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ZDNet>网络频道>ZD评测>UNIX系统 目标文件初探

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UNIX(R) 系统中运行的程序遵守一种称为目标文件格式的精心设计。了解更多关于目标文件格式的内容,以及可以用来研究系统中目标文件的工具。

来源:chinaitlab 2007年12月27日

关键字:系统安全 unix

UNIX(R) 系统中运行的程序遵守一种称为目标文件格式的精心设计。了解更多关于目标文件格式的内容,以及可以用来研究系统中目标文件的工具。

    计算机编程的最新技术将一种特殊的人性与一组特殊的工具结合在一起,用以生产出对其他人非常有帮助的一种神奇的产品,即软件。计算机程序员是一群注重细节的人,他们可以处理计算机中各种各样的困难。计算机的要求非常苛刻,并且不能容忍其中存在任何的偏差。毫无疑问,无论您的个性如何以及在工作中使用了何种辅助工具,计算机程序的编写都是非常困难的。

    在 UNIX® 和 Linux® 中,任何事物都是文件。您可以认为,UNIX 和 Linux 编程实际上是编写处理各种文件的代码。系统由许多类型的文件组成,但目标文件具有一种特殊的设计,提供了灵活和多样的用途。

    目标文件是包含带有附加地址和值的助记符号的路线图。这些符号可以用来对各种代码段和数据段进行命名,包括经过初始化的和未初始化的。它们也可以用来定位嵌入的调试信息,就像语义 Web,非常适合由程序进行阅读。

行业工具

    计算机编程中使用的工具包括代码编辑器,如 vi 或 Emacs,您可以使用这些工具输入和编辑希望计算机在完成所需任务时执行的指令,以及编译器和连接器,它们可以生成真正实现这些目标的机器代码。

    高级的工具,称为集成调试环境 (IDE),它以统一的外观集成了不同工具的功能。IDE 使得编辑器、编译器、连接器和调试器之间的界限变得很模糊。因此,为了更深入地研究和了解系统,在使用集成的套件之前,最好先单独地使用这些工具。(注意:IDE 也通常被称为集成开发环境。)

    编译器可以将您在代码编辑器中创建的文本转换为目标文件。最初,目标文件被称为代码的中间表示形式,因为它用作连接编辑器(即连接器)的输入,而连接编辑器最终完成整个任务并生成可执行的程序作为输出。

     从代码到可执行代码的转换过程经过了良好的定义并实现了自动化,而目标文件是这个链中有机的连接性环节。在这个转换过程中,目标文件作为连接编辑器所使用的映象,使得它们能够解析各种符号并将不同的代码和数据段连接在一起形成统一的整体。

历史

   计算机编程领域中存在许多著名的目标文件格式。DOS 系列包括 COM、OBJ 和 EXE 格式。UNIX 和 Linux 使用 a.out、COFF 和 ELF。Microsoft® Windows® 使用可移植的执行文件 (PE) 格式,而 Macintosh 使用 PEF、Mach-O 和其他文件格式。

    最初,各种类型的计算机具有自己独特的目标文件格式,但随着 UNIX 和其他在不同硬件平台上提供可移植性的操作系统的出现,一些常用的文件格式上升为通用的标准。其中包括 a.out、COFF 和 ELF 格式。

    要了解目标文件,需要一组可以读取目标文件中不同部分并以更易于读取的格式显示这些内容的工具。本文将讨论这些工具中比较重要的方面。但首先,您必须创建一个工作台,并在其中建立一个研究对象。

工作台

    启动一个 xterm 会话,让我们先创建一个空白的工作台,并开始对目标文件进行研究。下面的命令创建了一个目录,可以将目标文件放到该目录中进行研究:

cd
mkdir src
cd src
mkdir hw
cd hw

然后,使用您最喜欢的代码编辑器,在 $HOME/src/hw 目录中输入清单 1 中的程序,并命名为 hw.c。


清单 1. hw.c 程序

    要使用 UNIX 工具库中提供的各种工具,可以将这个简单的“Hello World”程序作为研究的对象。您将学习构建和查看目标文件的输出,而不是使用任何快捷方法直接创建可执行文件(的确有许多这样的快捷方法)。

文件格式

    C 编译器的正常输出是用于您所指定的目标处理器的汇编代码。汇编代码是汇编器的输入,在缺省情况下,汇编器将生成所有目标文件的祖先,即 a.out 文件。这个名称本身表示汇编输出 (Assembler Output)。要创建 a.out 文件,可以在 xterm 窗口中输入下面的命令:


    注意:如果出现了任何错误或者没有创建 a.out 文件,那么您可能需要检查自己的系统或源文件 (hw.c),以找出其中的错误。还需要检查是否已将 cc 定义为运行您的 C/C++ 编译器。

    最新的 C 编译器将编译和汇编步骤组合成一个步骤。您可以指定不同开关选项以查看 C 编译器的汇编输出。通过输入下面的命令,您可以看到 C 编译器的汇编输出:

cc -S hw.c

     这个命令生成了一个新的文件 hw.s,其中包含您通常无法看到的汇编输入文本,因为编译器在缺省情况下将生成 a.out 文件。正如所预期的,UNIX 汇编程序可以对这种输入文件进行汇编,以生成 a.out 文件。

UNIX 特定的工具

    假定编译过程一切顺利,那么在该目录中就有了一个 a.out 文件,下面让我们来对其进行研究。有许多可用于研究目标文件的有价值的工具,下面便是其中一组:

  • nm:列出目标文件中的符号。
  • objdump:显示目标文件中的详细信息。
  • readelf:显示关于 ELF 目标文件的信息。

    列表中的第一个工具是 nm,它可以列出目标文件中的符号。如果您输入 nm 命令,您将注意到在缺省情况下,它会寻找一个名为 a.out 的文件。如果没有找到该文件,这个工具会给出相应的提示。然而,如果该工具找到了编译器创建的 a.out 文件,它将显示类似清单 2 的清单。


清单 2. nm 命令的输出
	
08049594 A __bss_start
080482e4 t call_gmon_start
08049594 b completed.4463
08049498 d __CTOR_END__
08049494 d __CTOR_LIST__
08049588 D __data_start
08049588 W data_start
0804842c t __do_global_ctors_aux
0804830c t __do_global_dtors_aux
0804958c D __dso_handle
080494a0 d __DTOR_END__
0804949c d __DTOR_LIST__
080494a8 d _DYNAMIC
08049594 A _edata
08049598 A _end
08048458 T _fini
08049494 a __fini_array_end
08049494 a __fini_array_start
08048478 R _fp_hw
0804833b t frame_dummy
08048490 r __FRAME_END__
08049574 d _GLOBAL_OFFSET_TABLE_
         w __gmon_start__
08048308 T __i686.get_pc_thunk.bx
08048278 T _init
08049494 a __init_array_end
08049494 a __init_array_start
0804847c R _IO_stdin_used
080494a4 d __JCR_END__
080494a4 d __JCR_LIST__
         w _Jv_RegisterClasses
080483e1 T __libc_csu_fini
08048390 T __libc_csu_init
         U __libc_start_main@@GLIBC_2.0
08048360 T main
08049590 d p.4462
         U puts@@GLIBC_2.0
080482c0 T _start

