目录
连接和加载
》1.静态库是如何形成可执行程序的---弱化
• .symtab节 :SymbolTable符号表，就是源码里面那些函数名、变量名和代码的对应关系。
字符串表
只要记录起始、偏移量
char类型数组
ELF header 包含了理解整个文件所需的所有关键信息。
操作系统拿着ELF首先要判定这个是ELF格式
通过Magic随机值判定
[user1@iZ5waahoxw3q2bZ ~]$ readelf -h /usr/bin/ls
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: 0x4042d4
Start of program headers: 64 (bytes into file)
Start of section headers: 115696 (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: 31
Section header string table index: 30Entry point address: 0x68c0----- 入口地址
静态链接
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ cat code.c
#include<stdio.h>
void run()
{
printf("running...\n");
}
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ cat hello.c
#include<stdio.h>
void run();
int main()
{
printf("hello world!\n");
run();
return 0;
}
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ gcc -c *.c
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ readelf -h code.o
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: REL (Relocatable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 664 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 13
Section header string table index: 12
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ readelf -h hello.o
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: REL (Relocatable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 728 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 13
Section header string table index: 12
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ readelf -S hello.o
There are 13 section headers, starting at offset 0x2d8:
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
000000000000001f 0000000000000000 AX 0 0 1
[ 2] .rela.text RELA 0000000000000000 00000210
0000000000000048 0000000000000018 I 10 1 8
[ 3] .data PROGBITS 0000000000000000 0000005f
0000000000000000 0000000000000000 WA 0 0 1
[ 4] .bss NOBITS 0000000000000000 0000005f
0000000000000000 0000000000000000 WA 0 0 1
[ 5] .rodata PROGBITS 0000000000000000 0000005f
000000000000000d 0000000000000000 A 0 0 1
[ 6] .comment PROGBITS 0000000000000000 0000006c
000000000000002e 0000000000000001 MS 0 0 1
[ 7] .note.GNU-stack PROGBITS 0000000000000000 0000009a
0000000000000000 0000000000000000 0 0 1
[ 8] .eh_frame PROGBITS 0000000000000000 000000a0
0000000000000038 0000000000000000 A 0 0 8
[ 9] .rela.eh_frame RELA 0000000000000000 00000258
0000000000000018 0000000000000018 I 10 8 8
[10] .symtab SYMTAB 0000000000000000 000000d8
0000000000000120 0000000000000018 11 9 8
[11] .strtab STRTAB 0000000000000000 000001f8
0000000000000017 0000000000000000 0 0 1
[12] .shstrtab STRTAB 0000000000000000 00000270
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)
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ readelf -S code.o
There are 13 section headers, starting at offset 0x298:
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
0000000000000010 0000000000000000 AX 0 0 1
[ 2] .rela.text RELA 0000000000000000 000001e8
0000000000000030 0000000000000018 I 10 1 8
[ 3] .data PROGBITS 0000000000000000 00000050
0000000000000000 0000000000000000 WA 0 0 1
[ 4] .bss NOBITS 0000000000000000 00000050
0000000000000000 0000000000000000 WA 0 0 1
[ 5] .rodata PROGBITS 0000000000000000 00000050
000000000000000b 0000000000000000 A 0 0 1
[ 6] .comment PROGBITS 0000000000000000 0000005b
000000000000002e 0000000000000001 MS 0 0 1
[ 7] .note.GNU-stack PROGBITS 0000000000000000 00000089
0000000000000000 0000000000000000 0 0 1
[ 8] .eh_frame PROGBITS 0000000000000000 00000090
0000000000000038 0000000000000000 A 0 0 8
[ 9] .rela.eh_frame RELA 0000000000000000 00000218
0000000000000018 0000000000000018 I 10 8 8
[10] .symtab SYMTAB 0000000000000000 000000c8
0000000000000108 0000000000000018 11 9 8
[11] .strtab STRTAB 0000000000000000 000001d0
0000000000000011 0000000000000000 0 0 1
[12] .shstrtab STRTAB 0000000000000000 00000230
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)
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ gcc -o main.exe *.o
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ ll
total 28
-rw-rw-r-- 1 user1 user1 62 May 17 15:44 code.c
-rw-rw-r-- 1 user1 user1 1496 May 17 15:45 code.o
-rw-rw-r-- 1 user1 user1 103 May 17 15:44 hello.c
-rw-rw-r-- 1 user1 user1 1560 May 17 15:45 hello.o
-rwxrwxr-x 1 user1 user1 8504 May 17 15:46 main.exe
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ ./main.exe
hello world!
