Programmer self-cultivation ch7 Dynamic Link

1. DSO, Dynamic Shared Objects

[serverate: /local/root/c/ch7 ]>> cat lib.h#ifndef LIB_H#define LIB_Hvoid foo(int i);#endif[serverate: /local/root/c/ch7 ]>> cat lib.c#include <stdio.h>void fun(int i){        printf("printing from lib.so %d\n", i);        sleep(-1);}>> cat p1.c#include "lib.h"int main(){        fun(1);        return 0;}[serverate: /local/root/c/ch7 ]>> cat p2.c#include "lib.h"int main(){        fun(2);        return 0;}test 1:[serverate: /local/root/c/ch7 ]>> gcc -m32 -shared -o lib.so lib.c[serverate: /local/root/c/ch7 ]>> gcc -m32 -o p1 p1.c ./lib.so[serverate: /local/root/c/ch7 ]>> gcc -m32 -o p2 p2.c ./lib.so>> ./p1 &[2]     24559[serverate: /local/root/c/ch7 ]>> cd /proc/24559[serverate: /proc/24559 ]>> cat maps08048000-08049000 r-xp 00000000 08:06 21915275                           /local/root/c/ch7/p108049000-0804a000 rw-p 00000000 08:06 21915275                           /local/root/c/ch7/p1f7dca000-f7dcb000 rw-p f7dca000 00:00 0f7dcb000-f7f14000 r-xp 00000000 08:01 504034                             /lib32/libc-2.7.sof7f14000-f7f15000 r--p 00149000 08:01 504034                             /lib32/libc-2.7.sof7f15000-f7f17000 rw-p 0014a000 08:01 504034                             /lib32/libc-2.7.sof7f17000-f7f1a000 rw-p f7f17000 00:00 0f7f2a000-f7f2b000 rw-p f7f2a000 00:00 0f7f2b000-f7f2c000 r-xp 00000000 08:06 21915273                           /local/root/c/ch7/lib.sof7f2c000-f7f2d000 rw-p 00000000 08:06 21915273                           /local/root/c/ch7/lib.sof7f2d000-f7f30000 rw-p f7f2d000 00:00 0f7f30000-f7f4d000 r-xp 00000000 08:01 504031                             /lib32/ld-2.7.sof7f4d000-f7f4f000 rw-p 0001c000 08:01 504031                             /lib32/ld-2.7.soffca8000-ffcbd000 rw-p 7ffffffea000 00:00 0                              [stack]ffffe000-fffff000 r-xp ffffe000 00:00 0                                  [vdso][serverate: /local/root/c/ch7 ]>> ./p2 &[1]     24575[serverate: /local/root/c/ch7 ]>> cd /proc/24575[serverate: /proc/24575 ]>> cat maps08048000-08049000 r-xp 00000000 08:06 21915277                           /local/root/c/ch7/p208049000-0804a000 rw-p 00000000 08:06 21915277                           /local/root/c/ch7/p2f7e11000-f7e12000 rw-p f7e11000 00:00 0f7e12000-f7f5b000 r-xp 00000000 08:01 504034                             /lib32/libc-2.7.sof7f5b000-f7f5c000 r--p 00149000 08:01 504034                             /lib32/libc-2.7.sof7f5c000-f7f5e000 rw-p 0014a000 08:01 504034                             /lib32/libc-2.7.sof7f5e000-f7f61000 rw-p f7f5e000 00:00 0f7f71000-f7f72000 rw-p f7f71000 00:00 0f7f72000-f7f73000 r-xp 00000000 08:06 21915273                           /local/root/c/ch7/lib.sof7f73000-f7f74000 rw-p 00000000 08:06 21915273                           /local/root/c/ch7/lib.sof7f74000-f7f77000 rw-p f7f74000 00:00 0f7f77000-f7f94000 r-xp 00000000 08:01 504031                             /lib32/ld-2.7.sof7f94000-f7f96000 rw-p 0001c000 08:01 504031                             /lib32/ld-2.7.soffed1000-ffee6000 rw-p 7ffffffea000 00:00 0                              [stack]ffffe000-fffff000 r-xp ffffe000 00:00 0                                  [vdso]test 2:[serverate: /local/root/c/ch7 ]>> gcc -fPIC -m32 -shared -o lib.so lib.c[serverate: /local/root/c/ch7 ]>> gcc -m32 -o p1 p1.c ./lib.so[serverate: /local/root/c/ch7 ]>> gcc -m32 -o p2 p2.c ./lib.so

