linux-insides-zh/Misc/linkers.md

介绍

During the writing of the linux-insides book I have received many emails with questions related to the linker script and linker-related subjects. So I've decided to write this to cover some aspects of the linker and the linking of object files.

在写 linux-insides 一书的过程中，我收到了很多邮件询问关于链接器和链接器脚本的问题。所以我决定写这篇文章来介绍链接器和目标文件的链接方面的知识。

If we open the Linker page on Wikipedia, we will see following definition:

如果我们打开维基百科的 链接器 页，我们将会看到如下定义：

In computer science, a linker or link editor is a computer program that takes one or more object files generated by a compiler and combines them into a single executable file, library file, or another object file.

在计算机科学中，链接器（英文：Linker），是一个计算机程序，它将一个或多个由编译器生成的目标文件链接为一个单独的可执行文件，库文件或者另外一个目标文件

If you've written at least one program on C in your life, you will have seen files with the *.o extension. These files are object files. Object files are blocks of machine code and data with placeholder addresses that reference data and functions in other object files or libraries, as well as a list of its own functions and data. The main purpose of the linker is collect/handle the code and data of each object file, turning it into the final executable file or library. In this post we will try to go through all aspects of this process. Let's start.

如果你曾经用 C 写过至少一个程序，那你就已经见过以 *.o 扩展名结尾的文件了。这些文件是目标文件。目标文件是一块块的机器码和数据，其数据包含了引用其他目标文件或库的数据和函数的占位符地址，也包括其自身的函数和数据列表。链接器的主要目的就是收集/处理每一个目标文件的代码和数据，将它们转成最终的可执行文件或者库。在这篇文章里，我们会试着研究这个流程的各个方面。开始吧。

链接流程

Let's create a simple project with the following structure:

让我们按以下结构创建一个项目：

*-linkers
*--main.c
*--lib.c
*--lib.h

Our main.c source code file contains:

我们的 main.c 源文件包含了：

#include <stdio.h>

#include "lib.h"

int main(int argc, char **argv) {
	printf("factorial of 5 is: %d\n", factorial(5));
	return 0;
}

The lib.c file contains:

lib.c 文件包含了：

int factorial(int base) {
	int res,i = 1;
	
	if (base == 0) {
		return 1;
	}

	while (i <= base) {
		res *= i;
		i++;
	}

	return res;
}

And the lib.h file contains:

lib.h 文件包含了：

#ifndef LIB_H
#define LIB_H

int factorial(int base);

#endif

Now let's compile only the main.c source code file with:

现在让我们用以下命令单独编译 main.c 源码：

$ gcc -c main.c

If we look inside the outputted object file with the nm util, we will see the following output:

如果我们用 nm 工具查看输出的目标文件，我们将会看到如下输出：

$ nm -A main.o
main.o:                 U factorial
main.o:0000000000000000 T main
main.o:                 U printf

The nm util allows us to see the list of symbols from the given object file. It consists of three columns: the first is the name of the given object file and the address of any resolved symbols. The second column contains a character that represents the status of the given symbol. In this case the U means undefined and the T denotes that the symbols are placed in the .text section of the object. The nm utility shows us here that we have three symbols in the main.c source code file:

nm 工具让我们能够看到给定目标文件的符号表列表。其包含了三列：第一列是该目标文件的名称和解析得到的符号地址。第二列包含了一个表示该符号状态的字符。这里 U 表示 未定义， T 表示该符号被置于 .text 段。在这里， nm 工具向我们展示了 main.c 文件里包含的三个符号：

• factorial - the factorial function defined in the lib.c source code file. It is marked as undefined here because we compiled only the main.c source code file, and it does not know anything about code from the lib.c file for now;

• main - the main function;

• printf - the function from the glibc library. main.c does not know anything about it for now either.

• factorial - 在 lib.c 文件中定义的阶乘函数。因为我们只编译了 main.c，所以其不知道任何有关 lib.c 文件的事；

• main - 主函数;

• printf - 来自 glibc 库的函数。 main.c 同样不知道任何与其相关的事。

What can we understand from the output of nm so far? The main.o object file contains the local symbol main at address 0000000000000000 (it will be filled with correct address after is is linked), and two unresolved symbols. We can see all of this information in the disassembly output of the main.o object file:

目前我们可以从 nm 的输出中了解哪些事情呢？ main.o 目标文件包含了在地址 0000000000000000 处的本地变量 main （在被链接后其将会被赋予正确的地址），以及两个无法解析的符号。我们可以从 main.o 的反汇编输出中了解这些信息：