running...•⽆论是自己的.o,还是静态库中的.o，本质都是把.o⽂件进行连接的过程
• 所以：研究静态链接，本质就是研究.o是如何链接的
objdump对目标文件进行反汇编
objdump -d命令：将代码段（.text）进行反汇编查看
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ objdump -d code.o
code.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <run>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: bf 00 00 00 00 mov $0x0,%edi
9: e8 00 00 00 00 callq e <run+0xe>
e: 5d pop %rbp
f: c3 retq
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ objdump -d code.o >code.s
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ objdump -d hello.o > hello.s
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ ll
total 36
-rw-rw-r-- 1 user1 user1 62 May 17 15:44 code.c
-rw-rw-r-- 1 user1 user1 1496 May 17 15:45 code.o
-rw-rw-r-- 1 user1 user1 348 May 17 15:48 code.s
-rw-rw-r-- 1 user1 user1 103 May 17 15:44 hello.c
-rw-rw-r-- 1 user1 user1 1560 May 17 15:45 hello.o
-rw-rw-r-- 1 user1 user1 491 May 17 15:49 hello.s
-rwxrwxr-x 1 user1 user1 8504 May 17 15:46 main.exe
我们可以看到这⾥的call指令，它们分别对应之前调用的printf和run函数，但是你会发现他们的跳转地址都被设成了0。那这是为什么呢？
其实就是在编译hello.c 的时候，编译器是完全不知道printf 和run 函数的存在的，⽐如他们
位于内存的哪个区块，代码长什么样都是不知道的。因此，编译器只能将这两个函数的跳转地址先暂时设为0。
**这个地址会在哪个时候被修正？链接的时候！**为了让链接器将来在链接时能够正确定位到这些被修正的地址，在代码块（.data）中还存在⼀个重定位表，这张表将来在链接的时候，就会根据表⾥记录的地址将其修正。
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ readelf -s code.o
Symbol table '.symtab' contains 11 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS code.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 16 FUNC GLOBAL DEFAULT 1 run
10: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND puts
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ readelf -s hello.o
Symbol table '.symtab' contains 12 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS hello.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 31 FUNC GLOBAL DEFAULT 1 main
10: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND puts
11: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND run当两个.o还没有链接，所以他们的方法都是未定义的。所以我们将来怎么讲两个.o进行链接？
hello.o拿着自己UND符号去找能找到一个定义的run函数。两个.o的puts又跟库函数进行链接找到。
其实就是拿自己未定义的符号去其他区域找。
[user1@iZ5waahoxw3q2bZ 26-5-18.3]$ readelf -s main.exe
Symbol table '.dynsym' contains 4 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FUNC GLOBAL DEFAULT UND puts@GLIBC_2.2.5 (2)
2: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.2.5 (2)
3: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
Symbol table '.symtab' contains 66 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000400238 0 SECTION LOCAL DEFAULT 1
2: 0000000000400254 0 SECTION LOCAL DEFAULT 2
3: 0000000000400274 0 SECTION LOCAL DEFAULT 3
4: 0000000000400298 0 SECTION LOCAL DEFAULT 4
5: 00000000004002b8 0 SECTION LOCAL DEFAULT 5
6: 0000000000400318 0 SECTION LOCAL DEFAULT 6
7: 0000000000400356 0 SECTION LOCAL DEFAULT 7
8: 0000000000400360 0 SECTION LOCAL DEFAULT 8
9: 0000000000400380 0 SECTION LOCAL DEFAULT 9
10: 0000000000400398 0 SECTION LOCAL DEFAULT 10
11: 00000000004003c8 0 SECTION LOCAL DEFAULT 11
12: 00000000004003f0 0 SECTION LOCAL DEFAULT 12
13: 0000000000400420 0 SECTION LOCAL DEFAULT 13
14: 0000000000400430 0 SECTION LOCAL DEFAULT 14
15: 00000000004005c4 0 SECTION LOCAL DEFAULT 15
16: 00000000004005d0 0 SECTION LOCAL DEFAULT 16
17: 00000000004005f8 0 SECTION LOCAL DEFAULT 17