2. PIC

>> cat pic.cstatic int a;extern int b;extern void ext();void bar(){        a = 1;        b = 2;}void foo(){        bar();        ext();}[serverate: /local/root/c/ch7 ]>> gcc -m32 -fPIC -shared -o pic.so pic.c[serverate: /local/root/c/ch7 ]>> objdump -d pic.sopic.so:     file format elf32-i386Disassembly of section .init:000002dc <_init>: 2dc:   55                      push   %ebp 2dd:   89 e5                   mov    %esp,%ebp 2df:   53                      push   %ebx 2e0:   83 ec 04                sub    $0x4,%esp 2e3:   e8 00 00 00 00          call   2e8 <_init+0xc> 2e8:   5b                      pop    %ebx 2e9:   81 c3 b0 12 00 00       add    $0x12b0,%ebx 2ef:   8b 93 f0 ff ff ff       mov    -0x10(%ebx),%edx 2f5:   85 d2                   test   %edx,%edx 2f7:   74 05                   je     2fe <_init+0x22> 2f9:   e8 2e 00 00 00          call   32c <__gmon_start__@plt> 2fe:   e8 cd 00 00 00          call   3d0 <frame_dummy> 303:   e8 58 01 00 00          call   460 <__do_global_ctors_aux> 308:   58                      pop    %eax 309:   5b                      pop    %ebx 30a:   c9                      leave 30b:   c3                      retDisassembly of section .plt:0000030c <bar@plt-0x10>: 30c:   ff b3 04 00 00 00       pushl  0x4(%ebx) 312:   ff a3 08 00 00 00       jmp    *0x8(%ebx) 318:   00 00                   add    %al,(%eax)        ...0000031c <bar@plt>: 31c:   ff a3 0c 00 00 00       jmp    *0xc(%ebx) 322:   68 00 00 00 00          push   $0x0 327:   e9 e0 ff ff ff          jmp    30c <_init+0x30>0000032c <__gmon_start__@plt>: 32c:   ff a3 10 00 00 00       jmp    *0x10(%ebx) 332:   68 08 00 00 00          push   $0x8 337:   e9 d0 ff ff ff          jmp    30c <_init+0x30>0000033c <_Jv_RegisterClasses@plt>: 33c:   ff a3 14 00 00 00       jmp    *0x14(%ebx) 342:   68 10 00 00 00          push   $0x10 347:   e9 c0 ff ff ff          jmp    30c <_init+0x30>0000034c <ext@plt>: 34c:   ff a3 18 00 00 00       jmp    *0x18(%ebx) 352:   68 18 00 00 00          push   $0x18 357:   e9 b0 ff ff ff          jmp    30c <_init+0x30>0000035c <__cxa_finalize@plt>: 35c:   ff a3 1c 00 00 00       jmp    *0x1c(%ebx) 362:   68 20 00 00 00          push   $0x20 367:   e9 a0 ff ff ff          jmp    30c <_init+0x30>Disassembly of section .text:00000370 <__do_global_dtors_aux>: 370:   55                      push   %ebp 371:   89 e5                   mov    %esp,%ebp 373:   53                      push   %ebx 374:   e8 91 00 00 00          call   40a <__i686.get_pc_thunk.bx> 379:   81 c3 1f 12 00 00       add    $0x121f,%ebx 37f:   83 ec 04                sub    $0x4,%esp 382:   80 bb 28 00 00 00 00    cmpb   $0x0,0x28(%ebx) 389:   