$ objdump -S main.o

main.o:     file format elf64-x86-64
Disassembly of section .text:

0000000000000000 <main>:
   0:	55                   	push   %rbp
   1:	48 89 e5             	mov    %rsp,%rbp
   4:	48 83 ec 10          	sub    $0x10,%rsp
   8:	89 7d fc             	mov    %edi,-0x4(%rbp)
   b:	48 89 75 f0          	mov    %rsi,-0x10(%rbp)
   f:	bf 05 00 00 00       	mov    $0x5,%edi
  14:	e8 00 00 00 00       	callq  19 <main+0x19>
  19:	89 c6                	mov    %eax,%esi
  1b:	bf 00 00 00 00       	mov    $0x0,%edi
  20:	b8 00 00 00 00       	mov    $0x0,%eax
  25:	e8 00 00 00 00       	callq  2a <main+0x2a>
  2a:	b8 00 00 00 00       	mov    $0x0,%eax
  2f:	c9                   	leaveq 
  30:	c3                   	retq   

Here we are interested only in the two callq operations. The two callq operations contain linker stubs, or the function name and offset from it to the next instruction. These stubs will be updated to the real addresses of the functions. We can see these functions' names with in the following objdump output:

这里我们只关注两个 callq 操作。这两个 callq 操作包含了 链接器存根，或者函数的名称和其相对当前的下一条指令的偏移。这些存根将会被更新到函数的真实地址。我们可以在下面的 objdump 输出看到这些函数的名字：

$ objdump -S -r main.o

...
  14:	e8 00 00 00 00       	callq  19 <main+0x19>
  15: R_X86_64_PC32	               factorial-0x4
  19:	89 c6                	mov    %eax,%esi
...
  25:	e8 00 00 00 00       	callq  2a <main+0x2a>
  26:   R_X86_64_PC32	               printf-0x4
  2a:	b8 00 00 00 00       	mov    $0x0,%eax
...

The -r or --reloc flags of the objdump util print the relocation entries of the file. Now let's look in more detail at the relocation process.

objdump 工具中的 -r 或 --reloc 选项会打印文件的 重定位 条目。现在让我们更加深入重定位流程。

重定位

Relocation is the process of connecting symbolic references with symbolic definitions. Let's look at the previous snippet from the objdump output:

重定位是连接符号引用和符号定义的流程。让我们看看前一段 objdump 的输出：

  14:	e8 00 00 00 00       	callq  19 <main+0x19>
  15:   R_X86_64_PC32	               factorial-0x4
  19:	89 c6                	mov    %eax,%esi

Note the e8 00 00 00 00 on the first line. The e8 is the opcode of the call, and the remainder of the line is a relative offset. So the e8 00 00 00 00 contains a one-byte operation code followed by a four-byte address. Note that the 00 00 00 00 is 4-bytes. Why only 4-bytes if an address can be 8-bytes in a x86_64 (64-bit) machine? Actually we compiled the main.c source code file with the -mcmodel=small! From the gcc man page:

注意第一行的 e8 00 00 00 00 。e8 是 call 的 操作码 ，这一行的剩余部分是一个相对偏移。所以 e8 00 00 00 包含了一个单字节操作码，跟着一个四字节地址。注意 00 00 00 00 是 4 个字节。为什么只有 4 字节 而不是 x86_64 64 位机器上的 8 字节地址？其实我们用了 -mcmodel=small 选项来编译 main.c ！从 gcc 的指南上看：

-mcmodel=small

Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model.

-mcmodel=small

为小代码模型生成代码: 目标程序及其符号必须被链接到低于 2GB 的地址空间。指针是 64 位的。程序可以被动态或静态的链接。这是默认的代码模型。

Of course we didn't pass this option to the gcc when we compiled the main.c, but it is the default. We know that our program will be linked in the lower 2 GB of the address space from the gcc manual extract above. Four bytes is therefore enough for this. So we have opcode of the call instruction and an unknown address. When we compile main.c with all its dependencies to an executable file, and then look at the factorial call we see:

当然，我们在编译时并没有将这一选项传给 gcc ，但是这是默认的。从上面摘录的 gcc 指南我们知道，我们的程序会被链接到低于 2 GB 的地址空间。因此 4 字节已经足够。所以我们有了 call 指令和一个未知的地址。当我们编译 main.c 以及它的依赖形成一个可执行文件时，关注阶乘函数的调用，我们看到：

$ gcc main.c lib.c -o factorial | objdump -S factorial | grep factorial

factorial:     file format elf64-x86-64
...
...
0000000000400506 <main>:
	40051a:	e8 18 00 00 00       	callq  400537 <factorial>
...
...
0000000000400537 <factorial>:
	400550:	75 07                	jne    400559 <factorial+0x22>
	400557:	eb 1b                	jmp    400574 <factorial+0x3d>
	400559:	eb 0e                	jmp    400569 <factorial+0x32>
	40056f:	7e ea                	jle    40055b <factorial+0x24>
...
...

