对PIC(position independent code)的探究

示例:

static  int a=2;extern int b;extern void test();int glo=2;void bar(){        a=1;        b=2;        glo=3;}void foo(){        bar();        test();}

在这个例子中 ,有局部变量a 外部全局变量b 本地全局变量glo 本地全局函数bar() 外部全局函数test()
编译:

gcc -m32 -fPIC -c test.c -o test.o

汇编代码 objdump -d test.o:

Disassembly of section .text:00000000 <bar>:   0:   55                      push   %ebp   1:   89 e5                   mov    %esp,%ebp   3:   e8 fc ff ff ff          call   4 <bar+0x4>   8:   05 01 00 00 00          add    $0x1,%eax   d:   c7 80 00 00 00 00 01    movl   $0x1,0x0(%eax)  14:   00 00 00   17:   8b 90 00 00 00 00       mov    0x0(%eax),%edx  1d:   c7 02 02 00 00 00       movl   $0x2,(%edx)  23:   8b 80 00 00 00 00       mov    0x0(%eax),%eax  29:   c7 00 03 00 00 00       movl   $0x3,(%eax)  2f:   90                      nop  30:   5d                      pop    %ebp  31:   c3                      ret    00000032 <foo>:  32:   55                      push   %ebp  33:   89 e5                   mov    %esp,%ebp  35:   53                      push   %ebx  36:   83 ec 04                sub    $0x4,%esp  39:   e8 fc ff ff ff          call   3a <foo+0x8>  3e:   81 c3 02 00 00 00       add    $0x2,%ebx  44:   e8 fc ff ff ff          call   45 <foo+0x13>  49:   e8 fc ff ff ff          call   4a <foo+0x18>  4e:   90                      nop  4f:   83 c4 04                add    $0x4,%esp  52:   5b                      pop    %ebx  53:   5d                      pop    %ebp  54:   c3                      ret    Disassembly of section .text.__x86.get_pc_thunk.ax:00000000 <__x86.get_pc_thunk.ax>:   0:   8b 04 24                mov    (%esp),%eax   3:   c3                      ret    Disassembly of section .text.__x86.get_pc_thunk.bx:00000000 <__x86.get_pc_thunk.bx>:   0:   8b 1c 24                mov    (%esp),%ebx   3:   c3                      ret    