18: 0000000000400638 0 SECTION LOCAL DEFAULT 18
19: 0000000000600e10 0 SECTION LOCAL DEFAULT 19
20: 0000000000600e18 0 SECTION LOCAL DEFAULT 20
21: 0000000000600e20 0 SECTION LOCAL DEFAULT 21
22: 0000000000600e28 0 SECTION LOCAL DEFAULT 22
23: 0000000000600ff8 0 SECTION LOCAL DEFAULT 23
24: 0000000000601000 0 SECTION LOCAL DEFAULT 24
25: 0000000000601028 0 SECTION LOCAL DEFAULT 25
26: 000000000060102c 0 SECTION LOCAL DEFAULT 26
27: 0000000000000000 0 SECTION LOCAL DEFAULT 27
28: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
29: 0000000000600e20 0 OBJECT LOCAL DEFAULT 21 __JCR_LIST__
30: 0000000000400460 0 FUNC LOCAL DEFAULT 14 deregister_tm_clones
31: 0000000000400490 0 FUNC LOCAL DEFAULT 14 register_tm_clones
32: 00000000004004d0 0 FUNC LOCAL DEFAULT 14 __do_global_dtors_aux
33: 000000000060102c 1 OBJECT LOCAL DEFAULT 26 completed.6355
34: 0000000000600e18 0 OBJECT LOCAL DEFAULT 20 __do_global_dtors_aux_fin
35: 00000000004004f0 0 FUNC LOCAL DEFAULT 14 frame_dummy
36: 0000000000600e10 0 OBJECT LOCAL DEFAULT 19 __frame_dummy_init_array_
37: 0000000000000000 0 FILE LOCAL DEFAULT ABS code.c
38: 0000000000000000 0 FILE LOCAL DEFAULT ABS hello.c
39: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
40: 0000000000400748 0 OBJECT LOCAL DEFAULT 18 __FRAME_END__
41: 0000000000600e20 0 OBJECT LOCAL DEFAULT 21 __JCR_END__
42: 0000000000000000 0 FILE LOCAL DEFAULT ABS
43: 0000000000600e18 0 NOTYPE LOCAL DEFAULT 19 __init_array_end
44: 0000000000600e28 0 OBJECT LOCAL DEFAULT 22 _DYNAMIC
45: 0000000000600e10 0 NOTYPE LOCAL DEFAULT 19 __init_array_start
46: 00000000004005f8 0 NOTYPE LOCAL DEFAULT 17 __GNU_EH_FRAME_HDR
47: 0000000000601000 0 OBJECT LOCAL DEFAULT 24 _GLOBAL_OFFSET_TABLE_
48: 00000000004005c0 2 FUNC GLOBAL DEFAULT 14 __libc_csu_fini
49: 0000000000601028 0 NOTYPE WEAK DEFAULT 25 data_start
50: 0000000000000000 0 FUNC GLOBAL DEFAULT UND puts@@GLIBC_2.2.5
51: 000000000060102c 0 NOTYPE GLOBAL DEFAULT 25 _edata
52: 000000000040051d 16 FUNC GLOBAL DEFAULT 14 run
53: 00000000004005c4 0 FUNC GLOBAL DEFAULT 15 _fini
54: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@@GLIBC_
55: 0000000000601028 0 NOTYPE GLOBAL DEFAULT 25 __data_start
56: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
57: 00000000004005d8 0 OBJECT GLOBAL HIDDEN 16 __dso_handle
58: 00000000004005d0 4 OBJECT GLOBAL DEFAULT 16 _IO_stdin_used
59: 0000000000400550 101 FUNC GLOBAL DEFAULT 14 __libc_csu_init
60: 0000000000601030 0 NOTYPE GLOBAL DEFAULT 26 _end
61: 0000000000400430 0 FUNC GLOBAL DEFAULT 14 _start
62: 000000000060102c 0 NOTYPE GLOBAL DEFAULT 26 __bss_start
63: 000000000040052d 31 FUNC GLOBAL DEFAULT 14 main
64: 0000000000601030 0 OBJECT GLOBAL HIDDEN 25 __TMC_END__
65: 00000000004003c8 0 FUNC GLOBAL DEFAULT 11 _init两个.o进行合并之后，在最终的可执行程序中，就找到了run
0000000000001149：其实是地址，后⾯说FUNC:表示run符号类型是个函数16：就是run函数所在的section被合并最终的那⼀个section中了,16就是下标
**最终：
- 两个.o的代码段合并到了⼀起，并进行了统⼀的编址
- 链接的时候，会修改.o中没有确定的函数地址，在合并完成之后，进行相关call地址，完成代码调用**
静态链接就是把库中的.o进行合并，和上述过程⼀样
所以链接其实就是将编译之后的所有目标文件连同用到的⼀些静态库运行时库组合，拼装成⼀个独立的可执行文件 。其中就包括我们之前提到的地址修正，当所有模块组合在⼀起之后，链接器会根据我们的.o⽂件或者静态库中的重定位表找到那些需要被重定位的函数全局变量，从⽽修正它们的地址。这其实就是静态链接的过程。
所以，链接过程中会涉及到对.o中外部符号进行地址重定位。这是一种链接重定位
所以.o为什么叫做可重定位目标文件，.o链接时地址会被重新修改
感谢你的观看，期待我们下次再见!