As we can see in the previous output, the address of the main function is 0x0000000000400506. Why it does not start from 0x0? You may already know that standard C programs are linked with the glibc C standard library (assuming the -nostdlib was not passed to the gcc). The compiled code for a program includes constructor functions to initialize data in the program when the program is started. These functions need to be called before the program is started, or in another words before the main function is called. To make the initialization and termination functions work, the compiler must output something in the assembler code to cause those functions to be called at the appropriate time. Execution of this program will start from the code placed in the special .init section. We can see this in the beginning of the objdump output:

在前面的输出中我们可以看到， main 函数的地址是 0x0000000000400506。为什么它不是从 0x0 开始的呢？你可能已经知道标准 C 程序是使用 glibc 的 C 标准库链接的（假设参数 -nostdlib 没有被传给 gcc ）。编译后的程序代码包含了用于在程序启动时初始化程序中数据的构造函数。这些函数需要在程序启动前被调用，或者说在 main 函数之前被调用。为了让初始化和终止函数起作用，编译器必须在汇编代码中输出一些让这些函数在正确时间被调用的代码。执行这个程序将会启动位于特殊的 .init 段的代码。我们可以从以下的 objdump 输出中看出：

objdump -S factorial | less

factorial:     file format elf64-x86-64

Disassembly of section .init:

00000000004003a8 <_init>:
  4003a8:       48 83 ec 08             sub    $0x8,%rsp
  4003ac:       48 8b 05 a5 05 20 00    mov    0x2005a5(%rip),%rax        # 600958 <_DYNAMIC+0x1d0>

Not that it starts at the 0x00000000004003a8 address relative to the glibc code. We can check it also in the ELF output by running readelf:

注意其开始于相对 glibc 代码偏移 0x00000000004003a8 的地址。我们也可以运行 readelf ，在 ELF 输出中检查：

$ readelf -d factorial | grep \(INIT\)
 0x000000000000000c (INIT)               0x4003a8

So, the address of the main function is 0000000000400506 and is offset from the .init section. As we can see from the output, the address of the factorial function is 0x0000000000400537 and binary code for the call of the factorial function now is e8 18 00 00 00. We already know that e8 is opcode for the call instruction, the next 18 00 00 00 (note that address represented as little endian for x86_64, so it is 00 00 00 18) is the offset from the callq to the factorial function:

所以， main 函数的地址是 0000000000400506 ，为相对于 .init 段的偏移地址。我们可以从输出中看出，factorial 函数的地址是 0x0000000000400537 ，并且现在调用 factorial 函数的二进制代码是 e8 18 00 00 00。我们已经知道 e8 是 call 指令的操作码，接下来的 18 00 00 00 （注意 x86_64中地址是小头存储的，所以是 00 00 00 18 ）是从 callq 到 factorial 函数的偏移。

>>> hex(0x40051a + 0x18 + 0x5) == hex(0x400537)
True

So we add 0x18 and 0x5 to the address of the call instruction. The offset is measured from the address of the following instruction. Our call instruction is 5-bytes long (e8 18 00 00 00) and the 0x18 is the offset of the call after the factorial function. A compiler generally creates each object file with the program addresses starting at zero. But if a program is created from multiple object files, these will overlap.

所以我们把 0x18和 0x5 加到 call 指令的地址上。偏移是从接下来一条指令开始算起的。我们的调用指令是 5 字节长（e8 18 00 00 00）并且 0x18 是从 factorial 函数之后的调用算起的偏移。编译器一般按程序地址从零开始创建目标文件。但是如果程序由多个目标文件生成，这些地址会重叠。

What we have seen in this section is the relocation process. This process assigns load addresses to the various parts of the program, adjusting the code and data in the program to reflect the assigned addresses.

我们在这一段看到的是 重定位 流程。这个流程为程序中各个部分赋予加载地址，调整程序中的代码和数据以反映出赋值的地址。

Ok, now that we know a little about linkers and relocation it is time to learn more about linkers by linking our object files.