   这些包含可执行代码的段称为正文段。同样地,数据段包含了不可执行的信息或数据。另一种类型的段,称为 BSS 段,它包含以符号数据开头的块。

    对于 nm 命令列出的每个符号,它们的值使用十六进制来表示(缺省行为),并且在该符号前面加上了一个表示符号类型的编码字符。常见的各种编码包括:A 表示绝对 (absolute),这意味着不能将该值更改为其他的连接;B 表示 BSS 段中的符号;而 C 表示引用未初始化的数据的一般符号。

    可以将目标文件中所包含的不同的部分划分为段。段可以包含可执行代码、符号名称、初始数据值和许多其他类型的数据。有关这些类型的数据的详细信息,可以阅读 UNIX 中 nm 的 man 页面,其中按照该命令输出中的字符编码分别对每种类型进行了描述。

细节,细节…

    在目标文件阶段,即使是一个简单的 Hello World 程序,其中也包含了大量的细节信息。nm 程序可用于列举符号及其类型和值,但是,要更仔细地研究目标文件中这些命名段的内容,需要使用功能更强大的工具。

    其中两种功能强大的工具是 objdump 和 readelf 程序。通过输入下面的命令,您可以看到目标文件中包含可执行代码的每个段的汇编清单。对于这么一个小的程序,编译器生成了这么多的代码,真的很令人惊异!

objdump -d a.out

    这个命令生成的输出如清单 3 所示。每个可执行代码段将在需要特定的事件时执行,这些事件包括库的初始化和该程序本身主入口点。


清单 3. objdump 命令的输出
	
a.out:     file format elf32-i386

Disassembly of section .init:

08048278 <_init>:
 8048278:       55                      push   %ebp
 8048279:       89 e5                   mov    %esp,%ebp
 804827b:       83 ec 08                sub    $0x8,%esp
 804827e:       e8 61 00 00 00          call   80482e4 <call_gmon_start>
 8048283:       e8 b3 00 00 00          call   804833b <frame_dummy>
 8048288:       e8 9f 01 00 00          call   804842c <__do_global_ctors_aux>
 804828d:       c9                      leave
 804828e:       c3                      ret
Disassembly of section .plt:

08048290 <puts@plt-0x10>:
 8048290:       ff 35 78 95 04 08       pushl  0x8049578
 8048296:       ff 25 7c 95 04 08       jmp    *0x804957c
 804829c:       00 00                   add    %al,(%eax)
        ...

080482a0 <puts@plt>:
 80482a0:       ff 25 80 95 04 08       jmp    *0x8049580
 80482a6:       68 00 00 00 00          push   $0x0
 80482ab:       e9 e0 ff ff ff          jmp    8048290 <_init+0x18>

080482b0 <__libc_start_main@plt>:
 80482b0:       ff 25 84 95 04 08       jmp    *0x8049584
 80482b6:       68 08 00 00 00          push   $0x8
 80482bb:       e9 d0 ff ff ff          jmp    8048290 <_init+0x18>
Disassembly of section .text:

080482c0 <_start>:
 80482c0:       31 ed                   xor    %ebp,%ebp
 80482c2:       5e                      pop    %esi
 80482c3:       89 e1                   mov    %esp,%ecx
 80482c5:       83 e4 f0                and    $0xfffffff0,%esp
 80482c8:       50                      push   %eax
 80482c9:       54                      push   %esp
 80482ca:       52                      push   %edx
 80482cb:       68 e1 83 04 08          push   $0x80483e1
 80482d0:       68 90 83 04 08          push   $0x8048390
 80482d5:       51                      push   %ecx
 80482d6:       56                      push   %esi
 80482d7:       68 60 83 04 08          push   $0x8048360
 80482dc:       e8 cf ff ff ff          call   80482b0 <__libc_start_main@plt>
 80482e1:       f4                      hlt
 80482e2:       90                      nop
 80482e3:       90                      nop
080482e4 <call_gmon_start>:
 80482e4:       55                      push   %ebp
 80482e5:       89 e5                   mov    %esp,%ebp
 80482e7:       53                      push   %ebx
 80482e8:       e8 1b 00 00 00          call   8048308 <__i686.get_pc_thunk.bx>
 80482ed:       81 c3 87 12 00 00       add    $0x1287,%ebx
 80482f3:       83 ec 04                sub    $0x4,%esp
 80482f6:       8b 83 fc ff ff ff       mov    0xfffffffc(%ebx),%eax
 80482fc:       85 c0                   test   %eax,%eax
 80482fe:       74 02                   je     8048302 <call_gmon_start+0x1e>
 8048300:       ff d0                   call   *%eax
 8048302:       83 c4 04                add    $0x4,%esp
 8048305:       5b                      pop    %ebx
 8048306:       5d                      pop    %ebp
 8048307:       c3                      ret

08048308 <__i686.get_pc_thunk.bx>:
 8048308:       8b 1c 24                mov    (%esp),%ebx
 804830b:       c3                      ret

0804830c <__do_global_dtors_aux>:
 804830c:       55                      push   %ebp
 804830d:       89 e5                   mov    %esp,%ebp
 804830f:       83 ec 08                sub    $0x8,%esp
 8048312:       80 3d 94 95 04 08 00    cmpb   $0x0,0x8049594
 8048319:       74 0c                   je     8048327 <__do_global_dtors_aux+0x1b>
 804831b:       eb 1c                   jmp    8048339 <__do_global_dtors_aux+0x2d>
 804831d:       83 c0 04                add    $0x4,%eax
 8048320:       a3 90 95 04 08          mov    %eax,0x8049590
 8048325:       ff d2                   call   *%edx
 8048327:       a1 90 95 04 08          mov    0x8049590,%eax
 804832c:       8b 10                   mov    (%eax),%edx
 804832e:       85 d2                   test   %edx,%edx
 8048330:       75 eb                   jne    804831d <__do_global_dtors_aux+0x11>
 8048332:       c6 05 94 95 04 08 01    movb   $0x1,0x8049594
 8048339:       c9                      leave
 804833a:       c3                      ret

0804833b <frame_dummy>:
 804833b:       55                      push   %ebp
 804833c:       89 e5                   mov    %esp,%ebp
 804833e:       83 ec 08                sub    $0x8,%esp
 8048341:       a1 a4 94 04 08          mov    0x80494a4,%eax
 8048346:       85 c0                   test   %eax,%eax
 8048348:       74 12                   je     804835c <frame_dummy+0x21>
 804834a:       b8 00 00 00 00          mov    $0x0,%eax
 804834f:       85 c0                   test   %eax,%eax
 8048351:       74 09                   je     804835c <frame_dummy+0x21>
 8048353:       c7 04 24 a4 94 04 08    movl   $0x80494a4,(%esp)
 804835a:       ff d0                   call   *%eax
 804835c:       c9                      leave
 804835d:       c3                      ret
 804835e:       90                      nop
 804835f:       90                      nop

