计算机系统篇之链接(12):Chapter 7 Linking 习题与解答

Posted csstormq

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计算机系统篇之链接(12):Chapter 7 Linking 习题与解答

Author:stormQ

Monday, 15. July 2019 11:18AM



习题









解答

7.6 解答

swap.c:

extern int buf[];

int *bufp0 = &buf[0];
static int *bufp1;

static void incr()

  static int count = 0;
  count++;


void swap()

  int temp;

  incr();
  bufp1 = &buf[1];
  temp = *bufp0;
  *bufp0 = *bufp1;
  *bufp1 = temp;

生成可重定位目标文件 swap.o:

$ gcc -c swap.c

查看 swap.o 的符号表:

$ readelf -s swap.o

Symbol table '.symtab' contains 14 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS swap.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     5: 0000000000000000     8 OBJECT  LOCAL  DEFAULT    5 bufp1
     6: 0000000000000000    22 FUNC    LOCAL  DEFAULT    1 incr
     7: 0000000000000008     4 OBJECT  LOCAL  DEFAULT    5 count.1760
     8: 0000000000000000     0 SECTION LOCAL  DEFAULT    7 
     9: 0000000000000000     0 SECTION LOCAL  DEFAULT    8 
    10: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
    11: 0000000000000000     8 OBJECT  GLOBAL DEFAULT    3 bufp0
    12: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND buf
    13: 0000000000000016    74 FUNC    GLOBAL DEFAULT    1 swap

查看 swap.o 的表头:

$ readelf -S swap.o
There are 13 section headers, starting at offset 0x420:

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
       0000000000000060  0000000000000000  AX       0     0     1
  [ 2] .rela.text        RELA             0000000000000000  000002b8
       00000000000000c0  0000000000000018   I      11     1     8
  [ 3] .data             PROGBITS         0000000000000000  000000a0
       0000000000000008  0000000000000000  WA       0     0     8
  [ 4] .rela.data        RELA             0000000000000000  00000378
       0000000000000018  0000000000000018   I      11     3     8
  [ 5] .bss              NOBITS           0000000000000000  000000a8
       000000000000000c  0000000000000000  WA       0     0     8
  [ 6] .comment          PROGBITS         0000000000000000  000000a8
       0000000000000033  0000000000000001  MS       0     0     1
  [ 7] .note.GNU-stack   PROGBITS         0000000000000000  000000db
       0000000000000000  0000000000000000           0     0     1
  [ 8] .eh_frame         PROGBITS         0000000000000000  000000e0
       0000000000000058  0000000000000000   A       0     0     8
  [ 9] .rela.eh_frame    RELA             0000000000000000  00000390
       0000000000000030  0000000000000018   I      11     8     8
  [10] .shstrtab         STRTAB           0000000000000000  000003c0
       000000000000005e  0000000000000000           0     0     1
  [11] .symtab           SYMTAB           0000000000000000  00000138
       0000000000000150  0000000000000018          12    11     8
  [12] .strtab           STRTAB           0000000000000000  00000288
       000000000000002d  0000000000000000           0     0     1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
  I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

因此,该题的解答为:

符号swap.symtab 条目?符号类型定义符号的模块
bufyesexternm.o.data
bufp0yesglobalswap.o.data
bufp1yeslocalswap.o.bss
swapyesglobalswap.o.text
tempno---
incryeslocalswap.o.text
countyeslocalswap.o.bss

返回题目


7.7 解答

a)先执行原来的代码

bar5.c:

double x;

void f()

    x = -0.0;

foo5.c:

#include <stdio.h>

void f();

int x = 15213;
int y = 15212;

int main()

    f();
    printf("x = 0x%x(%d) y = 0x%x(%d)\\n", x, x, y, y);
    return 0;

**注意:**这里调整了书中变量xy的定义顺序。因为在我们的系统中(X86-64 Linux, Little-endian)中,按照上述定义顺序时,x的地址是 0x601038,y的地址是 0x60103c。这一点可以通过下文查看可执行目标文件的符号表得到验证。