好了，现在我们知道了一点关于链接器和重定位的知识，是时候通过链接我们的目标文件来来学习更多关于链接器的知识了。

GNU 链接器

As you can understand from the title, I will use GNU linker or just ld in this post. Of course we can use gcc to link our factorial project:

如标题所说，在这篇文章中，我将会使用 GNU 链接器 或者说 ld 。当然我们可以使用 gcc 来链接我们的 factorial 项目：

$ gcc main.c lib.o -o factorial

and after it we will get executable file - factorial as a result:

在这之后，作为结果我们将会得到可执行文件—— factorial：

./factorial 
factorial of 5 is: 120

But gcc does not link object files. Instead it uses collect2 which is just wrapper for the GNU ld linker:

但是 gcc 不会链接目标文件。取而代之，其会使用 GUN ld 链接器的包装—— collect2。

~$ /usr/lib/gcc/x86_64-linux-gnu/4.9/collect2 --version
collect2 version 4.9.3
/usr/bin/ld --version
GNU ld (GNU Binutils for Debian) 2.25
...
...
...

Ok, we can use gcc and it will produce executable file of our program for us. But let's look how to use GNU ld linker for the same purpose. First of all let's try to link these object files with the following example:

好，我们可以使用 gcc 并且其会为我们的程序生成可执行文件。但是让我们看看如何使用 GUN ld 实现相同的目的。首先，让我们尝试用如下样例链接这些目标文件：

ld main.o lib.o -o factorial

Try to do it and you will get following error:

尝试一下，你将会得到如下错误：

$ ld main.o lib.o -o factorial
ld: warning: cannot find entry symbol _start; defaulting to 00000000004000b0
main.o: In function `main':
main.c:(.text+0x26): undefined reference to `printf'

Here we can see two problems:

这里我们可以看到两个问题：

• Linker can't find _start symbol;

• Linker does not know anything about printf function.

• 链接器无法找到 _start 符号；

• 链接器对 printf 一无所知。

First of all let's try to understand what is this _start entry symbol that appears to be required for our program to run? When I started to learn programming I learned that the main function is the entry point of the program. I think you learned this too :) But it actually isn't the entry point, it's _start instead. The _start symbol is defined in the crt1.o object file. We can find it with the following command:

首先，让我们尝试理解好像是我们程序运行所需要的 _start 入口符号是什么。当我开始学习编程时，我知道了 main 函数是程序的入口点。我认为你们也是如此认为的 :) 但实际上这不是入口点，_start 才是。 _start 符号被 crt1.0 所定义。我们可以用如下指令发现它：

$ objdump -S /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o

/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <_start>:
   0:	31 ed                	xor    %ebp,%ebp
   2:	49 89 d1             	mov    %rdx,%r9
   ...
   ...
   ...

We pass this object file to the ld command as its first argument (see above). Now let's try to link it and will look on result:

我们将该目标文件作为第一个参数传递给 ld 指令（如上所示）。现在让我们尝试链接它，会得到如下结果：

ld /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o \
main.o lib.o -o factorial

/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o: In function `_start':
/tmp/buildd/glibc-2.19/csu/../sysdeps/x86_64/start.S:115: undefined reference to `__libc_csu_fini'
/tmp/buildd/glibc-2.19/csu/../sysdeps/x86_64/start.S:116: undefined reference to `__libc_csu_init'
/tmp/buildd/glibc-2.19/csu/../sysdeps/x86_64/start.S:122: undefined reference to `__libc_start_main'
main.o: In function `main':
main.c:(.text+0x26): undefined reference to `printf'

Unfortunately we will see even more errors. We can see here old error about undefined printf and yet another three undefined references:

不幸的是，我们甚至会看到更多报错。我们可以在这里看到关于未定义 printf 的旧错误以及另外三个未定义的引用：

• __libc_csu_fini
• __libc_csu_init
• __libc_start_main

The _start symbol is defined in the sysdeps/x86_64/start.S assembly file in the glibc source code. We can find following assembly code lines there:

_start 符号被定义在 glibc 源文件的汇编文件 sysdeps/x86_64/start.S 中。我们可以在那里找到如下汇编代码：

mov $__libc_csu_fini, %R8_LP
mov $__libc_csu_init, %RCX_LP
...
call __libc_start_main

Here we pass address of the entry point to the .init and .fini section that contain code that starts to execute when the program is ran and the code that executes when program terminates. And in the end we see the call of the main function from our program. These three symbols are defined in the csu/elf-init.c source code file. The following two object files:

这里我们传递了 .init 和 .fini 段的入口点地址，它们包含了程序开始和结束时被执行的代码。并且在结尾我们看到对我们程序的 main 函数的调用。这三个符号被定义在源文件 csu/elf-init.c 中。如下两个目标文件：

• crtn.o;
• crti.o.

define the function prologs/epilogs for the .init and .fini sections (with the _init and _fini symbols respectively).