08048360 <main>:
 8048360:       55                      push   %ebp
 8048361:       89 e5                   mov    %esp,%ebp
 8048363:       83 ec 08                sub    $0x8,%esp
 8048366:       83 e4 f0                and    $0xfffffff0,%esp
 8048369:       b8 00 00 00 00          mov    $0x0,%eax
 804836e:       83 c0 0f                add    $0xf,%eax
 8048371:       83 c0 0f                add    $0xf,%eax
 8048374:       c1 e8 04                shr    $0x4,%eax
 8048377:       c1 e0 04                shl    $0x4,%eax
 804837a:       29 c4                   sub    %eax,%esp
 804837c:       c7 04 24 80 84 04 08    movl   $0x8048480,(%esp)
 8048383:       e8 18 ff ff ff          call   80482a0 <puts@plt>
 8048388:       b8 00 00 00 00          mov    $0x0,%eax
 804838d:       c9                      leave
 804838e:       c3                      ret
 804838f:       90                      nop

08048390 <__libc_csu_init>:
 8048390:       55                      push   %ebp
 8048391:       89 e5                   mov    %esp,%ebp
 8048393:       57                      push   %edi
 8048394:       56                      push   %esi
 8048395:       31 f6                   xor    %esi,%esi
 8048397:       53                      push   %ebx
 8048398:       e8 6b ff ff ff          call   8048308 <__i686.get_pc_thunk.bx>
 804839d:       81 c3 d7 11 00 00       add    $0x11d7,%ebx
 80483a3:       83 ec 0c                sub    $0xc,%esp
 80483a6:       e8 cd fe ff ff          call   8048278 <_init>
 80483ab:       8d 83 20 ff ff ff       lea    0xffffff20(%ebx),%eax
 80483b1:       8d 93 20 ff ff ff       lea    0xffffff20(%ebx),%edx
 80483b7:       89 45 f0                mov    %eax,0xfffffff0(%ebp)
 80483ba:       29 d0                   sub    %edx,%eax
 80483bc:       c1 f8 02                sar    $0x2,%eax
 80483bf:       39 c6                   cmp    %eax,%esi
 80483c1:       73 16                   jae    80483d9 <__libc_csu_init+0x49>
 80483c3:       89 d7                   mov    %edx,%edi
 80483c5:       ff 14 b2                call   *(%edx,%esi,4)
 80483c8:       8b 45 f0                mov    0xfffffff0(%ebp),%eax
 80483cb:       83 c6 01                add    $0x1,%esi
 80483ce:       29 f8                   sub    %edi,%eax
 80483d0:       89 fa                   mov    %edi,%edx
 80483d2:       c1 f8 02                sar    $0x2,%eax
 80483d5:       39 c6                   cmp    %eax,%esi
 80483d7:       72 ec                   jb     80483c5 <__libc_csu_init+0x35>
 80483d9:       83 c4 0c                add    $0xc,%esp
 80483dc:       5b                      pop    %ebx
 80483dd:       5e                      pop    %esi
 80483de:       5f                      pop    %edi
 80483df:       5d                      pop    %ebp
 80483e0:       c3                      ret

080483e1 <__libc_csu_fini>:
 80483e1:       55                      push   %ebp
 80483e2:       89 e5                   mov    %esp,%ebp
 80483e4:       83 ec 18                sub    $0x18,%esp
 80483e7:       89 5d f4                mov    %ebx,0xfffffff4(%ebp)
 80483ea:       e8 19 ff ff ff          call   8048308 <__i686.get_pc_thunk.bx>
 80483ef:       81 c3 85 11 00 00       add    $0x1185,%ebx
 80483f5:       89 75 f8                mov    %esi,0xfffffff8(%ebp)
 80483f8:       89 7d fc                mov    %edi,0xfffffffc(%ebp)
 80483fb:       8d b3 20 ff ff ff       lea    0xffffff20(%ebx),%esi
 8048401:       8d bb 20 ff ff ff       lea    0xffffff20(%ebx),%edi
 8048407:       29 fe                   sub    %edi,%esi
 8048409:       c1 fe 02                sar    $0x2,%esi
 804840c:       eb 03                   jmp    8048411 <__libc_csu_fini+0x30>
 804840e:       ff 14 b7                call   *(%edi,%esi,4)
 8048411:       83 ee 01                sub    $0x1,%esi
 8048414:       83 fe ff                cmp    $0xffffffff,%esi
 8048417:       75 f5                   jne    804840e <__libc_csu_fini+0x2d>
 8048419:       e8 3a 00 00 00          call   8048458 <_fini>
 804841e:       8b 5d f4                mov    0xfffffff4(%ebp),%ebx
 8048421:       8b 75 f8                mov    0xfffffff8(%ebp),%esi
 8048424:       8b 7d fc                mov    0xfffffffc(%ebp),%edi
 8048427:       89 ec                   mov    %ebp,%esp
 8048429:       5d                      pop    %ebp
 804842a:       c3                      ret
 804842b:       90                      nop

0804842c <__do_global_ctors_aux>:
 804842c:       55                      push   %ebp
 804842d:       89 e5                   mov    %esp,%ebp
 804842f:       53                      push   %ebx
 8048430:       83 ec 04                sub    $0x4,%esp
 8048433:       a1 94 94 04 08          mov    0x8049494,%eax
 8048438:       83 f8 ff                cmp    $0xffffffff,%eax
 804843b:       74 12                   je     804844f <__do_global_ctors_aux+0x23>
 804843d:       bb 94 94 04 08          mov    $0x8049494,%ebx
 8048442:       ff d0                   call   *%eax
 8048444:       8b 43 fc                mov    0xfffffffc(%ebx),%eax
 8048447:       83 eb 04                sub    $0x4,%ebx
 804844a:       83 f8 ff                cmp    $0xffffffff,%eax
 804844d:       75 f3                   jne    8048442 <__do_global_ctors_aux+0x16>
 804844f:       83 c4 04                add    $0x4,%esp
 8048452:       5b                      pop    %ebx
 8048453:       5d                      pop    %ebp
 8048454:       c3                      ret
 8048455:       90                      nop
 8048456:       90                      nop
 8048457:       90                      nop
Disassembly of section .fini:

08048458 <_fini>:
 8048458:       55                      push   %ebp
 8048459:       89 e5                   mov    %esp,%ebp
 804845b:       53                      push   %ebx
 804845c:       e8 a7 fe ff ff          call   8048308 <__i686.get_pc_thunk.bx>
 8048461:       81 c3 13 11 00 00       add    $0x1113,%ebx
 8048467:       83 ec 04                sub    $0x4,%esp
 804846a:       e8 9d fe ff ff          call   804830c <__do_global_dtors_aux>
 804846f:       83 c4 04                add    $0x4,%esp
 8048472:       5b                      pop    %ebx
 8048473:       5d                      pop    %ebp
 8048474:       c3                      ret