生成可执行目标文件——foo5:

$ gcc -o foo5 foo5.c bar5.c 
/usr/bin/x86_64-linux-gnu-ld: Warning: alignment 4 of symbol `x' in /tmp/ccCUovlG.o is smaller than 8 in /tmp/ccNJpz7k.o

注:gcc 版本为 6.5。

运行 foo5:

$ ./foo5 
x = 0x0(0) y = 0x80000000(-2147483648)

查看 foo5 的符号表:

$ readelf -s foo5

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 printf@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 71 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: 0000000000400358     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: 00000000004005f4     0 SECTION LOCAL  DEFAULT   15 
    16: 0000000000400600     0 SECTION LOCAL  DEFAULT   16 
    17: 0000000000400628     0 SECTION LOCAL  DEFAULT   17 
    18: 0000000000400668     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: 0000000000601040     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: 00000000004004a0     0 FUNC    LOCAL  DEFAULT   14 register_tm_clones
    32: 00000000004004e0     0 FUNC    LOCAL  DEFAULT   14 __do_global_dtors_aux
    33: 0000000000601040     1 OBJECT  LOCAL  DEFAULT   26 completed.7593
    34: 0000000000600e18     0 OBJECT  LOCAL  DEFAULT   20 __do_global_dtors_aux_fin
    35: 0000000000400500     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 foo5.c
    38: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS bar5.c
    39: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    40: 0000000000400778     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: 0000000000400628     0 NOTYPE  LOCAL  DEFAULT   17 __GNU_EH_FRAME_HDR
    47: 0000000000601000     0 OBJECT  LOCAL  DEFAULT   24 _GLOBAL_OFFSET_TABLE_
    48: 00000000004005f0     2 FUNC    GLOBAL DEFAULT   14 __libc_csu_fini
    49: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTab
    50: 0000000000601028     0 NOTYPE  WEAK   DEFAULT   25 data_start
    51: 0000000000601040     0 NOTYPE  GLOBAL DEFAULT   25 _edata
    52: 0000000000601038     4 OBJECT  GLOBAL DEFAULT   25 x
    53: 00000000004005f4     0 FUNC    GLOBAL DEFAULT   15 _fini
    54: 0000000000400567    23 FUNC    GLOBAL DEFAULT   14 f
    55: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND printf@@GLIBC_2.2.5
    56: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@@GLIBC_
    57: 0000000000601028     0 NOTYPE  GLOBAL DEFAULT   25 __data_start
    58: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
    59: 0000000000601030     0 OBJECT  GLOBAL HIDDEN    25 __dso_handle
    60: 0000000000400600     4 OBJECT  GLOBAL DEFAULT   16 _IO_stdin_used
    61: 0000000000400580   101 FUNC    GLOBAL DEFAULT   14 __libc_csu_init
    62: 0000000000601048     0 NOTYPE  GLOBAL DEFAULT   26 _end
    63: 0000000000400430    42 FUNC    GLOBAL DEFAULT   14 _start
    64: 000000000060103c     4 OBJECT  GLOBAL DEFAULT   25 y
    65: 0000000000601040     0 NOTYPE  GLOBAL DEFAULT   26 __bss_start
    66: 0000000000400526    65 FUNC    GLOBAL DEFAULT   14 main
    67: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
    68: 0000000000601040     0 OBJECT  GLOBAL HIDDEN    25 __TMC_END__
    69: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
    70: 00000000004003c8     0 FUNC    GLOBAL DEFAULT   11 _init

2)修改方案

修改方案:只将 bar5.c 中的 x 的作用域由全局作用域改为局部作用域,即用关键字static修饰。

修改后的 bar5.c:

static double x;

void f()

    x = -0.0;

foo5.c 与原来的保持一致。

编译并运行:

$ gcc -o foo5 foo5.c bar5.c
$ ./foo5 
x = 0x3b6d(15213) y = 0x3b6c(15212)

查看 foo5 的符号表:

$ readelf -s foo5

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 printf@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 72 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: 0000000000400358     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: 00000000004005f4     0 SECTION LOCAL  DEFAULT   15 
    16: 0000000000400600     0 SECTION LOCAL  DEFAULT   16 
    17: 0000000000400628     0 SECTION LOCAL  DEFAULT   17 
    18: 0000000000400668     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: 0000000000601040     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: 00000000004004a0     0 FUNC    LOCAL  DEFAULT   14 register_tm_clones
    32: 00000000004004e0     0 FUNC    LOCAL  DEFAULT   14 __do_global_dtors_aux
    33: 0000000000601040     1 OBJECT  LOCAL  DEFAULT   26 completed.7593
    34: 0000000000600e18     0 OBJECT  LOCAL  DEFAULT   20 __do_global_dtors_aux_fin
    35: 0000000000400500     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 foo5.c
    38: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS bar5.c
    39: 0000000000601048     8 OBJECT  LOCAL  DEFAULT   26 x
    40: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    41: 0000000000400778     0 OBJECT  LOCAL  DEFAULT   18 __FRAME_END__
    42: 0000000000600e20     0 OBJECT  LOCAL  DEFAULT   21 __JCR_END__
    43: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS 
    44: 0000000000600e18     0 NOTYPE  LOCAL  DEFAULT   19 __init_array_end
    45: 0000000000600e28     0 OBJECT  LOCAL  DEFAULT   22 _DYNAMIC
    46: 0000000000600e10     0 NOTYPE  LOCAL  DEFAULT   19 __init_array_start
    47: 0000000000400628     0 NOTYPE  LOCAL  DEFAULT   17 __GNU_EH_FRAME_HDR
    48: 0000000000601000     0 OBJECT  LOCAL  DEFAULT   24 _GLOBAL_OFFSET_TABLE_
    49: 00000000004005f0     2 FUNC    GLOBAL DEFAULT   14 __libc_csu_fini
    50: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTab
    51: 0000000000601028     0 NOTYPE  WEAK   DEFAULT   25 data_start
    52: 0000000000601040     0 NOTYPE  GLOBAL DEFAULT   25 _edata
    53: 0000000000601038     4 OBJECT  GLOBAL DEFAULT   25 x
    54: 00000000004005f4     0 FUNC    GLOBAL DEFAULT   15 _fini
    55: 0000000000400567    23 FUNC    GLOBAL DEFAULT   14 f
    56: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND printf@@GLIBC_2.2.5
    57: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@@GLIBC_
    58: 0000000000601028     0 NOTYPE  GLOBAL DEFAULT   25 __data_start
    59: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
    60: 0000000000601030     0 OBJECT  GLOBAL HIDDEN    25 __dso_handle
    61: 0000000000400600     4 OBJECT  GLOBAL DEFAULT   16 _IO_stdin_used
    62: 0000000000400580   101 FUNC    GLOBAL DEFAULT   14 __libc_csu_init
    63: 0000000000601050     0 NOTYPE  GLOBAL DEFAULT   26 _end
    64: 0000000000400430    42 FUNC    GLOBAL DEFAULT   14 _start
    65: 000000000060103c     4 OBJECT  GLOBAL DEFAULT   25 y
    66: 0000000000601040     0 NOTYPE  GLOBAL DEFAULT   26 __bss_start
    67: 0000000000400526    65 FUNC    GLOBAL DEFAULT   14 main
    68: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
    69: 0000000000601040     0 OBJECT  GLOBAL HIDDEN    25 __TMC_END__
    70: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
    71: 00000000004003c8     0 FUNC    GLOBAL DEFAULT   11 _init