定义了 .init 和 .fini 段的开端和尾声（分别为符号 _init 和 _fini ）。

The crtn.o object file contains these .init and .fini sections:

crtn.o 目标文件包含了 .init 和 .fini 这些段：

$ objdump -S /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crtn.o

0000000000000000 <.init>:
   0:	48 83 c4 08          	add    $0x8,%rsp
   4:	c3                   	retq   

Disassembly of section .fini:

0000000000000000 <.fini>:
   0:	48 83 c4 08          	add    $0x8,%rsp
   4:	c3                   	retq   

And the crti.o object file contains the _init and _fini symbols. Let's try to link again with these two object files:

且 crti.o 目标文件包含了符号 _init 和 _fini。让我们再次尝试链接这两个目标文件：

$ ld \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crti.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crtn.o main.o lib.o \
-o factorial

And anyway we will get the same errors. Now we need to pass -lc option to the ld. This option will search for the standard library in the paths present in the $LD_LIBRARY_PATH environment variable. Let's try to link again wit the -lc option:

当然，我们会得到相同的错误。现在我们需要把 -lc 选项传递给 ld 。这个选项将会在环境变量 $LD_LIBRARY_PATH 指定的目录中搜索标准库。让我们再次尝试用 -lc 选项链接：

$ ld \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crti.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crtn.o main.o lib.o -lc \
-o factorial

Finally we get an executable file, but if we try to run it, we will get strange results:

最后我们获得了一个可执行文件，但是如果我们尝试运行它，我们会遇到奇怪的结果：

$ ./factorial 
bash: ./factorial: No such file or directory

What's the problem here? Let's look on the executable file with the readelf util:

这里除了什么问题？让我们用 readelf 工具看看这个可执行文件：

$ readelf -l factorial 

Elf file type is EXEC (Executable file)
Entry point 0x4003c0
There are 7 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000400040 0x0000000000400040
                 0x0000000000000188 0x0000000000000188  R E    8
  INTERP         0x00000000000001c8 0x00000000004001c8 0x00000000004001c8
                 0x000000000000001c 0x000000000000001c  R      1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x0000000000000610 0x0000000000000610  R E    200000
  LOAD           0x0000000000000610 0x0000000000600610 0x0000000000600610
                 0x00000000000001cc 0x00000000000001cc  RW     200000
  DYNAMIC        0x0000000000000610 0x0000000000600610 0x0000000000600610
                 0x0000000000000190 0x0000000000000190  RW     8
  NOTE           0x00000000000001e4 0x00000000004001e4 0x00000000004001e4
                 0x0000000000000020 0x0000000000000020  R      4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     10

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

Note on the strange line:

注意这奇怪的一行：

  INTERP         0x00000000000001c8 0x00000000004001c8 0x00000000004001c8
                 0x000000000000001c 0x000000000000001c  R      1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]

The .interp section in the elf file holds the path name of a program interpreter or in another words the .interp section simply contains an ascii string that is the name of the dynamic linker. The dynamic linker is the part of Linux that loads and links shared libraries needed by an executable when it is executed, by copying the content of libraries from disk to RAM. As we can see in the output of the readelf command it is placed in the /lib64/ld-linux-x86-64.so.2 file for the x86_64 architecture. Now let's add the -dynamic-linker option with the path of ld-linux-x86-64.so.2 to the ld call and will see the following results:

elf 文件的 .interp 段保存了一个程序解释器的路径名，或者说 .interp 段就包含了一个动态链接器名字的 ascii 字符串。动态链接器是 Linux 的一部分，其通过将库的内容从磁盘复制到内存中以加载和链接一个可执行文件被执行所需要的动态链接库。我们可以从 readelf 命令的输出中看到，针对 x86_64 架构，其被放在 /lib64/ld-linux-x86-64.so.2。现在让我们把 ld-linux-x86-64.so.2 的路径和 -dynamic-linker 选项一起传递给 ld 调用，然后会看到如下结果：

$ gcc -c main.c lib.c

$ ld \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crti.o \
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crtn.o main.o lib.o \
-dynamic-linker /lib64/ld-linux-x86-64.so.2 \
-lc -o factorial

Now we can run it as normal executable file:

现在我们可以像普通可执行文件一样执行它了：

$ ./factorial

factorial of 5 is: 120

It works! With the first line we compile the main.c and the lib.c source code files to object files. We will get the main.o and the lib.o after execution of the gcc:

成功了！在第一行，我们把源文件 main.c 和 lib.c 编译成目标文件。执行 gcc 之后我们将会获得 main.o 和 lib.o：

$ file lib.o main.o
lib.o:  ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
main.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

and after this we link object files of our program with the needed system object files and libraries. We just saw a simple example of how to compile and link a C program with the gcc compiler and GNU ld linker. In this example we have used a couple command line options of the GNU linker, but it supports much more command line options than -o, -dynamic-linker, etc... Moreover GNU ld has its own language that allows to control the linking process. In the next two paragraphs we will look into it.

