Question

我正在调查共享库的重定位，并遇到了一些奇怪的问题。请考虑以下代码：

int myglob;

int ml_util_func(int p)
{
    return p + 2;
}

int ml_func2(int a, int b)
{
    int c = ml_util_func(a);
    return c + b + myglob;
}

我使用gcc -shared将其编译为非PIC 共享库。我在运行x86的32位Ubuntu上执行此操作。

生成的.so有一个重定位条目，用于调用ml_util_func中的ml_func2。以下是objdump -dR -Mintel上ml_func2的输出：

0000050d <ml_func2>:
 50d:   55                      push   ebp
 50e:   89 e5                   mov    ebp,esp
 510:   83 ec 14                sub    esp,0x14
 513:   8b 45 08                mov    eax,DWORD PTR [ebp+0x8]
 516:   89 04 24                mov    DWORD PTR [esp],eax
 519:   e8 fc ff ff ff          call   51a <ml_func2+0xd>
                        51a: R_386_PC32 ml_util_func
 51e:   89 45 fc                mov    DWORD PTR [ebp-0x4],eax
 521:   8b 45 0c                mov    eax,DWORD PTR [ebp+0xc]
 524:   8b 55 fc                mov    edx,DWORD PTR [ebp-0x4]
 527:   01 c2                   add    edx,eax
 529:   a1 00 00 00 00          mov    eax,ds:0x0
                        52a: R_386_32   myglob
 52e:   8d 04 02                lea    eax,[edx+eax*1]
 531:   c9                      leave  
 532:   c3                      ret    
 533:   90                      nop

请注意R_386_PC32指令上的call重定位。

现在，我的问题是为什么需要重新安置？ e8在x86上是“call relative ...”，并且由于ml_util_func在同一个对象中定义，因此链接器肯定可以计算它与调用之间的相对偏移量而不会将其保留为动态装载机？

有趣的是，如果ml_util_func被声明为static，则重定位将消失，链接器将正确计算并插入偏移量。什么是ml_util_func也被导出使链接器变得懒惰呢？

P.S。：我故意使用非PIC代码，以了解加载时重定位。

Answer 1

无法找到原因，但这是binutils对此的评论：

binutils-2.11.90-20010705-src.tar.gz / bfd / elf32-i386.c：679

      /* If we are creating a shared library, and this is a reloc
         against a global symbol, or a non PC relative reloc
         against a local symbol, then we need to copy the reloc
         into the shared library.  However, if we are linking with
         -Bsymbolic, we do not need to copy a reloc against a
         global symbol which is defined in an object we are

我认为，这个重定位是为了允许用户重载库中的任何全局符号而创建的。并且，似乎-Bsymbolic禁用此功能，并且不会从库本身生成符号的重定位。

http://www.rocketaware.com/man/man1/ld.1.htm

<强> -Bsymbolic 此选项会导致输出中的所有符号引用在此链接编辑会话中解决。唯一剩下的运行时间搬迁要求是基地相对重新安置，即关于加载地址的转换。未能解决任何符号引用都会导致报告错误。

各种-B模式和限制（C ++）的更长描述在这里：

http://developers.sun.com/sunstudio/documentation/ss12/mr/man1/CC.1.html

-B的结合的

           Specifies whether a library binding for linking is
           symbolic, dynamic (shared), or static (nonshared).

           -Bdynamic is the default.  You can use the -B
           option several times on a command line.

           For more information on the -Bbinding option, see
           the ld(1) man page and the Solaris documentation.


           -Bdynamic directs the link editor to look for
           liblib.so files. Use this option if you want
           shared library bindings for linking.  If the
           liblib.so files are not found, it looks for
           liblib.a files.

           -Bstatic directs the link editor to look only for
           liblib.a files. The .a suffix indicates that the
           file is static, that is, nonshared.  Use this
           option if you want nonshared library bindings for
           linking.

           -Bsymbolic forces symbols to be resolved within a
           shared library if possible, even when a symbol is
           already defined elsewhere. For an explanation of
           -Bsymbolic, see the ld(1) man page.

           This option and its arguments are passed to the
           linker, ld.  If you compile and link in separate
           steps and are using the -Bbinding option, you must
           include the option in the link step.

           Warning:

           Never use -Bsymbolic with programs containing C++
           code, use linker scoping instead. See the C++
           User's Guide for more information on linker scop-
           ing. See also the -xldscope option.

           With -Bsymbolic, references in different modules
           can bind to different copies of what is supposed
           to be one global object.

           The exception mechanism relies on comparing
           addresses. If you have two copies of something,
           their addresses won't compare equal, and the
           exception mechanism can fail because the exception
           mechanism relies on comparing what are supposed to
           be unique addresses.

Answer 2

请注意，对象不一定是整个链接的块。有一些方法可以将符号放在单独的部分中，这些部分可以放在最终的.exe中，具体取决于它是否被代码引用。（搜索-gc-sections链接器选项，以及相关节生成gcc选项）

如果没有使用任何部分，可能根本就没有微观优化。

共享库中的重定位条目

2 个答案: