c语言学习笔记十六

共享库

命名规范:
通常带有符号链接 ls -l /lib
...
libipq_pic.so.0 libulockmgr.so.1
libipq_pic.so.0.0.0 libulockmgr.so.1.0.1
libipq.so.0 libusb-0.1.so.4
libipq.so.0.0.0 libusb-0.1.so.4.4.4
...
共享库文件名的组成：
real name 库文件
soname 符号链接的名字,共享库的主版本号

示例代码：
/*soname:libstack.so.1
*real name: libstack.so.1.0
*/
gcc -shared -Wl,-soname,libstack.so.1 -o libstack.so.1.0 test20.o


test21.o test22.o test23.o
linker name gcc的-L选项指定linker name所在的目录
创建linker name 的符号链接
ln -s libstack.so.1.0 libstack.so
gcc test24.c -L. -lstack -Istack -o main





共享库在编译时要加-fPIC选项,
1 编译目标文件命令为：
gcc -c -fPIC stack/test20.c stack/test21.c stack/test22.c stack/test23.c
反汇编结果为：
test21.o: file format elf32-i386




Disassembly of section .text:


00000000 <push>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 53 push %ebx
4: 83 ec 04 sub $0x4,%esp
7: e8 fc ff ff ff call 8 <push+0x8>
c: 81 c3 02 00 00 00 add $0x2,%ebx
12: 8b 45 08 mov 0x8(%ebp),%eax
15: 88 45 f8 mov %al,-0x8(%ebp)
/* top的地址为：0x0(%ebx)*/
18: 8b 83 00 00 00 00 mov 0x0(%ebx),%eax
1e: 8b 00 mov (%eax),%eax
20: 8d 50 01 lea 0x1(%eax),%edx
23: 8b 83 00 00 00 00 mov 0x0(%ebx),%eax
29: 89 10 mov %edx,(%eax)
2b: 8b 83 00 00 00 00 mov 0x0(%ebx),%eax
31: 8b 00 mov (%eax),%eax
33: 8b 93 00 00 00 00 mov 0x0(%ebx),%edx
39: 0f b6 4d f8 movzbl -0x8(%ebp),%ecx
3d: 88 0c 02 mov %cl,(%edx,%eax,1)
40: 83 c4 04 add $0x4,%esp
43: 5b pop %ebx
44: 5d pop %ebp
45: c3 ret


查看符号表：readelf -a test21.o
Relocation section '.rel.text' at offset 0x42c contains 6 entries:
Offset Info Type Sym.Value Sym. Name
00000008 00000a02 R_386_PC32 00000000 __i686.get_pc_thunk.bx
0000000e 00000b0a R_386_GOTPC 00000000 _GLOBAL_OFFSET_TABLE_
/*top和stack对应的记录不是R_386_32,而是R_386_GOT32*/
0000001a 00000c03 R_386_GOT32 00000000 top
00000025 00000c03 R_386_GOT32 00000000 top
0000002d 00000c03 R_386_GOT32 00000000 top
00000035 00000d03 R_386_GOT32 00000000 stack












不加-fPIC进行编译，反汇编结果为:


test21.o: file format elf32-i386




Disassembly of section .text:


00000000 <push>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 83 ec 04 sub $0x4,%esp
6: 8b 45 08 mov 0x8(%ebp),%eax
9: 88 45 fc mov %al,-0x4(%ebp)
/*top地址为0x0，准备在重定位时修改 */
c: a1 00 00 00 00 mov 0x0,%eax
11: 83 c0 01 add $0x1,%eax
14: a3 00 00 00 00 mov %eax,0x0
19: a1 00 00 00 00 mov 0x0,%eax
1e: 0f b6 55 fc movzbl -0x4(%ebp),%edx
22: 88 90 00 00 00 00 mov %dl,0x0(%eax)
28: c9 leave
29: c3 ret



使用readelf -a test21.o 查看符号表
Relocation section '.rel.text' at offset 0x324 contains 4 entries:
/*标出指令中有四处需要重定位修改*/
Offset Info Type Sym.Value Sym. Name
0000000d 00000801 R_386_32 00000000 top
00000015 00000801 R_386_32 00000000 top
0000001a 00000801 R_386_32 00000000 top
00000024 00000901 R_386_32 00000000 stack

编译链接生成可发执行文件：
gcc -g test24.c test20.o test21.o test22.o test23.o -Istack -o test24

反汇编结果为：
080483d0 <push>:
80483d0: 55 push %ebp
80483d1: 89 e5 mov %esp,%ebp
80483d3: 83 ec 04 sub $0x4,%esp
80483d6: 8b 45 08 mov 0x8(%ebp),%eax
80483d9: 88 45 fc mov %al,-0x4(%ebp)
80483dc: a1 10 a0 04 08 mov 0x804a010,%eax
80483e1: 83 c0 01 add $0x1,%eax
/*0x0被修改为0x804a010*/
80483e4: a3 10 a0 04 08 mov %eax,0x804a010
80483e9: a1 10 a0 04 08 mov 0x804a010,%eax
80483ee: 0f b6 55 fc movzbl -0x4(%ebp),%edx
/*0x0被修改为0x804a040*/
80483f2: 88 90 40 a0 04 08 mov %dl,0x804a040(%eax)
80483f8: c9 leave
80483f9: c3 ret
80483fa: 90 nop
80483fb: 90 nop