在这之后，我们用所需的系统目标文件和库连链接我们的程序。我们刚看了一个简单的关于如何用 gcc 编译器和 GNU ld 链接器编译和链接一个 C 程序的样例。在这个样例中，我们使用了一些 GNU linker 的命令行选项，但是除了 -o、-dynamic-linker 等，它还支持其他很多选项。此外，GNU ld 还拥有其自己的语言来控制链接过程。在接下来的两个段落中我们深入讨论。

实用的 GNU 链接器命令行选项

As I already wrote and as you can see in the manual of the GNU linker, it has big set of the command line options. We've seen a couple of options in this post: -o <output> - that tells ld to produce an output file called output as the result of linking, -l<name> that adds the archive or object file specified by the name, -dynamic-linker that specifies the name of the dynamic linker. Of course ld supports much more command line options, let's look at some of them.

正如我之前所说，你也可以从 GNU linker 的指南看到，其拥有大量的命令行选项。我们已经在这篇文章见到一些： -o <output> - 告诉 ld 将链接结果输出成一个叫做 output 的文件，-l<name> - 通过文件名添加指定存档或者目标文件，-dynamic-linker 通过名字指定动态链接器。当然， ld 支持更多选项，让我们看看其中的一些。

The first useful command line option is @file. In this case the file specifies filename where command line options will be read. For example we can create file with the name linker.ld, put there our command line arguments from the previous example and execute it with:

第一个实用的选项是 @file 。在这里 file 指定了命令行选项将读取的文件名。比如我们可以创建一个叫做 linker.ld 的文件，把我们上一个例子里面的命令行参数放进去然后执行：

$ ld @linker.ld

The next command line option is -b or --format. This command line option specifies format of the input object files ELF, DJGPP/COFF and etc. There is a command line option for the same purpose but for the output file: --oformat=output-format.

下一个命令行选项是 -b 或 --format。这个命令行选项指定了输入的目标文件的格式是 ELF, DJGPP/COFF 等。针对输出文件也有相同功能的选项 --oformat=output-format 。

The next command line option is --defsym. Full format of this command line option is the --defsym=symbol=expression. It allows to create global symbol in the output file containing the absolute address given by expression. We can find following case where this command line option can be useful: in the Linux kernel source code and more precisely in the Makefile that is related to the kernel decompression for the ARM architecture - arch/arm/boot/compressed/Makefile, we can find following definition:

下一个命令行选项是 --defsym 。该选项的完整格式是 --defsym=symbol=expression 。它允许在输出文件中创建包含了由表达式给出了绝对地址的全局符号。在下面的例子中，我们会发现这个命令行选项很实用：在 Linux 内核源码中关于 ARM 架构内核解压的 Makefile - arch/arm/boot/compressed/Makefile，我们可以找到如下定义：

LDFLAGS_vmlinux = --defsym _kernel_bss_size=$(KBSS_SZ)

As we already know, it defines the _kernel_bss_size symbol with the size of the .bss section in the output file. This symbol will be used in the first assembly file that will be executed during kernel decompressing:

正如我们所知，其在输出文件中用 .bss 段的大小定义了 _kernel_bss_size 符号。这个符号将会作为第一个 汇编文件 在内核解压阶段被执行：

ldr r5, =_kernel_bss_size

The next command line options is the -shared that allows us to create shared library. The -M or -map <filename> command line option prints the linking map with the information about symbols. In our case:

下一个选项是 -shared ，其允许我们创建共享库。-M 或者说 -map <filename> 命令行选项会打印带符号信息的链接映射内容。在这里是：

$ ld -M @linker.ld
...
...
...
.text           0x00000000004003c0      0x112
 *(.text.unlikely .text.*_unlikely .text.unlikely.*)
 *(.text.exit .text.exit.*)
 *(.text.startup .text.startup.*)
 *(.text.hot .text.hot.*)
 *(.text .stub .text.* .gnu.linkonce.t.*)
 .text          0x00000000004003c0       0x2a /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o
...
...
...
 .text          0x00000000004003ea       0x31 main.o
                0x00000000004003ea                main
 .text          0x000000000040041b       0x3f lib.o
                0x000000000040041b                factorial

Of course the GNU linker support standard command line options: --help and --version that print common help of the usage of the ld and its version. That's all about command line options of the GNU linker. Of course it is not the full set of command line options supported by the ld util. You can find the complete documentation of the ld util in the manual.