**注意:**修改后的可执行目标文件foo5中符号名称为x的符号有两个。一个是全局作用域的,一个是局部作用域的。而修改前的foo5中符号名称为x的符号只有一个。

其他修改方案:只将 foo5.c 中的 x 的作用域由全局作用域改为局部作用域,即用关键字static修饰。关于该方案的可行性,有兴趣的可以自己验证一下。

返回题目


7.8 解答

a)A 中 Module 1 和 Module 2 的代码

m1_a.c(Module 1):

int main()


m2_a.c(Module 2):

static int main = 1;
int p2()


**注意:**这里将原书中 Module 2 里面的]改为;,应该是原书作者的笔误。

生成可重定位目标文件m1_a.o

$ gcc -c m1_a.c

生成可重定位目标文件m2_a.o

$ gcc -c m2_a.c

生成可执行目标文件m1_a

$ gcc -o m1_a m1_a.o m2_a.o

成功生成了可执行目标文件m1_a,即未出现链接错误。

查看可重定位目标文件m1_a.o的符号表:

$ readelf -s m1_a.o

Symbol table '.symtab' contains 9 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS m1_a.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
     8: 0000000000000000    11 FUNC    GLOBAL DEFAULT    1 main

可以看出,Module 1 中的符号引用main的定义在 Module 1 中的.textsection 中。

查看可重定位目标文件m2_a.o的符号表:

$ readelf -s m2_a.o

Symbol table '.symtab' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS m2_a.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     5: 0000000000000000     4 OBJECT  LOCAL  DEFAULT    2 main
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
     8: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
     9: 0000000000000000     7 FUNC    GLOBAL DEFAULT    1 p2

可以看出,Module 2 中的符号引用main的定义在 Module 2 中的.datasection 中。

b)B 中 Module 1 和 Module 2 的代码

m1_b.c(Module 1):

int x;
void main()


m2_b.c(Module 2):

double x;
int p2()


生成可重定位目标文件m1_b.o

$ gcc -c m1_b.c

生成可重定位目标文件m2_b.o

$ gcc -c m2_b.c

生成可执行目标文件m1_b

$ gcc -o m1_b m1_b.o m2_b.o

成功生成了可执行目标文件m1_b,即未出现链接错误。

查看可重定位目标文件m1_b.o的符号表:

$ readelf -s m1_b.o

Symbol table '.symtab' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS m1_b.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
     8: 0000000000000004     4 OBJECT  GLOBAL DEFAULT  COM x
     9: 0000000000000000     7 FUNC    GLOBAL DEFAULT    1 main

可以看出,Module 1 中的符号引用x被标记为COMMON

查看可重定位目标文件m2_b.o的符号表:

$ readelf -s m2_b.o

Symbol table '.symtab' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS m2_b.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
     8: 0000000000000008     8 OBJECT  GLOBAL DEFAULT  COM x
     9: 0000000000000000     7 FUNC    GLOBAL DEFAULT    1 p2

可以看出,Module 2 中的符号引用x也被标记为COMMON

因此,符号引用x的定义在 Module 1 或 Module 2 中任意选择一个。

c)C 中 Module 1 和 Module 2 的代码

m1_c.c(Module 1):

int x = 1;
void main()


m2_c.c(Module 2):

double x = 1.0;
int p2()


生成可重定位目标文件m1_c.o

$ gcc -c m1_c.c

生成可重定位目标文件m2_c.o

$ gcc -c m2_c.c

生成可执行目标文件m1_c

$ gcc -o m1_c m1_c.o m2_c.o
m2_c.o:(.data+0x0): multiple definition of `x'
m1_c.o:(.data+0x0): first defined here
/usr/bin/x86_64-linux-gnu-ld: Warning: size of symbol `x' changed from 4 in m1_c.o to 8 in m2_c.o
collect2: error: ld returned 1 exit status

可以看出,在生成可执行目标文件m1_c时,出现了链接错误。

查看可重定位目标文件m1_c.o的符号表:

$ readelf -s m1_c.o

Symbol table '.symtab' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS m1_c.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
     8: 0000000000000000     4 OBJECT  GLOBAL DEFAULT    2 x
     9: 0000000000000000     7 FUNC    GLOBAL DEFAULT    1 main

可以看出,Module 1 中的符号引用x的定义在 Module 1 中,并且为全局作用域。

查看可重定位目标文件m2_c.o的符号表:

$ readelf -s m2_c.o

Symbol table '.symtab' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS m2_c.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
     8: 0000000000000000     8 OBJECT  GLOBAL DEFAULT    2 x
     9: 0000000000000000     7 FUNC    GLOBAL DEFAULT    1 p2

可以看出,Module 2 中的符号引用x的定义在 Module 2 中,并且为全局作用域。

也就是说,Module 1 和 Module 2 中重复定义了全局变量x。所以,在生成可执行目标文件m1_c时,出现了链接错误。

因此,该题的解答为:

题目解答
A
  • REF(main.1)->DEF(main.1)
  • REF(main.2)->DEF(main.2)
B
  • REF(x.1)->DEF(UNKNOWN)
  • REF(x.2)->DEF(UNKNOWN)
C
  • REF(x.1)->DEF(ERROR)
  • REF(x.2)->DEF(ERROR)

返回题目


7.9 解答

a)生成并执行可执行目标文件

foo6.c:

void p2();

int main()

    p2();
    return 0;

bar6.c:

char main;

void p2()

    printf("main=0x%x\\n", main);

生成可重定位目标文件foo6.o

$ gcc -c foo6.c

生成可重定位目标文件bar6.o

$ gcc -c bar6.c

生成并执行可执行目标文件foo6

$ gcc -o foo6 foo6.o bar6.o
$ ./foo6
main=0x55

**注意:**在我们的系统中(X86-64 Linux, Little-endian),按照上述步骤生成可执行目标文件foo6时,打印的main变量的值为 0x55,而不是题目中的 0x48。要想打印的main变量的值为题目中的 0x48,生成可执行目标文件foo6_Og的方式应该为gcc -o foo6_Og bar6.c foo6.c -Og

查看foo6_Ogmain函数的汇编代码(部分):

$ objdump -d foo6_Og 

foo6_Og:     file format elf64-x86-64

# 省略...

Disassembly of section .text:

# 省略...

000000000040056a <main>:
  40056a:	48 83 ec 08          	sub    $0x8,%rsp
  40056e:	b8 00 00 00 00       	mov    $0x0,%eax
  400573:	e8 ce ff ff ff       	callq  400546 <p2>
  400578:	b8 00 00 00 00       	mov    $0x0,%eax
  40057d:	48 83 c4 08          	add    $0x8,%rsp
  400581:	c3                   	retq   
  400582:	66 2e 0f 1f 84 00 00 	nopw   %cs:0x0(%rax,%rax,1)
  400589:	00 00 00 
  40058c:	0f 1f 40 00          	nopl   0x0(%rax)

# 省略...

可以看出,可执行目标文件foo6_Ogmain函数的第一个字节的值为 0x48,符合题目中的 0x48。

b)为什么执行可执行目标文件foo6时,打印的main变量的值为 0x55

查看可重定位目标文件foo6.o的符号表:

$ readelf -s foo6.o

Symbol table '.symtab' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS foo6.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    6 
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    7 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     8: 0000000000000000    21 FUNC    GLOBAL DEFAULT    1 main
     9: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND p2

可以看出,foo6.c中符号引用main所对应的符号定义在foo6.o.textsection 中。

查看可重定位目标文件bar6.o的符号表:

$ readelf -s bar6.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 bar6.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: 0000000000000001     1 OBJECT  GLOBAL DEFAULT  COM main
    10: 0000000000000000    34 FUNC    GLOBAL DEFAULT    1 p2
    11: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND printf