    对于那些着迷于底层编程细节的程序员来说,这是一个功能非常强大的工具,可用于研究编译器和汇编器的输出。细节信息,比如这段代码中所显示的这些信息,可以揭示有关本地处理器本身运行方式的很多内容。对该处理器制造商提供的技术文档进行深入的研究,您可以收集关于一些有价值的信息,通过这些信息可以深入地了解内部的运行机制,因为功能程序提供了清晰的输出。

    类似地,readelf 程序也可以清楚地列出目标文件中的内容。输入下面的命令,您将可以看到这一点:

readelf -all a.out

    这个命令生成的输出如清单 4 所示。ELF Header 为该文件中所有段入口显示了详细的摘要。在列举出这些 Header 中的内容之前,您可以看到 Header 的具体数目。在研究一个较大的目标文件时,该信息可能非常有用。


清单 4. readelf 命令的输出
	
ELF Header:
  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:                              EXEC (Executable file)
  Machine:                           Intel 80386
  Version:                           0x1
  Entry point address:               0x80482c0
  Start of program headers:          52 (bytes into file)
  Start of section headers:          3504 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         7
  Size of section headers:           40 (bytes)
  Number of section headers:         34
  Section header string table index: 31

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .interp           PROGBITS        08048114 000114 000013 00   A  0   0  1
  [ 2] .note.ABI-tag     NOTE            08048128 000128 000020 00   A  0   0  4
  [ 3] .hash             HASH            08048148 000148 00002c 04   A  4   0  4
  [ 4] .dynsym           DYNSYM          08048174 000174 000060 10   A  5   1  4
  [ 5] .dynstr           STRTAB          080481d4 0001d4 00005e 00   A  0   0  1
  [ 6] .gnu.version      VERSYM          08048232 000232 00000c 02   A  4   0  2
  [ 7] .gnu.version_r    VERNEED         08048240 000240 000020 00   A  5   1  4
  [ 8] .rel.dyn          REL             08048260 000260 000008 08   A  4   0  4
  [ 9] .rel.plt          REL             08048268 000268 000010 08   A  4  11  4
  [10] .init             PROGBITS        08048278 000278 000017 00  AX  0   0  1
  [11] .plt              PROGBITS        08048290 000290 000030 04  AX  0   0  4
  [12] .text             PROGBITS        080482c0 0002c0 000198 00  AX  0   0  4
  [13] .fini             PROGBITS        08048458 000458 00001d 00  AX  0   0  1
  [14] .rodata           PROGBITS        08048478 000478 000015 00   A  0   0  4
  [15] .eh_frame         PROGBITS        08048490 000490 000004 00   A  0   0  4
  [16] .ctors            PROGBITS        08049494 000494 000008 00  WA  0   0  4
  [17] .dtors            PROGBITS        0804949c 00049c 000008 00  WA  0   0  4
  [18] .jcr              PROGBITS        080494a4 0004a4 000004 00  WA  0   0  4
  [19] .dynamic          DYNAMIC         080494a8 0004a8 0000c8 08  WA  5   0  4
  [20] .got              PROGBITS        08049570 000570 000004 04  WA  0   0  4
  [21] .got.plt          PROGBITS        08049574 000574 000014 04  WA  0   0  4
  [22] .data             PROGBITS        08049588 000588 00000c 00  WA  0   0  4
  [23] .bss              NOBITS          08049594 000594 000004 00  WA  0   0  4
  [24] .comment          PROGBITS        00000000 000594 000126 00      0   0  1
  [25] .debug_aranges    PROGBITS        00000000 0006c0 000088 00      0   0  8
  [26] .debug_pubnames   PROGBITS        00000000 000748 000025 00      0   0  1
  [27] .debug_info       PROGBITS        00000000 00076d 00022b 00      0   0  1
  [28] .debug_abbrev     PROGBITS        00000000 000998 000076 00      0   0  1
  [29] .debug_line       PROGBITS        00000000 000a0e 0001bb 00      0   0  1
  [30] .debug_str        PROGBITS        00000000 000bc9 0000bf 01  MS  0   0  1
  [31] .shstrtab         STRTAB          00000000 000c88 000127 00      0   0  1
  [32] .symtab           SYMTAB          00000000 001300 000520 10     33  63  4
  [33] .strtab           STRTAB          00000000 001820 0002d2 00      0   0  1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings)
  I (info), L (link order), G (group), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

There are no section groups in this file.

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  PHDR           0x000034 0x08048034 0x08048034 0x000e0 0x000e0 R E 0x4
  INTERP         0x000114 0x08048114 0x08048114 0x00013 0x00013 R   0x1
      [Requesting program interpreter: /lib/ld-linux.so.2]
  LOAD           0x000000 0x08048000 0x08048000 0x00494 0x00494 R E 0x1000
  LOAD           0x000494 0x08049494 0x08049494 0x00100 0x00104 RW  0x1000
  DYNAMIC        0x0004a8 0x080494a8 0x080494a8 0x000c8 0x000c8 RW  0x4
  NOTE           0x000128 0x08048128 0x08048128 0x00020 0x00020 R   0x4
  GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0x4

 Section to Segment mapping:
  Segment Sections...
   00
   01     .interp
   02     .interp .note.ABI-tag .hash .dynsym .dynstr .gnu.version 
          .gnu.version_r .rel.dyn .rel.plt .init .plt .text .fini .rodata .eh_frame
   03     .ctors .dtors .jcr .dynamic .got .got.plt .data .bss
   04     .dynamic
   05     .note.ABI-tag
   06

 
Dynamic section at offset 0x4a8 contains 20 entries:
  Tag        Type                         Name/Value
 0x00000001 (NEEDED)                     Shared library: [libc.so.6]
 0x0000000c (INIT)                       0x8048278
 0x0000000d (FINI)                       0x8048458
 0x00000004 (HASH)                       0x8048148
 0x00000005 (STRTAB)                     0x80481d4
 0x00000006 (SYMTAB)                     0x8048174
 0x0000000a (STRSZ)                      94 (bytes)
 0x0000000b (SYMENT)                     16 (bytes)
 0x00000015 (DEBUG)                      0x0
 0x00000003 (PLTGOT)                     0x8049574
 0x00000002 (PLTRELSZ)                   16 (bytes)
 0x00000014 (PLTREL)                     REL
 0x00000017 (JMPREL)                     0x8048268
 0x00000011 (REL)                        0x8048260
 0x00000012 (RELSZ)                      8 (bytes)
 0x00000013 (RELENT)                     8 (bytes)
 0x6ffffffe (VERNEED)                    0x8048240
 0x6fffffff (VERNEEDNUM)                 1
 0x6ffffff0 (VERSYM)                     0x8048232
 0x00000000 (NULL)                       0x0

Relocation section '.rel.dyn' at offset 0x260 contains 1 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
08049570  00000506 R_386_GLOB_DAT    00000000   __gmon_start__

Relocation section '.rel.plt' at offset 0x268 contains 2 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
08049580  00000107 R_386_JUMP_SLOT   00000000   puts
08049584  00000207 R_386_JUMP_SLOT   00000000   __libc_start_main

There are no unwind sections in this file.