当然，GNU 链接器 支持标准的命令行选项：--help 和 --version 能够打印 ld 的命令帮助、使用方法和版本。以上就是所有关于 GNU 链接器 命令行选项的内容。当然这不是 ld 工具支持的所有命令行选项。你可以在指南中找到 ld 工具的完整文档。

链接器控制语言

As I wrote previously, ld has support for its own language. It accepts Linker Command Language files written in a superset of AT&T's Link Editor Command Language syntax, to provide explicit and total control over the linking process. Let's look on its details.

如我之前所说， ld 支持它自己的语言。它接受由一种 AT&T 链接器控制语法的超集编写的链接器控制语言文件，以提供对链接过程明确且完全的控制。接下来让我们关注其中细节。

With the linker language we can control:

我们可以通过链接器语言控制：

• input files;

• output files;

• file formats

• addresses of sections;

• etc...

• 输入文件；

• 输出文件；

• 文件格式；

• 段的地址；

• 其他更多...

Commands written in the linker control language are usually placed in a file called linker script. We can pass it to ld with the -T command line option. The main command in a linker script is the SECTIONS command. Each linker script must contain this command and it determines the map of the output file. The special variable . contains current position of the output. Let's write a simple assembly program and we will look at how we can use a linker script to control linking of this program. We will take a hello world program for this example:

用链接器控制语言编写的命令通常被放在一个被称作链接器脚本的文件中。我们可以通过命令行选项 -T 将其传递给 ld 。一个链接器脚本的主要命令是 SECTIONS 指令。每个链接器脚本必须包含这个指令，并且其决定了输出文件的 映射 。特殊变量 . 包含了当前输出的位置。让我们写一个简单的汇编程序，然后看看如何使用链接器脚本来控制程序的链接。我们将会使用一个 hello world 程序作为样例。

.data
	msg	.ascii "hello, world!",`\n`

.text

	global	_start
  
_start:
	mov    $1,%rax
	mov    $1,%rdi
	mov    $msg,%rsi
	mov    $14,%rdx
	syscall

	mov    $60,%rax
	mov    $0,%rdi
	syscall

We can compile and link it with the following commands:

我们可以用以下命令编译并链接：

$ as -o hello.o hello.asm
$ ld -o hello hello.o

Our program consists from two sections: .text contains code of the program and .data contains initialized variables. Let's write simple linker script and try to link our hello.asm assembly file with it. Our script is:

我们的程序包含了两个段： .text 包含了程序的代码， .data 段包含了被初始化的变量。让我们写一个简单的链接脚本，然后尝试用它来链接我们的 hello.asm 汇编文件。我们的脚本是：

/*
 * Linker script for the factorial
 */
OUTPUT(hello) 
OUTPUT_FORMAT("elf64-x86-64")
INPUT(hello.o)

SECTIONS
{
	. = 0x200000;
	.text : {
	      *(.text)
	}

	. = 0x400000;
	.data : {
	      *(.data)
	}
}

On the first three lines you can see a comment written in C style. After it the OUTPUT and the OUTPUT_FORMAT commands specify the name of our executable file and its format. The next command, INPUT, specifies the input file to the ld linker. Then, we can see the main SECTIONS command, which, as I already wrote, must be present in every linker script. The SECTIONS command represents the set and order of the sections which will be in the output file. At the beginning of the SECTIONS command we can see following line . = 0x200000. I already wrote above that . command points to the current position of the output. This line says that the code should be loaded at address 0x200000 and the line . = 0x400000 says that data section should be loaded at address 0x400000. The second line after the . = 0x200000 defines .text as an output section. We can see *(.text) expression inside it. The * symbol is wildcard that matches any file name. In other words, the *(.text) expression says all .text input sections in all input files. We can rewrite it as hello.o(.text) for our example. After the following location counter . = 0x400000, we can see definition of the data section.

