C语言链接脚本浅析

前言

哎,链接脚本
在深入底层的时候还是需要了解一下的

实例

本文以aarch64架构下gcc的链接脚本为例(和x86略有不同)
源文件:src.c

int func()
{ return 6;}
int main()
{ return 0;}

链接成可执行文件

aarch64-arco-linux-gnu-gcc -c src.c -o src.o
aarch64-arco-linux-gnu-ld src.o -o src --verbose > exe.lds

exe.lds

/* Script for -z combreloc */
/* Copyright (C) 2014-2023 Free Software Foundation, Inc.
   Copying and distribution of this script, with or without modification,
   are permitted in any medium without royalty provided the copyright
   notice and this notice are preserved.  */
OUTPUT_FORMAT("elf64-littleaarch64", "elf64-bigaarch64",
	      "elf64-littleaarch64")
OUTPUT_ARCH(aarch64)
ENTRY(_start)
SEARCH_DIR("=/home/arco/x-tools/aarch64-arco-linux-gnu/aarch64-arco-linux-gnu/lib64"); SEARCH_DIR("=/usr/local/lib64"); SEARCH_DIR("=/lib64"); SEARCH_DIR("=/usr/lib64"); SEARCH_DIR("=/home/arco/x-tools/aarch64-arco-linux-gnu/aarch64-arco-linux-gnu/lib"); SEARCH_DIR("=/usr/local/lib"); SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
SECTIONS
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}

链接成动态库

aarch64-arco-linux-gnu-gcc -c src.c -o src.o
aarch64-arco-linux-gnu-ld src.o -o src --verbose > so.lds

so.lds

/* Script for -shared -z combreloc */
/* Copyright (C) 2014-2023 Free Software Foundation, Inc.
   Copying and distribution of this script, with or without modification,
   are permitted in any medium without royalty provided the copyright
   notice and this notice are preserved.  */
OUTPUT_FORMAT("elf64-littleaarch64", "elf64-bigaarch64",
	      "elf64-littleaarch64")
OUTPUT_ARCH(aarch64)
ENTRY(_start)
SEARCH_DIR("=/home/arco/x-tools/aarch64-arco-linux-gnu/aarch64-arco-linux-gnu/lib64"); SEARCH_DIR("=/usr/local/lib64"); SEARCH_DIR("=/lib64"); SEARCH_DIR("=/usr/lib64"); SEARCH_DIR("=/home/arco/x-tools/aarch64-arco-linux-gnu/aarch64-arco-linux-gnu/lib"); SEARCH_DIR("=/usr/local/lib"); SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
SECTIONS
{
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}

内容浅析

常用语法

OUTPUT

输出格式

OUTPUT_ARCH

架构

ENTRY

入口点,如果你不想用_start作为入口点,可以使用-e 换
但一般没必要,不用入口点,c运行时环境都用不了

MEMORY

内存分布定义,这里一样没用到

SEARCH_DIR

库文件搜索路径,优先级低于-L

SECTIONS

段列表定义

SEGMENG_START

SEGMENT_START(“a”, 0xb),定义.a段的起始地址为0xb,并将此地址返回
(这个段名称不会形成对应的符号)

SIZEOF_HEADERS

输出文件头的大小(没仔细看,也许就是ELF文件头的大小)

PROVIDE

重点来了,满篇都是这个
作用: 定义一个’输入文件内被引用但没定义’的符号,相当于在符号表中定义一个全局变量,其他源文件可以操作这个符号来读写对应的内存(这个符号本身不占用内存|地址,链接脚本中的声明一个PROVIDE不会使地址计数器增加)

冲突: 当其他源文件已经声明了同名强符号时,链接器会优先使用外部强符号(外部弱符号不会覆盖PROVIDE的符号(你可以extern一个试试

保留: PROVIDE声明的符号有些可以在符号表中被找到,有些则不行
1、未被引用的符号可能会被链接器回收(默认是–no-gc-sections,但有些符号确实会丢,可能因为它们不在段里)
2、如果PROVIDE声明的符号被其他源文件引用,那么最终生成的文件就会包含这个符号,例如__executable_start。nm查看不到的,但如果在源文件里extern一下,就有了。