可以看出,bar6.c中符号引用main所对应的符号定义还未确定。

查看可执行目标文件foo6的汇编代码(部分):

$ objdump -d foo6

foo6:     file format elf64-x86-64

# 省略...

Disassembly of section .text:

# 省略...

0000000000400526 <main>:
  400526:	55                   	push   %rbp
  400527:	48 89 e5             	mov    %rsp,%rbp
  40052a:	b8 00 00 00 00       	mov    $0x0,%eax
  40052f:	e8 07 00 00 00       	callq  40053b <p2>
  400534:	b8 00 00 00 00       	mov    $0x0,%eax
  400539:	5d                   	pop    %rbp
  40053a:	c3                   	retq

# 省略...

可以看出,main函数的汇编代码中第一个字节的值为 0x55,与打印的main变量的值相同。也就是说,main变量符号引用所对应的符号定义为main函数的符号。所以,执行可执行目标文件foo6时,打印的main变量的值为 0x55。另外,可以通过 gdb 调试验证main 变量符号引用的地址就是 main 函数的地址,具体过程如下:

$ gcc -o foo6_g foo6.c bar6.c -g
$ gdb -q ./foo6_g
Reading symbols from ./foo6_g...done.
(gdb) start
Temporary breakpoint 1 at 0x40052a: file foo6.c, line 7.
Starting program: /home/xuxiaoqiang/tx/7/7.6/foo6_g 

Temporary breakpoint 1, main () at foo6.c:7
7	    p2();
(gdb) l
2	
3	void p2();
4	
5	int main()
6	
7	    p2();
8	    return 0;
9	
(gdb) b bar6.c:7
Breakpoint 2 at 0x40053f: file bar6.c, line 7.
(gdb) c
Continuing.

Breakpoint 2, p2 () at bar6.c:7
7	    printf("main=0x%x\\n", main);
(gdb) x/bx main
0x400526 <main>:	0x55
(gdb) disas main
Dump of assembler code for function main:
   0x0000000000400526 <+0>:	push   %rbp
   0x0000000000400527 <+1>:	mov    %rsp,%rbp
   0x000000000040052a <+4>:	mov    $0x0,%eax
   0x000000000040052f <+9>:	callq  0x40053b <p2>
   0x0000000000400534 <+14>:	mov    $0x0,%eax
   0x0000000000400539 <+19>:	pop    %rbp
   0x000000000040053a <+20>:	retq
End of assembler dump.
(gdb) 

可以看出,在bar6.c中打印main变量的值时,main变量的地址为 0x400526,即main函数的地址。

返回题目


7.10 解答

对于 A:p.o libx.a

对于 B:p.o libx.a liby.a libx.a

对于 C:p.o libx.a liby.a libz.a libx.a

返回题目


7.11 解答

a)生成可执行目标文件foo6作为研究案例

具体生成步骤与 7.9解答 中的一致。

b)查看可执行目标文件foo6的程序表

$ readelf -l foo6

Elf file type is EXEC (Executable file)
Entry point 0x400430
There are 9 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000400040 0x0000000000400040
                 0x00000000000001f8 0x00000000000001f8  R E    8
  INTERP         0x0000000000000238 0x0000000000400238 0x0000000000400238
                 0x000000000000001c 0x000000000000001c  R      1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x0000000000000744 0x0000000000000744  R E    200000
  LOAD           0x0000000000000e10 0x0000000000600e10 0x0000000000600e10
                 0x0000000000000228 0x0000000000000230  RW     200000
  DYNAMIC        0x0000000000000e28 0x0000000000600e28 0x0000000000600e28
                 0x00000000000001d0 0x00000000000001d0  RW     8
  NOTE           0x0000000000000254 0x0000000000400254 0x0000000000400254
                 0x0000000000000044 0x0000000000000044  R      4
  GNU_EH_FRAME   0x00000000000005f0 0x00000000004005f0 0x00000000004005f0
                 0x000000000000003c 0x000000000000003c  R      4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     10
  GNU_RELRO      0x0000000000000e10 0x0000000000600e10 0x0000000000600e10
                 0x00000000000001f0 0x00000000000001f0  R      1