Symbol table '.dynsym' contains 6 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000   378 FUNC    GLOBAL DEFAULT  UND puts@GLIBC_2.0 (2)
     2: 00000000   230 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.0 (2)
     3: 0804847c     4 OBJECT  GLOBAL DEFAULT   14 _IO_stdin_used
     4: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
     5: 00000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__

Symbol table '.symtab' contains 82 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 08048114     0 SECTION LOCAL  DEFAULT    1
     2: 08048128     0 SECTION LOCAL  DEFAULT    2
     3: 08048148     0 SECTION LOCAL  DEFAULT    3
     4: 08048174     0 SECTION LOCAL  DEFAULT    4
     5: 080481d4     0 SECTION LOCAL  DEFAULT    5
     6: 08048232     0 SECTION LOCAL  DEFAULT    6
     7: 08048240     0 SECTION LOCAL  DEFAULT    7
     8: 08048260     0 SECTION LOCAL  DEFAULT    8
     9: 08048268     0 SECTION LOCAL  DEFAULT    9
    10: 08048278     0 SECTION LOCAL  DEFAULT   10
    11: 08048290     0 SECTION LOCAL  DEFAULT   11
    12: 080482c0     0 SECTION LOCAL  DEFAULT   12
    13: 08048458     0 SECTION LOCAL  DEFAULT   13
    14: 08048478     0 SECTION LOCAL  DEFAULT   14
    15: 08048490     0 SECTION LOCAL  DEFAULT   15
    16: 08049494     0 SECTION LOCAL  DEFAULT   16
    17: 0804949c     0 SECTION LOCAL  DEFAULT   17
    18: 080494a4     0 SECTION LOCAL  DEFAULT   18
    19: 080494a8     0 SECTION LOCAL  DEFAULT   19
    20: 08049570     0 SECTION LOCAL  DEFAULT   20
    21: 08049574     0 SECTION LOCAL  DEFAULT   21
    22: 08049588     0 SECTION LOCAL  DEFAULT   22
    23: 08049594     0 SECTION LOCAL  DEFAULT   23
    24: 00000000     0 SECTION LOCAL  DEFAULT   24
    25: 00000000     0 SECTION LOCAL  DEFAULT   25
    26: 00000000     0 SECTION LOCAL  DEFAULT   26
    27: 00000000     0 SECTION LOCAL  DEFAULT   27
    28: 00000000     0 SECTION LOCAL  DEFAULT   28
    29: 00000000     0 SECTION LOCAL  DEFAULT   29
    30: 00000000     0 SECTION LOCAL  DEFAULT   30
    31: 00000000     0 SECTION LOCAL  DEFAULT   31
    32: 00000000     0 SECTION LOCAL  DEFAULT   32
    33: 00000000     0 SECTION LOCAL  DEFAULT   33
    34: 00000000     0 FILE    LOCAL  DEFAULT  ABS abi-note.S
    35: 00000000     0 FILE    LOCAL  DEFAULT  ABS ../sysdeps/i386/elf/start
    36: 00000000     0 FILE    LOCAL  DEFAULT  ABS init.c
    37: 00000000     0 FILE    LOCAL  DEFAULT  ABS initfini.c
    38: 00000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.6
    39: 080482e4     0 FUNC    LOCAL  DEFAULT   12 call_gmon_start
    40: 00000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    41: 08049494     0 OBJECT  LOCAL  DEFAULT   16 __CTOR_LIST__
    42: 0804949c     0 OBJECT  LOCAL  DEFAULT   17 __DTOR_LIST__
    43: 080494a4     0 OBJECT  LOCAL  DEFAULT   18 __JCR_LIST__
    44: 08049594     1 OBJECT  LOCAL  DEFAULT   23 completed.4463
    45: 08049590     0 OBJECT  LOCAL  DEFAULT   22 p.4462
    46: 0804830c     0 FUNC    LOCAL  DEFAULT   12 __do_global_dtors_aux
    47: 0804833b     0 FUNC    LOCAL  DEFAULT   12 frame_dummy
    48: 00000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    49: 08049498     0 OBJECT  LOCAL  DEFAULT   16 __CTOR_END__
    50: 080494a0     0 OBJECT  LOCAL  DEFAULT   17 __DTOR_END__
    51: 08048490     0 OBJECT  LOCAL  DEFAULT   15 __FRAME_END__
    52: 080494a4     0 OBJECT  LOCAL  DEFAULT   18 __JCR_END__
    53: 0804842c     0 FUNC    LOCAL  DEFAULT   12 __do_global_ctors_aux
    54: 00000000     0 FILE    LOCAL  DEFAULT  ABS initfini.c
    55: 00000000     0 FILE    LOCAL  DEFAULT  ABS /build/buildd/glibc-2.3.6
    56: 00000000     0 FILE    LOCAL  DEFAULT  ABS hw.c
    57: 080494a8     0 OBJECT  LOCAL  HIDDEN   19 _DYNAMIC
    58: 08049494     0 NOTYPE  LOCAL  HIDDEN  ABS __fini_array_end
    59: 08049494     0 NOTYPE  LOCAL  HIDDEN  ABS __fini_array_start
    60: 08049494     0 NOTYPE  LOCAL  HIDDEN  ABS __init_array_end
    61: 08049574     0 OBJECT  LOCAL  HIDDEN   21 _GLOBAL_OFFSET_TABLE_
    62: 08049494     0 NOTYPE  LOCAL  HIDDEN  ABS __init_array_start
    63: 08048478     4 OBJECT  GLOBAL DEFAULT   14 _fp_hw
    64: 0804958c     0 OBJECT  GLOBAL HIDDEN   22 __dso_handle
    65: 080483e1    74 FUNC    GLOBAL DEFAULT   12 __libc_csu_fini
    66: 00000000   378 FUNC    GLOBAL DEFAULT  UND puts@@GLIBC_2.0
    67: 08048278     0 FUNC    GLOBAL DEFAULT   10 _init
    68: 080482c0     0 FUNC    GLOBAL DEFAULT   12 _start
    69: 08048390    81 FUNC    GLOBAL DEFAULT   12 __libc_csu_init
    70: 08049594     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start
    71: 08048360    47 FUNC    GLOBAL DEFAULT   12 main
    72: 00000000   230 FUNC    GLOBAL DEFAULT  UND __libc_start_main@@GLIBC_
    73: 08049588     0 NOTYPE  WEAK   DEFAULT   22 data_start
    74: 08048458     0 FUNC    GLOBAL DEFAULT   13 _fini
    75: 08049594     0 NOTYPE  GLOBAL DEFAULT  ABS _edata
    76: 08048308     0 FUNC    GLOBAL HIDDEN   12 __i686.get_pc_thunk.bx
    77: 08049598     0 NOTYPE  GLOBAL DEFAULT  ABS _end
    78: 0804847c     4 OBJECT  GLOBAL DEFAULT   14 _IO_stdin_used
    79: 08049588     0 NOTYPE  GLOBAL DEFAULT   22 __data_start
    80: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
    81: 00000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__