75 38                   jne    3c3 <__do_global_dtors_aux+0x53> 38b:   8b 83 fc ff ff ff       mov    -0x4(%ebx),%eax 391:   85 c0                   test   %eax,%eax 393:   74 1b                   je     3b0 <__do_global_dtors_aux+0x40> 395:   8b 83 20 00 00 00       mov    0x20(%ebx),%eax 39b:   89 04 24                mov    %eax,(%esp) 39e:   e8 b9 ff ff ff          call   35c <__cxa_finalize@plt> 3a3:   eb 0b                   jmp    3b0 <__do_global_dtors_aux+0x40> 3a5:   83 c0 04                add    $0x4,%eax 3a8:   89 83 24 00 00 00       mov    %eax,0x24(%ebx) 3ae:   ff d2                   call   *%edx 3b0:   8b 83 24 00 00 00       mov    0x24(%ebx),%eax 3b6:   8b 10                   mov    (%eax),%edx 3b8:   85 d2                   test   %edx,%edx 3ba:   75 e9                   jne    3a5 <__do_global_dtors_aux+0x35> 3bc:   c6 83 28 00 00 00 01    movb   $0x1,0x28(%ebx) 3c3:   83 c4 04                add    $0x4,%esp 3c6:   5b                      pop    %ebx 3c7:   c9                      leave 3c8:   c3                      ret 3c9:   8d b4 26 00 00 00 00    lea    0x0(%esi),%esi000003d0 <frame_dummy>: 3d0:   55                      push   %ebp 3d1:   89 e5                   mov    %esp,%ebp 3d3:   53                      push   %ebx 3d4:   e8 31 00 00 00          call   40a <__i686.get_pc_thunk.bx> 3d9:   81 c3 bf 11 00 00       add    $0x11bf,%ebx 3df:   83 ec 04                sub    $0x4,%esp 3e2:   8b 8b 2c ff ff ff       mov    -0xd4(%ebx),%ecx 3e8:   85 c9                   test   %ecx,%ecx 3ea:   74 18                   je     404 <frame_dummy+0x34> 3ec:   8b 93 f4 ff ff ff       mov    -0xc(%ebx),%edx 3f2:   85 d2                   test   %edx,%edx 3f4:   74 0e                   je     404 <frame_dummy+0x34> 3f6:   8d 83 2c ff ff ff       lea    -0xd4(%ebx),%eax 3fc:   89 04 24                mov    %eax,(%esp) 3ff:   e8 38 ff ff ff          call   33c <_Jv_RegisterClasses@plt> 404:   83 c4 04                add    $0x4,%esp 407:   5b                      pop    %ebx 408:   c9                      leave 409:   c3                      ret0000040a <__i686.get_pc_thunk.bx>: 40a:   8b 1c 24                mov    (%esp),%ebx 40d:   c3                      ret 40e:   90                      nop 40f:   90                      nop00000410 <bar>: 410:   55                      push   %ebp 411:   89 e5                   mov    %esp,%ebp 413:   e8 40 00 00 00          call   458 <__i686.get_pc_thunk.cx> 418:   81 c1 80 11 00 00       add    $0x1180,%ecx 41e:   c7 81 2c 00 00 00 01    movl   $0x1,0x2c(%ecx) 425:   00 00 00 428:   8b 81 f8 ff ff ff       mov    -0x8(%ecx),%eax 42e:   c7 00 02 00 00 00       movl   $0x2,(%eax) 