 Section to Segment mapping:
  Segment Sections...
   00     
   01     .interp 
   02     .interp .note.ABI-tag .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .plt.got .text .fini .rodata .eh_frame_hdr .eh_frame 
   03     .init_array .fini_array .jcr .dynamic .got .got.plt .data .bss 
   04     .dynamic 
   05     .note.ABI-tag .note.gnu.build-id 
   06     .eh_frame_hdr 
   07     
   08     .init_array .fini_array .jcr .dynamic .got 

可以看出,数据段(data segment)的起始地址为 0x0000000000600e10,目标文件中的段大小为 0x0000000000000228,内存中的段大小为 0x0000000000000230。也就是题目中所说的程序头部表表明数据段占用了内存中 0x230 个字节。然后,其中只有开始的 0x228 字节来自可执行目标文件的节

c)是什么引起了这种差异?

查看可执行目标文件foo6的符号表:

$ readelf -S foo6
There are 31 section headers, starting at offset 0x1a18:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .interp           PROGBITS         0000000000400238  00000238
       000000000000001c  0000000000000000   A       0     0     1
  [ 2] .note.ABI-tag     NOTE             0000000000400254  00000254
       0000000000000020  0000000000000000   A       0     0     4
  [ 3] .note.gnu.build-i NOTE             0000000000400274  00000274
       0000000000000024  0000000000000000   A       0     0     4
  [ 4] .gnu.hash         GNU_HASH         0000000000400298  00000298
       000000000000001c  0000000000000000   A       5     0     8
  [ 5] .dynsym           DYNSYM           00000000004002b8  000002b8
       0000000000000060  0000000000000018   A       6     1     8
  [ 6] .dynstr           STRTAB           0000000000400318  00000318
       000000000000003f  0000000000000000   A       0     0     1
  [ 7] .gnu.version      VERSYM           0000000000400358  00000358
       0000000000000008  0000000000000002   A       5     0     2
  [ 8] .gnu.version_r    VERNEED          0000000000400360  00000360
       0000000000000020  0000000000000000   A       6     1     8
  [ 9] .rela.dyn         RELA             0000000000400380  00000380
       0000000000000018  0000000000000018   A       5     0     8
  [10] .rela.plt         RELA             0000000000400398  00000398
       0000000000000030  0000000000000018  AI       5    24     8
  [11] .init             PROGBITS         00000000004003c8  000003c8
       000000000000001a  0000000000000000  AX       0     0     4
  [12] .plt              PROGBITS         00000000004003f0  000003f0
       0000000000000030  0000000000000010  AX       0     0     16
  [13] .plt.got          PROGBITS         0000000000400420  00000420
       0000000000000008  0000000000000000  AX       0     0     8
  [14] .text             PROGBITS         0000000000400430  00000430
       00000000000001a2  0000000000000000  AX       0     0     16
  [15] .fini             PROGBITS         00000000004005d4  000005d4
       0000000000000009  0000000000000000  AX       0     0     4
  [16] .rodata           PROGBITS         00000000004005e0  000005e0
       000000000000000f  0000000000000000   A       0     0     4
  [17] .eh_frame_hdr     PROGBITS         00000000004005f0  000005f0
       000000000000003c  0000000000000000   A       0     0     4
  [18] .eh_frame         PROGBITS         0000000000400630  00000630
       0000000000000114  0000000000000000   A       0     0     8
  [19] .init_array       INIT_ARRAY       0000000000600e10  00000e10
       0000000000000008  0000000000000000  WA       0     0     8
  [20] .fini_array       FINI_ARRAY       0000000000600e18  00000e18
       0000000000000008  0000000000000000  WA       0     0     8
  [21] .jcr              PROGBITS         0000000000600e20  00000e20
       0000000000000008  0000000000000000  WA       0     0     8
  [22] .dynamic          DYNAMIC          0000000000600e28  00000e28
       00000000000001d0  0000000000000010  WA       6     0     8
  [23] .got              PROGBITS         0000000000600ff8  00000ff8
       0000000000000008  0000000000000008  WA       0     0     8
  [24] .got.plt          PROGBITS         0000000000601000  00001000
       0000000000000028  0000000000000008  WA       0     0     8
  [25] .data             PROGBITS         0000000000601028  00001028
       0000000000000010  0000000000000000  WA       0     0     8
  [26] .bss              NOBITS           0000000000601038  00001038
       0000000000000008  0000000000000000  WA       0     0     1
  [27] .comment          PROGBITS         0000000000000000  00001038
       0000000000000032  0000000000000001  MS       0     0     1
  [28] .shstrtab         STRTAB           0000000000000000  00001907
       000000000000010c  0000000000000000           0     0     1
  [29] .symtab           SYMTAB           0000000000000000  00001070
       0000000000000678  0000000000000018          30    48     8
  [30] .strtab           STRTAB           0000000000000000  000016e8
       000000000000021f  0000000000000000           0     0     1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
  I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