Histogram for bucket list length (total of 3 buckets):
 Length  Number     % of total  Coverage
      0  0          (  0.0%)
      1  1          ( 33.3%)     20.0%
      2  2          ( 66.7%)    100.0%

Version symbols section '.gnu.version' contains 6 entries:
 Addr: 0000000008048232  Offset: 0x000232  Link: 4 (.dynsym)
  000:   0 (*local*)       2 (GLIBC_2.0)     2 (GLIBC_2.0)     1 (*global*)
  004:   0 (*local*)       0 (*local*)

Version needs section '.gnu.version_r' contains 1 entries:
 Addr: 0x0000000008048240  Offset: 0x000240  Link to section: 5 (.dynstr)
  000000: Version: 1  File: libc.so.6  Cnt: 1
  0x0010:   Name: GLIBC_2.0  Flags: none  Version: 2

Notes at offset 0x00000128 with length 0x00000020:
  Owner         Data size       Description
  GNU           0x00000010      NT_VERSION (version)

正如从该输出中看到的,简单的 a.out Hello World 文件中包含了大量有价值的细节信息,包括版本信息、柱状图、各种符号类型的表格,等等。通过使用本文中介绍的这几种工具分析目标文件,您可以慢慢地对可执行程序进行研究。

    除了所有这些段之外,编译器可以将调试信息放入到目标文件中,并且还可以显示这些信息。输入下面的命令,仔细分析编译器的输出(假设您扮演了调试程序的角色):

readelf --debug-dump a.out | less

    这个命令生成的输出如清单 5 所示。调试工具,如 GDB,可以读取这些调试信息,并且当程序在调试器中运行的同时,您可以使用该工具显示更具描述性的标记,而不是对代码进行反汇编时的原始地址值。


清单 5. 该程序中的调试信息
	
The section .debug_aranges contains:

  Length:                   28
  Version:                  2
  Offset into .debug_info:  0
  Pointer Size:             4
  Segment Size:             0

    Address  Length
    080482c0 34
  Length:                   52
  Version:                  2
  Offset into .debug_info:  10b
  Pointer Size:             4
  Segment Size:             0

    Address  Length
    08048308 4
    08048458 18
    08048278 11
    080482e4 36
  Length:                   44
  Version:                  2
  Offset into .debug_info:  19b
  Pointer Size:             4
  Segment Size:             0

    Address  Length
    08048308 4
    0804846f 6
    0804828d 2

Contents of the .debug_pubnames section:

  Length:                              33
  Version:                             2
  Offset into .debug_info section:     122
  Size of area in .debug_info section: 145

    Offset      Name
    121                 _IO_stdin_used

The section .debug_info contains:

  Compilation Unit @ offset 0x0:
   Length:        118
   Version:       2
   Abbrev Offset: 0
   Pointer Size:  4
 <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
     DW_AT_stmt_list   : 0
     DW_AT_low_pc      : 0x80482c0
     DW_AT_high_pc     : 0x80482e2
     DW_AT_name        : ../sysdeps/i386/elf/start.S
     DW_AT_comp_dir    : /build/buildd/glibc-2.3.6/build-tree/glibc-2.3.6/csu
     DW_AT_producer    : GNU AS 2.16.91
     DW_AT_language    : 32769  (MIPS assembler)
  Compilation Unit @ offset 0x7a:
   Length:        141
   Version:       2
   Abbrev Offset: 20
   Pointer Size:  4
 <0><85>: Abbrev Number: 1 (DW_TAG_compile_unit)
     DW_AT_stmt_list   : 0x5b
     DW_AT_high_pc     : 0x80482e4
     DW_AT_low_pc      : 0x80482e4
     DW_AT_producer    : (indirect string, offset: 0x62): GNU C 3.4.6
     DW_AT_language    : 1      (ANSI C)
     DW_AT_name        : (indirect string, offset: 0x0): init.c
     DW_AT_comp_dir    : (indirect string, offset: 0x11): /build/buildd/...
 <1><9f>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x90): unsigned int
     DW_AT_byte_size   : 4
     DW_AT_encoding    : 7      (unsigned)
 <1><a6>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x54): unsigned char
     DW_AT_byte_size   : 1
     DW_AT_encoding    : 8      (unsigned char)
 <1><ad>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x9d): short unsigned int
     DW_AT_byte_size   : 2
     DW_AT_encoding    : 7      (unsigned)
 <1><b4>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x8b): long unsigned int
     DW_AT_byte_size   : 4
     DW_AT_encoding    : 7      (unsigned)
 <1><bb>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x56): signed char
     DW_AT_byte_size   : 1
     DW_AT_encoding    : 6      (signed char)
 <1><c2>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x7): short int
     DW_AT_byte_size   : 2
     DW_AT_encoding    : 5      (signed)
 <1><c9>: Abbrev Number: 3 (DW_TAG_base_type)
     DW_AT_name        : int
     DW_AT_byte_size   : 4
     DW_AT_encoding    : 5      (signed)
 <1><d0>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x46): long long int
     DW_AT_byte_size   : 8
     DW_AT_encoding    : 5      (signed)
 <1><d7>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x86): long long unsigned int
     DW_AT_byte_size   : 8
     DW_AT_encoding    : 7      (unsigned)
 <1><de>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x4b): long int
     DW_AT_byte_size   : 4
     DW_AT_encoding    : 5      (signed)
 <1><e5>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x90): unsigned int
     DW_AT_byte_size   : 4
     DW_AT_encoding    : 7      (unsigned)
 <1><ec>: Abbrev Number: 2 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x5d): char
     DW_AT_byte_size   : 1
     DW_AT_encoding    : 6      (signed char)
 <1><f3>: Abbrev Number: 4 (DW_TAG_variable)
     DW_AT_name        : (indirect string, offset: 0xb0): _IO_stdin_used
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 25
     DW_AT_type        : <105>
     DW_AT_external    : 1
     DW_AT_location    : 5 byte block: 3 7c 84 4 8      (DW_OP_addr: 804847c)
 <1><105>: Abbrev Number: 5 (DW_TAG_const_type)
     DW_AT_type        : <c9>
  Compilation Unit @ offset 0x10b:
   Length:        140
   Version:       2
   Abbrev Offset: 86
   Pointer Size:  4
 <0><116>: Abbrev Number: 1 (DW_TAG_compile_unit)
     DW_AT_stmt_list   : 0x82
     DW_AT_name        : /build/buildd/glibc-2.3.6/build-tree/i386-libc/csu/crti.S
     DW_AT_comp_dir    : /build/buildd/glibc-2.3.6/build-tree/glibc-2.3.6/csu
     DW_AT_producer    : GNU AS 2.16.91
     DW_AT_language    : 32769  (MIPS assembler)
  Compilation Unit @ offset 0x19b:
   Length:        140
   Version:       2
   Abbrev Offset: 102
   Pointer Size:  4
 <0><1a6>: Abbrev Number: 1 (DW_TAG_compile_unit)
     DW_AT_stmt_list   : 0x12f
     DW_AT_name        : /build/buildd/glibc-2.3.6/build-tree/i386-libc/csu/crtn.S
     DW_AT_comp_dir    : /build/buildd/glibc-2.3.6/build-tree/glibc-2.3.6/csu
     DW_AT_producer    : GNU AS 2.16.91
     DW_AT_language    : 32769  (MIPS assembler)
Contents of the .debug_abbrev section:

  Number TAG
   1      DW_TAG_compile_unit    [no children]
    DW_AT_stmt_list    DW_FORM_data4
    DW_AT_low_pc       DW_FORM_addr
    DW_AT_high_pc      DW_FORM_addr
    DW_AT_name         DW_FORM_string
    DW_AT_comp_dir     DW_FORM_string
    DW_AT_producer     DW_FORM_string
    DW_AT_language     DW_FORM_data2

  Number TAG

   1      DW_TAG_compile_unit    [has children]

    DW_AT_stmt_list    DW_FORM_data4
    DW_AT_high_pc      DW_FORM_addr
    DW_AT_low_pc       DW_FORM_addr
    DW_AT_producer     DW_FORM_strp
    DW_AT_language     DW_FORM_data1
    DW_AT_name         DW_FORM_strp
    DW_AT_comp_dir     DW_FORM_strp

   2      DW_TAG_base_type    [no children]

    DW_AT_name         DW_FORM_strp
    DW_AT_byte_size    DW_FORM_data1
    DW_AT_encoding     DW_FORM_data1

   3      DW_TAG_base_type    [no children]

    DW_AT_name         DW_FORM_string
    DW_AT_byte_size    DW_FORM_data1
    DW_AT_encoding     DW_FORM_data1

   4      DW_TAG_variable    [no children]

    DW_AT_name         DW_FORM_strp
    DW_AT_decl_file    DW_FORM_data1
    DW_AT_decl_line    DW_FORM_data1
    DW_AT_type         DW_FORM_ref4
    DW_AT_external     DW_FORM_flag
    DW_AT_location     DW_FORM_block1

   5      DW_TAG_const_type    [no children]

    DW_AT_type         DW_FORM_ref4
    Number TAG

   1      DW_TAG_compile_unit    [no children]

    DW_AT_stmt_list    DW_FORM_data4
    DW_AT_name         DW_FORM_string
    DW_AT_comp_dir     DW_FORM_string
    DW_AT_producer     DW_FORM_string
    DW_AT_language     DW_FORM_data2
    Number TAG

   1      DW_TAG_compile_unit    [no children]

    DW_AT_stmt_list    DW_FORM_data4
    DW_AT_name         DW_FORM_string
    DW_AT_comp_dir     DW_FORM_string
    DW_AT_producer     DW_FORM_string
    DW_AT_language     DW_FORM_data2


Dump of debug contents of section .debug_line:

  Length:                      87
  DWARF Version:               2
  Prologue Length:             50
  Minimum Instruction Length:  1
  Initial value of 'is_stmt':  1
  Line Base:                   -5
  Line Range:                  14
  Opcode Base:                 13
  (Pointer size:               4)

Opcodes:
  Opcode 1 has 0 args
  Opcode 2 has 1 args
  Opcode 3 has 1 args
  Opcode 4 has 1 args
  Opcode 5 has 1 args
  Opcode 6 has 0 args
  Opcode 7 has 0 args
  Opcode 8 has 0 args
  Opcode 9 has 1 args
  Opcode 10 has 0 args
  Opcode 11 has 0 args
  Opcode 12 has 1 args

The Directory Table:
  ../sysdeps/i386/elf

The File Name Table:
  Entry Dir     Time    Size    Name
  1     1       0       0       start.S

Line Number Statements:
  Extended opcode 2: set Address to 0x80482c0
  Advance Line by 64 to 65
  Copy
  Special opcode 38: advance Address by 2 to 0x80482c2 and Line by 5 to 70
  Special opcode 20: advance Address by 1 to 0x80482c3 and Line by 1 to 71
  Special opcode 39: advance Address by 2 to 0x80482c5 and Line by 6 to 77
  Special opcode 48: advance Address by 3 to 0x80482c8 and Line by 1 to 78
  Special opcode 24: advance Address by 1 to 0x80482c9 and Line by 5 to 83
  Special opcode 21: advance Address by 1 to 0x80482ca and Line by 2 to 85
  Advance Line by 24 to 109
  Special opcode 19: advance Address by 1 to 0x80482cb and Line by 0 to 109
  Special opcode 76: advance Address by 5 to 0x80482d0 and Line by 1 to 110
  Special opcode 77: advance Address by 5 to 0x80482d5 and Line by 2 to 112
  Special opcode 20: advance Address by 1 to 0x80482d6 and Line by 1 to 113
  Special opcode 21: advance Address by 1 to 0x80482d7 and Line by 2 to 115
  Special opcode 79: advance Address by 5 to 0x80482dc and Line by 4 to 119
  Special opcode 78: advance Address by 5 to 0x80482e1 and Line by 3 to 122
  Advance PC by 1 to 0x80482e2
  Extended opcode 1: End of Sequence


  Length:                      35
  DWARF Version:               2
  Prologue Length:             29
  Minimum Instruction Length:  1
  Initial value of 'is_stmt':  1
  Line Base:                   -5
  Line Range:                  14
  Opcode Base:                 13
  (Pointer size:               4)

Opcodes:
  Opcode 1 has 0 args
  Opcode 2 has 1 args
  Opcode 3 has 1 args
  Opcode 4 has 1 args
  Opcode 5 has 1 args
  Opcode 6 has 0 args
  Opcode 7 has 0 args
  Opcode 8 has 0 args
  Opcode 9 has 1 args
  Opcode 10 has 0 args
  Opcode 11 has 0 args
  Opcode 12 has 1 args

The Directory Table is empty.