elf信息: readelf -a test.o

ELF 头：  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:                              REL (可重定位文件)  Machine:                           Intel 80386  Version:                           0x1  入口点地址：               0x0  程序头起点：          0 (bytes into file)  Start of section headers:          1044 (bytes into file)  标志：             0x0  本头的大小：       52 (字节)  程序头大小：       0 (字节)  Number of program headers:         0  节头大小：         40 (字节)  节头数量：         16  字符串表索引节头： 13节头：  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al  [ 0]                   NULL            00000000 000000 000000 00      0   0  0  [ 1] .group            GROUP           00000000 000034 000008 04     14  15  4  [ 2] .group            GROUP           00000000 00003c 000008 04     14  19  4  [ 3] .text             PROGBITS        00000000 000044 000055 00  AX  0   0  1  [ 4] .rel.text         REL             00000000 0003ac 000048 08   I 14   3  4  [ 5] .data             PROGBITS        00000000 00009c 000008 00  WA  0   0  4  [ 6] .bss              NOBITS          00000000 0000a4 000000 00  WA  0   0  1  [ 7] .text.__x86.get_p PROGBITS        00000000 0000a4 000004 00 AXG  0   0  1  [ 8] .text.__x86.get_p PROGBITS        00000000 0000a8 000004 00 AXG  0   0  1  [ 9] .comment          PROGBITS        00000000 0000ac 00002e 01  MS  0   0  1  [10] .note.GNU-stack   PROGBITS        00000000 0000da 000000 00      0   0  1  [11] .eh_frame         PROGBITS        00000000 0000dc 000084 00   A  0   0  4  [12] .rel.eh_frame     REL             00000000 0003f4 000020 08   I 14  11  4  [13] .shstrtab         STRTAB          00000000 000160 000096 00      0   0  1  [14] .symtab           SYMTAB          00000000 0001f8 000150 10     15  13  4  [15] .strtab           STRTAB          00000000 000348 000062 00      0   0  1Key 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)COMDAT group section [    1] `.group' [__x86.get_pc_thunk.ax] contains 1 sections:   [Index]    Name   [    7]   .text.__x86.get_pc_thunk.axCOMDAT group section [    2] `.group' [__x86.get_pc_thunk.bx] contains 1 sections:   [Index]    Name   [    8]   .text.__x86.get_pc_thunk.bx本文件中没有程序头。重定位节 '.rel.text' 位于偏移量 0x3ac 含有 9 个条目： Offset     Info    Type            Sym.Value  Sym. Name00000004  00000f02 R_386_PC32        00000000   __x86.get_pc_thunk.ax00000009  0000100a R_386_GOTPC       00000000   _GLOBAL_OFFSET_TABLE_0000000f  00000309 R_386_GOTOFF      00000000   .data00000019  00001103 R_386_GOT32       00000000   b00000025  00000d03 R_386_GOT32       00000004   glo0000003a  00001302 R_386_PC32        00000000   __x86.get_pc_thunk.bx00000040  0000100a R_386_GOTPC       00000000   _GLOBAL_OFFSET_TABLE_00000045  00000e04 R_386_PLT32       00000000   bar0000004a  00001404 R_386_PLT32       00000000   test重定位节 '.rel.eh_frame' 位于偏移量 0x3f4 含有 4 个条目： Offset     Info    Type            Sym.Value  Sym. Name00000020  00000202 R_386_PC32        00000000   .text00000040  00000202 R_386_PC32        00000000   .text00000064  00000602 R_386_PC32        00000000   .text.__x86.get_pc_thu00000078  00000702 R_386_PC32        00000000   .text.__x86.get_pc_thuThe decoding of unwind sections for machine type Intel 80386 is not currently supported.Symbol table '.symtab' contains 21 entries:   Num:    Value  Size Type    Bind   Vis      Ndx Name     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND      1: 00000000     0 FILE    LOCAL  DEFAULT  ABS testpic.c     2: 00000000     0 SECTION LOCAL  DEFAULT    3      3: 00000000     0 SECTION LOCAL  DEFAULT    5      4: 00000000     0 SECTION LOCAL  DEFAULT    6      5: 00000000     4 OBJECT  LOCAL  DEFAULT    5 a     6: 00000000     0 SECTION LOCAL  DEFAULT    7      7: 00000000     0 SECTION LOCAL  DEFAULT    8      8: 00000000     0 SECTION LOCAL  DEFAULT   10      9: 00000000     0 SECTION LOCAL  DEFAULT   11     10: 00000000     0 SECTION LOCAL  DEFAULT    9     11: 00000000     0 SECTION LOCAL  DEFAULT    1     12: 00000000     0 SECTION LOCAL  DEFAULT    2     13: 00000004     4 OBJECT  GLOBAL DEFAULT    5 glo    14: 00000000    50 FUNC    GLOBAL DEFAULT    3 bar    15: 00000000     0 FUNC    GLOBAL HIDDEN     7 __x86.get_pc_thunk.ax    16: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND _GLOBAL_OFFSET_TABLE_    17: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND b    18: 00000032    35 FUNC    GLOBAL DEFAULT    3 foo    19: 00000000     0 FUNC    GLOBAL HIDDEN     8 __x86.get_pc_thunk.bx    20: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND testNo version information found in this file.

R_386_PC32 符号地址+重定位处值-重定位地址
R_386_GOTPC got.plt地址(GOT)+重定位处值-重定位地址 用于确定
R_386_GOTOFF 符号地址-GOT 局部变量位于.data中利用相对GOT的值修改局部变量值 用于全局静态变量
R_386_GOT32 符号地址(在.got中) 全局变量在.got中的偏移 全局变量
R_386_PLT32 函数在.plt地址(bar@plt)+重定位处值-重定位地址 函数