从步骤b)中打印的程序表可以看出,数据段包含的 sections 有:.init_array、.fini_array、.jcr .dynamic、.got、.got.plt、.data、.bss 。

这些 sections 的大小(字节) = 0x601038 + 0x8 - 0x60e10 = 0x230(0x601038 为 .bss section 在目标文件中偏移量,0x8 为 .bss section 的大小,0x60e10 为 …init_array section 在目标文件中偏移量)。

也就是说,这些 sections 在内存中会占用 0x230 字节,符合程序表中数据段占用的内存大小。由于数据段包含了.bsssection(大小为 8 字节),而该 section 不占用目标文件的空间。因此,只有 0x228 字节来自可执行目标文件的节。

返回题目


7.12 解答

根据公式 VALUE(reference) = ADDR(defined_symbol) + VALUE(r.addend) - ADDR(reference):

对于 A:

callq 指令中对 swap 的重定位引用的值 = swap 符号定义的运行时地址 + r.addend - swap 符号引用的运行时地址
	= 0x4004f8 + (-4) - (.text section 在可执行目标文件中的地址 + swap 符号引用在 .text section 中的偏移量)
	= 0x4004f8 + (-4) - (0x4004e0 + 0xa)
	= 0xa

对于 B:

callq 指令中对 swap 的重定位引用的值 = swap 符号定义的运行时地址 + r.addend - swap 符号引用的运行时地址
	= 0x400500 + (-4) - (.text section 在可执行目标文件中的地址 + swap 符号引用在 .text section 中的偏移量)
	= 0x400500 + (-4) - (0x4004d0 + 0xa)
	= 0x22

返回题目


7.13 解答

对于 A:

# 查看 libc.a 中可重定位目标文件的数量
$ ar t /usr/aarch64-linux-gnu/lib/libc.a | grep ".o" -c
1522
# 查看 libm.a 中可重定位目标文件的数量
$ ar t /usr/aarch64-linux-gnu/lib/libm.a | grep ".o" -c
431

对于 B:在我们的系统中(X86-64 Linux, Little-endian),gcc -Og 产生的可执行代码与 gcc -Og -g 产生的是相同的。

对于 C:

# 查看 gcc-6 使用的共享库
$ ldd /usr/bin/gcc-6
	linux-vdso.so.1 =>  (0x00007ffc6178d000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f58462a6000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f5846670000)
# 查看 g++-6 使用的共享库
$ ldd /usr/bin/g++-6
	linux-vdso.so.1 =>  (0x00007ffc129d4000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f779003f000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f7790409000)

返回题目

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