434:   c9                      leave 435:   c3                      ret00000436 <foo>: 436:   55                      push   %ebp 437:   89 e5                   mov    %esp,%ebp 439:   53                      push   %ebx 43a:   83 ec 04                sub    $0x4,%esp 43d:   e8 c8 ff ff ff          call   40a <__i686.get_pc_thunk.bx> 442:   81 c3 56 11 00 00       add    $0x1156,%ebx 448:   e8 cf fe ff ff          call   31c <bar@plt> 44d:   e8 fa fe ff ff          call   34c <ext@plt> 452:   83 c4 04                add    $0x4,%esp 455:   5b                      pop    %ebx 456:   c9                      leave 457:   c3                      ret00000458 <__i686.get_pc_thunk.cx>: 458:   8b 0c 24                mov    (%esp),%ecx 45b:   c3                      ret 45c:   90                      nop 45d:   90                      nop 45e:   90                      nop 45f:   90                      nop00000460 <__do_global_ctors_aux>: 460:   55                      push   %ebp 461:   89 e5                   mov    %esp,%ebp 463:   56                      push   %esi 464:   53                      push   %ebx 465:   e8 a0 ff ff ff          call   40a <__i686.get_pc_thunk.bx> 46a:   81 c3 2e 11 00 00       add    $0x112e,%ebx 470:   8d 83 20 ff ff ff       lea    -0xe0(%ebx),%eax 476:   8d 70 fc                lea    -0x4(%eax),%esi 479:   8b 40 fc                mov    -0x4(%eax),%eax 47c:   eb 09                   jmp    487 <__do_global_ctors_aux+0x27> 47e:   66 90                   xchg   %ax,%ax 480:   83 ee 04                sub    $0x4,%esi 483:   ff d0                   call   *%eax 485:   8b 06                   mov    (%esi),%eax 487:   83 f8 ff                cmp    $0xffffffff,%eax 48a:   75 f4                   jne    480 <__do_global_ctors_aux+0x20> 48c:   5b                      pop    %ebx 48d:   5e                      pop    %esi 48e:   c9                      leave 48f:   90                      nop 490:   c3                      ret 491:   90                      nop 492:   90                      nop 493:   90                      nopDisassembly of section .fini:00000494 <_fini>: 494:   55                      push   %ebp 495:   89 e5                   mov    %esp,%ebp 497:   53                      push   %ebx 498:   83 ec 04                sub    $0x4,%esp 49b:   e8 00 00 00 00          call   4a0 <_fini+0xc> 4a0:   5b                      pop    %ebx 4a1:   81 c3 f8 10 00 00       add    $0x10f8,%ebx 4a7:   e8 c4 fe ff ff          call   370 <__do_global_dtors_aux> 4ac:   59                      pop    %ecx 4ad:   5b                      pop    %ebx 4ae:   c9                      leave 4af:   c3                      ret

2.1 Internal call or redirection of the module
Call 31c <bar @ plt>

2.2 Module Internal data access
Data access between 2.3 modules
GOT: Global offset table
We can see from the following that the location of got is 00001588.

>> objdump -h pic.sopic.so:     file format elf32-i386Sections:Idx Name          Size      VMA       LMA       File off  Algn  0 .hash         00000048  000000b4  000000b4  000000b4  2**2                  CONTENTS, ALLOC, LOAD, READONLY, DATA  1 .dynsym       000000d0  000000fc  000000fc  000000fc  2**2                  CONTENTS, ALLOC, LOAD, READONLY, DATA  2 .dynstr       0000007b  000001cc  000001cc  000001cc  2**0                  CONTENTS, ALLOC, LOAD, READONLY, DATA  3 .gnu.version  0000001a  00000248  00000248  00000248  2**1                  CONTENTS, ALLOC, LOAD, READONLY, DATA  4 .gnu.version_r 00000020  00000264  00000264  00000264  2**2                  CONTENTS, ALLOC, LOAD, READONLY, DATA  5 .rel.dyn      00000030  00000284  00000284  00000284  2**2                  CONTENTS, ALLOC, LOAD, READONLY, DATA  6 .rel.plt      00000028  000002b4  000002b4  000002b4  2**2                  CONTENTS, ALLOC, LOAD, READONLY, DATA  7 .init         00000030  000002dc  000002dc  000002dc  2**2                  CONTENTS, ALLOC, LOAD, READONLY, CODE  8 .plt          00000060  0000030c  0000030c  0000030c  2**2                  CONTENTS, ALLOC, LOAD, READONLY, CODE  9 .text         00000124  00000370  00000370  00000370  2**4                  CONTENTS, ALLOC, LOAD, READONLY, CODE 10 .fini         0000001c  00000494  00000494  00000494  2**2                  CONTENTS, ALLOC, LOAD, READONLY, CODE 11 .eh_frame     00000004  000004b0  000004b0  000004b0  2**2                  CONTENTS, ALLOC, LOAD, READONLY, DATA 12 .ctors        00000008  000014b4  000014b4  000004b4  2**2                  CONTENTS, ALLOC, LOAD, DATA 13 .dtors        00000008  000014bc  000014bc  000004bc  2**2                  CONTENTS, ALLOC, LOAD, DATA 14 .jcr          00000004  000014c4  000014c4  000004c4  2**2                  CONTENTS, ALLOC, LOAD, DATA 15 .dynamic      000000c0  000014c8  000014c8  000004c8  2**2                  CONTENTS, ALLOC, LOAD, DATA 16 .got          00000010  00001588  00001588  00000588  2**2                  CONTENTS, ALLOC, LOAD, DATA 17 .got.plt      00000020  00001598  00001598  00000598  2**2                  CONTENTS, ALLOC, LOAD, DATA 18 .data         00000008  000015b8  000015b8  000005b8  2**2                  CONTENTS, ALLOC, LOAD, DATA 19 .bss          00000008  000015c0  000015c0  000005c0  2**2                  ALLOC 20 .comment      000000d2  00000000  00000000  000005c0  2**0                  CONTENTS, READONLY 21 .debug_aranges 00000050  00000000  00000000  00000698  2**3                  CONTENTS, READONLY, DEBUGGING 22 .debug_info   0000011c  00000000  00000000  000006e8  2**0                  CONTENTS, READONLY, DEBUGGING 23 .debug_abbrev 00000024  00000000  00000000  00000804  2**0                  CONTENTS, READONLY, DEBUGGING 24 .debug_line   00000104  00000000  00000000  00000828  2**0                  CONTENTS, READONLY, DEBUGGING 25 .debug_ranges 00000040  00000000  00000000  00000930  2**3                  CONTENTS, READONLY, DEBUGGING>> objdump -R pic.sopic.so:     file format elf32-i386DYNAMIC RELOCATION RECORDSOFFSET   TYPE              VALUE000015b8 R_386_RELATIVE    *ABS*000015bc R_386_RELATIVE    *ABS*00001588 R_386_GLOB_DAT    __gmon_start__0000158c R_386_GLOB_DAT    _Jv_RegisterClasses00001590 R_386_GLOB_DAT    b00001594 R_386_GLOB_DAT    __cxa_finalize000015a4 R_386_JUMP_SLOT   bar000015a8 R_386_JUMP_SLOT   __gmon_start__000015ac R_386_JUMP_SLOT   _Jv_RegisterClasses000015b0 R_386_JUMP_SLOT   ext000015b4 R_386_JUMP_SLOT   __cxa_finalize

The offset of variable B is 1590.
This corresponds to the code,

 428:   8b 81 f8 ff ff ff       mov    -0x8(%ecx),%eax 42e:   c7 00 02 00 00 00       movl   $0x2,(%eax)

2.4 inter-module calls and redirection
How to identify whether a DSO is PIC?

[serverate: /local/root/c/ch7 ]>> gcc -m32 -shared -o lib.so lib.c[serverate: /local/root/c/ch7 ]>> readelf -d lib.so |grep TEXTREL 0x00000016 (TEXTREL)                    0x0

If there is output, it is not PIC. The pic dso does not have a code segment to relocate the table address.
PIE: Position-Independent Executable-fPIE

3. global variables of shared modules
Question: If a shared object lib. so defines A global variable G, while process A and process B both use lib. so, will G in process B be affected when process A changes the value of the global variable G?
No, because when lib. when so is loaded by two processes, its data segment has an independent copy in each process. From this perspective, the global variables in the shared object are actually no different from those defined in the program. If it is two threads in the same process, you will see the other party's modifications to the global variables.