The File Name Table:
  Entry Dir     Time    Size    Name
  1     0       0       0       init.c

Line Number Statements:

  Length:                      169
  DWARF Version:               2
  Prologue Length:             80
  Minimum Instruction Length:  1
  Initial value of 'is_stmt':  1
  Line Base:                   -5
  Line Range:                  14
  Opcode Base:                 13
  (Pointer size:               4)
Opcodes:
  Opcode 1 has 0 args
  Opcode 2 has 1 args
  Opcode 3 has 1 args
  Opcode 4 has 1 args
  Opcode 5 has 1 args
  Opcode 6 has 0 args
  Opcode 7 has 0 args
  Opcode 8 has 0 args
  Opcode 9 has 1 args
  Opcode 10 has 0 args
  Opcode 11 has 0 args
  Opcode 12 has 1 args

The Directory Table:
  /build/buildd/glibc-2.3.6/build-tree/i386-libc/csu

The File Name Table:
  Entry Dir     Time    Size    Name
  1     1       0       0       crti.S

Line Number Statements:
  Extended opcode 2: set Address to 0x8048308
  Advance Line by 64 to 65
  Copy
  Special opcode 48: advance Address by 3 to 0x804830b and Line by 1 to 66
  Advance PC by 1 to 0x804830c
  Extended opcode 1: End of Sequence

  Extended opcode 2: set Address to 0x8048458
  Advance Line by 46 to 47
  Copy
  Special opcode 20: advance Address by 1 to 0x8048459 and Line by 1 to 48
  Special opcode 34: advance Address by 2 to 0x804845b and Line by 1 to 49
  Special opcode 20: advance Address by 1 to 0x804845c and Line by 1 to 50
  Special opcode 76: advance Address by 5 to 0x8048461 and Line by 1 to 51
  Special opcode 90: advance Address by 6 to 0x8048467 and Line by 1 to 52
  Advance PC by 3 to 0x804846a
  Extended opcode 1: End of Sequence

  Extended opcode 2: set Address to 0x8048278
  Advance Line by 31 to 32
  Copy
  Special opcode 20: advance Address by 1 to 0x8048279 and Line by 1 to 33
  Special opcode 34: advance Address by 2 to 0x804827b and Line by 1 to 34
  Special opcode 48: advance Address by 3 to 0x804827e and Line by 1 to 35
  Advance PC by 5 to 0x8048283
  Extended opcode 1: End of Sequence

  Extended opcode 2: set Address to 0x80482e4
  Advance Line by 10 to 11
  Copy
  Special opcode 20: advance Address by 1 to 0x80482e5 and Line by 1 to 12
  Special opcode 34: advance Address by 2 to 0x80482e7 and Line by 1 to 13
  Special opcode 20: advance Address by 1 to 0x80482e8 and Line by 1 to 14
  Special opcode 76: advance Address by 5 to 0x80482ed and Line by 1 to 15
  Special opcode 90: advance Address by 6 to 0x80482f3 and Line by 1 to 16
  Special opcode 48: advance Address by 3 to 0x80482f6 and Line by 1 to 17
  Special opcode 90: advance Address by 6 to 0x80482fc and Line by 1 to 18
  Special opcode 34: advance Address by 2 to 0x80482fe and Line by 1 to 19
  Special opcode 34: advance Address by 2 to 0x8048300 and Line by 1 to 20
  Special opcode 35: advance Address by 2 to 0x8048302 and Line by 2 to 22
  Special opcode 48: advance Address by 3 to 0x8048305 and Line by 1 to 23
  Special opcode 20: advance Address by 1 to 0x8048306 and Line by 1 to 24
  Special opcode 20: advance Address by 1 to 0x8048307 and Line by 1 to 25
  Advance PC by 1 to 0x8048308
  Extended opcode 1: End of Sequence


  Length:                      136
  DWARF Version:               2
  Prologue Length:             80
  Minimum Instruction Length:  1
  Initial value of 'is_stmt':  1
  Line Base:                   -5
  Line Range:                  14
  Opcode Base:                 13
  (Pointer size:               4)

Opcodes:
  Opcode 1 has 0 args
  Opcode 2 has 1 args
  Opcode 3 has 1 args
  Opcode 4 has 1 args
  Opcode 5 has 1 args
  Opcode 6 has 0 args
  Opcode 7 has 0 args
  Opcode 8 has 0 args
  Opcode 9 has 1 args
  Opcode 10 has 0 args
  Opcode 11 has 0 args
  Opcode 12 has 1 args

The Directory Table:
  /build/buildd/glibc-2.3.6/build-tree/i386-libc/csu

The File Name Table:
  Entry Dir     Time    Size    Name
  1     1       0       0       crtn.S

Line Number Statements:
  Extended opcode 2: set Address to 0x8048308
  Advance Line by 33 to 34
  Copy
  Special opcode 48: advance Address by 3 to 0x804830b and Line by 1 to 35
  Advance PC by 1 to 0x804830c
  Extended opcode 1: End of Sequence

  Extended opcode 2: set Address to 0x804846f
  Advance Line by 18 to 19
  Copy
  Special opcode 48: advance Address by 3 to 0x8048472 and Line by 1 to 20
  Special opcode 20: advance Address by 1 to 0x8048473 and Line by 1 to 21
  Special opcode 20: advance Address by 1 to 0x8048474 and Line by 1 to 22
  Advance PC by 1 to 0x8048475
  Extended opcode 1: End of Sequence

  Extended opcode 2: set Address to 0x804828d
  Advance Line by 9 to 10
  Copy
  Special opcode 20: advance Address by 1 to 0x804828e and Line by 1 to 11
  Advance PC by 1 to 0x804828f
  Extended opcode 1: End of Sequence


Contents of the .debug_str section:

  0x00000000 696e6974 2e630073 686f7274 20696e74 init.c.short int
  0x00000010 002f6275 696c642f 6275696c 64642f67 ./build/buildd/g
  0x00000020 6c696263 2d322e33 2e362f62 75696c64 libc-2.3.6/build
  0x00000030 2d747265 652f676c 6962632d 322e332e -tree/glibc-2.3.
  0x00000040 362f6373 75006c6f 6e67206c 6f6e6720 6/csu.long long
  0x00000050 696e7400 756e7369 676e6564 20636861 int.unsigned cha
  0x00000060 7200474e 55204320 332e342e 36202855 r.GNU C 3.4.6 (U
  0x00000070 62756e74 7520332e 342e362d 31756275 buntu 3.4.6-1ubu
  0x00000080 6e747532 29006c6f 6e67206c 6f6e6720 ntu2).long long
  0x00000090 756e7369 676e6564 20696e74 0073686f unsigned int.sho
  0x000000a0 72742075 6e736967 6e656420 696e7400 rt unsigned int.
  0x000000b0 5f494f5f 73746469 6e5f7573 656400   _IO_stdin_used.

可执行文件是目标文件

    在 UNIX 中,可执行文件是 目标文件,并且您可以像对 a.out 文件那样对它们进行分析。可以进行一次有益的练习,更改到 /bin 或 /local/bin 目录,然后针对一些您最常用的命令,如 pwd、ps、cat 或 rm,运行 nm、objdump 和 readelf。通常,在您编写需要某种功能的程序时,如果标准的工具已经提供了这个功能,那么通过运行 objdump -d <command>,可以查看这些工具究竟如何完成这项任务。

    如果您倾向于使用编译器和其他的语言工具,那么您可以对组成计算机系统的各种目标文件进行仔细研究,并且您将会发现这项工作是非常值得的。UNIX 操作系统具有许多层次,那些通过工具查看目标文件所公开的层次,非常接近底层硬件。通过这种方式,您可以真实地接触到系统。

结束语

    研究目标文件可以极大地加深您对 UNIX 操作系统的认识,并且可以更深入地了解如何对软件的源代码进行汇编。我鼓励您使用本文中介绍的目标文件工具对系统中 /bin 或 /local/bin 目录中的程序进行分析,仔细研究其输出结果,并找出您的硬件制造商所提供的系统文档。

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