫函数级别链接,gcc 参数-ffunction-sections or -fdata-sections
它把每个函数都放到单独的段,这样链接器针对函数级别的段进行链接，自然那些用不到的函数的段，就不会最终合并，也就可减少最终文件的大小，代价是编译和链接过程变慢,而且更复杂了，链接器也需要维护不同函数的调用关系，目标文件自然也增大，
符号决议(Symbol Resolution) + 重定位(Relocation)
先看结果. 比较下 a.o 和 ab.o 里,可以看到符号修正的情况:
objdump 可以 dump 指定的段:
a.o:
objdump -d a.o
0000000000000000 <main>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 48 83 ec 20 sub $0x20,%rsp
8: 89 7d ec mov %edi,-0x14(%rbp)
b: 48 89 75 e0 mov %rsi,-0x20(%rbp)
f: 64 48 8b 04 25 28 00 mov %fs:0x28,%rax
16: 00 00
18: 48 89 45 f8 mov %rax,-0x8(%rbp)
1c: 31 c0 xor %eax,%eax
1e: c7 45 f4 64 00 00 00 movl $0x64,-0xc(%rbp)
25: 48 8d 45 f4 lea -0xc(%rbp),%rax
29: 48 8d 35 00 00 00 00 lea 0x0(%rip),%rsi # 30 <main+0x30>
30: 48 89 c7 mov %rax,%rdi
33: b8 00 00 00 00 mov $0x0,%eax
38: e8 00 00 00 00 callq 3d <main+0x3d>
3d: 8b 05 00 00 00 00 mov 0x0(%rip),%eax # 43 <main+0x43>
43: 89 c6 mov %eax,%esi
45: 48 8d 3d 00 00 00 00 lea 0x0(%rip),%rdi # 4c <main+0x4c>
4c: b8 00 00 00 00 mov $0x0,%eax
51: e8 00 00 00 00 callq 56 <main+0x56>
56: b8 00 00 00 00 mov $0x0,%eax
5b: 48 8b 55 f8 mov -0x8(%rbp),%rdx
5f: 64 48 33 14 25 28 00 xor %fs:0x28,%rdx
66: 00 00
68: 74 05 je 6f <main+0x6f>
6a: e8 00 00 00 00 callq 6f <main+0x6f>
6f: c9 leaveq
70: c3 retq
line 29 : swap 调用前将 shared 压栈，此时 shared 地址为 0x00000000,显示并不是有效的地址;
line 38: 通过 callq 指令，调用 swap,但 swap 的地址也为 0x00000000,无意义;
再看符号重定位后的 elf 执行文件 ab:
objdump -d ab
00000000000006aa <main>:
6aa: 55 push %rbp
6ab: 48 89 e5 mov %rsp,%rbp
6ae: 48 83 ec 20 sub $0x20,%rsp
6b2: 89 7d ec mov %edi,-0x14(%rbp)
6b5: 48 89 75 e0 mov %rsi,-0x20(%rbp)
6b9: 64 48 8b 04 25 28 00 mov %fs:0x28,%rax
6c0: 00 00
6c2: 48 89 45 f8 mov %rax,-0x8(%rbp)
6c6: 31 c0 xor %eax,%eax
6c8: c7 45 f4 64 00 00 00 movl $0x64,-0xc(%rbp)
6cf: 48 8d 45 f4 lea -0xc(%rbp),%rax
6d3: 48 8d 35 36 09 20 00 lea 0x200936(%rip),%rsi # 201010 <shared>
6da: 48 89 c7 mov %rax,%rdi
6dd: b8 00 00 00 00 mov $0x0,%eax
6e2: e8 34 00 00 00 callq 71b <swap>
6e7: 8b 05 23 09 20 00 mov 0x200923(%rip),%eax # 201010 <shared>
6ed: 89 c6 mov %eax,%esi
6ef: 48 8d 3d de 00 00 00 lea 0xde(%rip),%rdi # 7d4 <_IO_stdin_used+0x4>
6f6: b8 00 00 00 00 mov $0x0,%eax
6fb: e8 80 fe ff ff callq 580 <printf@plt>
700: b8 00 00 00 00 mov $0x0,%eax
705: 48 8b 55 f8 mov -0x8(%rbp),%rdx
709: 64 48 33 14 25 28 00 xor %fs:0x28,%rdx
710: 00 00
712: 74 05 je 719 <main+0x6f>
714: e8 57 fe ff ff callq 570 <__stack_chk_fail@plt>
719: c9 leaveq
71a: c3 retq
line 6d3: shared 已经有具体的虚拟地址:0x00200936
line 6e2: swap 有具体的虚拟地址:0x00000034
具体怎么重定位的? 就是利用重定位表和段地址,符号的偏移值，修正规则等， 计算一般符号的虚拟地址.
来看重定位表，即 rela.开头的段:
➜ compile-link-lib objdump -r a.o
a.o: file format elf64-x86-64
RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
000000000000002c R_X86_64_PC32 shared-0x0000000000000004
0000000000000039 R_X86_64_PLT32 swap-0x0000000000000004
000000000000003f R_X86_64_PC32 shared-0x0000000000000004
0000000000000048 R_X86_64_PC32 .rodata-0x0000000000000004
0000000000000052 R_X86_64_PLT32 printf-0x0000000000000004
000000000000006b R_X86_64_PLT32 __stack_chk_fail-0x0000000000000004
RELOCATION RECORDS FOR [.eh_frame]:
OFFSET TYPE VALUE
0000000000000020 R_X86_64_PC32 .text
第 1 列的 offset 就是重定位入口在 a.o 中相对短起始的偏移位置.
第 2 列是重定位符号类型，决定了它的地址修正方式:
A:保存在被修正位置的值,
P:被修正的位置(相对于段开始的偏移量)
S:符号的实际地址 (符号肯定是存在的，它的实际位置是放在sysmtab中)
修正方式:
_32: 绝对地址寻址修正 S+A
_PC32: 相对地址寻址修正 S+A-P
_PLT32:
来看这个例子,shared 的修正方式是相对寻址修正.
- S 先看 shared 符号在符号表中的地址 S = 0x00201010
➜ compile-link-lib objdump -t ab |grep shared
0000000000201010 g O .data 0000000000000004 shared
- A 保存在被修正位置的值，也就是未重定位前的原值是 A = 0x00000000.这就是要修改的目标 一般都是 0x0.
- P 表示 shared 要被修正的位置,它位于 main 中，所以先找到 main 的实际地址
00000000000006aa g F .text 0000000000000071 main
修正的指令的偏移在重定位表里为 offset = 0x2c
所以被修正的位置值: P = 0x6aa + 0x2c
那么计算相对寻址的方式是:
modified_A = S + A - P - 4
计算结果:
>>> hex(0x201010 + 0x0 -(0x6aa+0x2c) - 4)
'0x200936'
最后看实际的代码，是正确的:
6d3: 48 8d 35 36 09 20 00 lea 0x200936(%rip),%rsi # 201010 <shared>
注意：这个 lea + rip 大有学问..
如果是绝对寻址,修正值直接就是符号表里的地址(when A=0x0),这是很自然的:
modified_A = S + A(A=0 usually)
相对寻址修正，这两个的区别自然也就是这个 P 上.
为何需要相对寻址修正?
也许在汇编过程中，编译器为了其他指令使用相对寻址的方式来寻访 access 的方便.
静态库链接
lib.a 就是一堆的.o，所以本质上和 obj 静态链接没有区别.