2 生成共享库:
gcc -shared -o libstack.so test20.o test21.o test22.o test23.o
查看反汇编结果：objdump -dS libstack.so
0000048c <push>:
48c: 55 push %ebp
48d: 89 e5 mov %esp,%ebp
48f: 53 push %ebx
490: 83 ec 04 sub $0x4,%esp
493: e8 ef ff ff ff call 487 <__i686.get_pc_thunk.bx>
498: 81 c3 5c 1b 00 00 add $0x1b5c,%ebx
49e: 8b 45 08 mov 0x8(%ebp),%eax
4a1: 88 45 f8 mov %al,-0x8(%ebp)
/*oxo(%ebx)被修改成-0xc(%ebx) 和0x8(%ebp),不是绝对地址*/
4a4: 8b 83 f4 ff ff ff mov -0xc(%ebx),%eax
4aa: 8b 00 mov (%eax),%eax
4ac: 8d 50 01 lea 0x1(%eax),%edx
4af: 8b 83 f4 ff ff ff mov -0xc(%ebx),%eax
4b5: 89 10 mov %edx,(%eax)
/*top和stack的绝对地址保存在一个地址列表中*/
4b7: 8b 83 f4 ff ff ff mov -0xc(%ebx),%eax
4bd: 8b 00 mov (%eax),%eax
4bf: 8b 93 fc ff ff ff mov -0x4(%ebx),%edx
4c5: 0f b6 4d f8 movzbl -0x8(%ebp),%ecx
4c9: 88 0c 02 mov %cl,(%edx,%eax,1)
4cc: 83 c4 04 add $0x4,%esp
4cf: 5b pop %ebx
4d0: 5d pop %ebp
4d1: c3 ret
4d2: 90 nop
4d3: 90 nop

3 与共离享库编编链接在一起
gcc test24.c -g -L. -lstack -Istack -o test24
4 运行可执行文件
yuezhenhua@ubuntu:/opt/sdk/tc$ ./test24
./test24: error while loading shared libraries: libstack.so: cannot open


shared object file: No such file or directory

运行时找不到依赖共享库



查看该程序所依赖的共享库:
ldd test24
返回结果为：
/*没有存在于文件系统中，由内核虚拟出来,负责处理系统调用*/
linux-gate.so.1 => (0x00b89000)
/*？路径没有找到*/
libstack.so => not found
/*路径是由ld-linux.so.2在作动态链接时搜索到的*/
libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0x00c04000)
/*动态链接器，路径在编译链接时指定*/
/lib/ld-linux.so.2 (0x008ea000)



解决办法1 ：
将libstack.so的绝对路径添加到/etc/ld.so.conf中
然后执行：sudo ldconfig -v



方法2：
将libstack.so复制到/usr/lib或是/lib 目录下
复制文件：sudo cp libstack.so /lib

方法3：(不推荐)
在编译可执行文件的时候,将libstack.so的路径写死在可执行文件中
gcc test24.c -g -L. -lstack -Istack -o test24 -Wl,-


rpath,/home/opt/sdk/tc
查看符号表：readlf -a test24
Dynamic section at offset 0xf18 contains 22 entries:
Tag Type Name/Value
0x00000001 (NEEDED) Shared library: [libstack.so]
0x00000001 (NEEDED) Shared library: [libc.so.6]
0x0000000f (RPATH) Library rpath: [/home/opt/sdk/tc]



动态链接的过程:

查看反汇编结果： objdump -dS test24


080484a4 <main>:
#include <stdio.h>
#include "test18.h"

int main(void){
80484a4: 55 push %ebp
80484a5: 89 e5 mov %esp,%ebp
80484a7: 83 e4 f0 and $0xfffffff0,%esp
80484aa: 83 ec 10 sub $0x10,%esp
/*push函数没有链接到可执行文件中*/
push('a');
80484ad: c7 04 24 61 00 00 00 movl $0x61,(%esp)
/*通过间接寻址来找到push函数的地址*/
80484b4: e8 27 ff ff ff call 80483e0 <push@plt>
return 0;
80484b9: b8 00 00 00 00 mov $0x0,%eax
}
80484be: c9 leave
80484bf: c3 ret



用gdb跟踪结果:gdb test24
/* 开始: start*/

(gdb) start
Temporary breakpoint 1 at 0x80484ad: file test24.c, line 5.
Starting program: /opt/sdk/tc/test24


Temporary breakpoint 1, main () at test24.c:5
5 push('a');

/*执行下一条：si*/
(gdb) si
0x080484b4 5 push('a');



/*执行下一条：si*/
(gdb) si
0x080483e0 in push@plt ()

/*查看地址0x080483e0 存放的数据*/
(gdb) x 0x080483e0
0x80483e0 <push@plt>: 0xa00825ff

(gdb) si
0x080483e6 in push@plt ()
(gdb) si
0x080483eb in push@plt ()
(gdb) si
0x080483b0 in ?? ()
(gdb) si
0x080483b6 in ?? ()
/*进入动态链接器*/
(gdb) si
0x00123c00 in ?? () from /lib/ld-linux.so.2