PROVIDE_HIDDEN

类似PROVIDE,但对外部(其他源文件)不可见,如果有引用这个符号,nm命令仍然可见
比如__rela_iplt_start,glibc/csu里有用

HIDDEN

声明成HIDDEN的符号不会输出到目标文件

KEEP

保证链接器不优化指定的段(section)

SORT

SORT(func sections)
	使用自定义函数对sections的段排序
SORT_NONE
	不排序
SORT_BY_NAME
	按名称排序
SORT_BY_ALIGNMENT
	按对齐排序

ALIGN

强制输出section地址对齐(大概吧,没验过)

ONLY_IF_RO

诸如此类的..判断条件,只有某个段时只读时才...

符号含义

:

定义输出段
例1:

.interp : { *(.interp) }

通配符匹配所有目标文件的.interp段,合并到输出文件的.interp段
例2:

.init : { KEEP (*(SORT_NONE(.init))) } =0x1f2003d5

保留.init段不被优化、不对个源文件的.init段排序、在结束地址处填充0x1f2003d5(hexdump可见)

…init : ()

当前地址计数器

{}

段内容分组

*

通配符

链接脚本分析

总结

[1] 大部分内容是在讲合并段,比如.interp
[2] 少部分内容是声明符号(提供地址)供c库使用

推测

[1] PROVIDE的作用是定义符号供使用,那么,没有PROVIDE的便不是符号,如果在链接脚本中也没有使用的话,我认为是无用的。比如__bss_start = .;删除之后仍然正常编译运行。
[2] SEGMENT_START定义的段名称仅供人类阅读,实际上没有这个符号,如. = SEGMENT_START(“text-segment”, 0x400000) + SIZEOF_HEADERS; 把text-segment改成别的字符串,链接结果毫无变化。

动态库和可执行文件链接脚本的区别

[1] 可执行文件PROVIDE了__executable_start这个符号
[2] exe.lds合并了.interp段,so.lds没处理
[3] exe.lds提供了几个PROVIDE_HIDDEN的符号,很关键
[4] so.lds PROVIDE了部分符号,但exe.lds没有。

实例

将程序同时编译成可执行的动态库(C语言
这篇文章是在x86下验证的,在aarch64下 有所不同

命令

aarch64-arco-linux-gnu-ld src.o \
/home/arco/x-tools/aarch64-arco-linux-gnu/aarch64-arco-linux-gnu/sysroot/usr/lib/../lib64/crt1.o \
/home/arco/x-tools/aarch64-arco-linux-gnu/aarch64-arco-linux-gnu/sysroot/usr/lib/../lib64/crti.o \
/home/arco/x-tools/aarch64-arco-linux-gnu/lib/gcc/aarch64-arco-linux-gnu/11.4.0/crtbegin.o \
/home/arco/x-tools/aarch64-arco-linux-gnu/lib/gcc/aarch64-arco-linux-gnu/11.4.0/crtend.o \
/home/arco/x-tools/aarch64-arco-linux-gnu/aarch64-arco-linux-gnu/sysroot/usr/lib/../lib64/crtn.o \
-L/home/arco/x-tools/aarch64-arco-linux-gnu/aarch64-arco-linux-gnu/sysroot/usr/lib/../lib64 \
-shared -lc -o src.so

在某环境下执行上述链接命令报错:
in function main
undefined reference to __init_array_start
undefined reference to __init_array_end

分析

注释掉__executable_start的PROVIDE后仍然可以正常链接,
影响链接的是PROVIDE_HIDDEN的__init_array_start和__init_array_end,
这两个符号在glibc/csu/elf-init.c里被extern使用了。
所以用so.lds将动态库链接的可执行,需要PROVIDE这几个符号

_start
	__libc_csu_init
	__libc_start_main

__libc_csu_init
	const size_t size = __init_array_end - __init_array_start
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