在前三行你可以看到 C 风格的注释。之后是 OUTPUT 和 OUTPUT_FORMAT 命令，指定了我们的可执行文件名称和格式。下一个指令，INPUT，指定了给 ld 的输入文件。接下来，我们可以看到主要的 SECTIONS 指令，正如我写的，它是必须存在于每个链接器脚本中。SECTIONS 命令表示了输出文件中的段的集合和顺序。在 SECTIONS 命令的开头，我们可以看到一行 . = 0x200000 。我上面已经写过，. 命令指向输出中的当前位置。这一行说明代码段应该被加载到地址 0x200000。. = 0x400000一行说明数据段应该被加载到地址0x400000 。. = 0x200000之后的第二行定义 .text 作为输出段。我们可以看到其中的 *(.text) 表达式。 * 符号是一个匹配任意文件名的通配符。换句话说，*(.text) 表达式代表所有输入文件中的所有 .text 输入段。在我们的样例中，我们可以将其重写为 hello.o(.text) 。在地址计数器 . = 0x400000 之后，我们可以看到数据段的定义。

We can compile and link it with the:

我们可以用以下语句进行编译和链接：

$ as -o hello.o hello.S && ld -T linker.script && ./hello
hello, world!

If we will look inside it with the objdump util, we can see that .text section starts from the address 0x200000 and the .data sections starts from the address 0x400000:

如果我们用 objdump 工具深入查看，我们可以看到 .text 段从地址 0x200000 开始， .data 段从 0x400000 开始：

$ objdump -D hello

Disassembly of section .text:

0000000000200000 <_start>:
  200000:	48 c7 c0 01 00 00 00 	mov    $0x1,%rax
  ...

Disassembly of section .data:

0000000000400000 <msg>:
  400000:	68 65 6c 6c 6f       	pushq  $0x6f6c6c65
  ...

Apart from the commands we have already seen, there are a few others. The first is the ASSERT(exp, message) that ensures that given expression is not zero. If it is zero, then exit the linker with an error code and print the given error message. If you've read about Linux kernel booting process in the linux-insides book, you may know that the setup header of the Linux kernel has offset 0x1f1. In the linker script of the Linux kernel we can find a check for this:

除了我们已经看到的命令，另外还有一些。首先是 ASSERT(exp, message) ，保证给定的表达式不为零。如果为零，那么链接器会退出同时返回错误码，打印错误信息。如果你已经阅读了 linux-insides 的 Linux 内核启动流程，你或许知道 Linux 内核的设置头的偏移为 0x1f1。在 Linux 内核的链接器脚本中，我们可以看到下面的校验：

. = ASSERT(hdr == 0x1f1, "The setup header has the wrong offset!");

The INCLUDE filename command allows to include external linker script symbols in the current one. In a linker script we can assign a value to a symbol. ld supports a couple of assignment operators:

INCLUDE filename 允许我们在当前的链接器脚本中包含外部符号。我们可以在一个链接器脚本中给一个符号赋值。 ld 支持一些赋值操作符：

• symbol = expression ;
• symbol += expression ;
• symbol -= expression ;
• symbol *= expression ;
• symbol /= expression ;
• symbol <<= expression ;
• symbol >>= expression ;
• symbol &= expression ;
• symbol |= expression ;

As you can note all operators are C assignment operators. For example we can use it in our linker script as:

正如你注意到的，所有操作符都是 C 赋值操作符。比如我们可以在我们的链接器脚本中使用：

START_ADDRESS = 0x200000;
DATA_OFFSET   = 0x200000;

SECTIONS
{
	. = START_ADDRESS;
	.text : {
	      *(.text)
	}

	. = START_ADDRESS + DATA_OFFSET;
	.data : {
	      *(.data)
	}
}

As you already may noted the syntax for expressions in the linker script language is identical to that of C expressions. Besides this the control language of the linking supports following builtin functions:

你可能已经注意到了链接器脚本中表达式的语法和 C 表达式相同。除此之外，这个链接控制语言还支持如下内嵌函数：

• ABSOLUTE - returns absolute value of the given expression;

• ADDR - takes the section and returns its address;

• ALIGN - returns the value of the location counter (. operator) that aligned by the boundary of the next expression after the given expression;

• DEFINED - returns 1 if the given symbol placed in the global symbol table and 0 in other way;

• MAX and MIN - return maximum and minimum of the two given expressions;

• NEXT - returns the next unallocated address that is a multiple of the give expression;

• SIZEOF - returns the size in bytes of the given named section.

• ABSOLUTE - 返回给定表达式的绝对值；

• ADDR - 接受段，返回其地址；

• ALIGN - 返回和给定表达式下一句的边界对齐的位置计数器（ . 操作符）的值；

• DEFINED - 如果给定符号在全局符号表中，返回 1，否则 0；

• MAX and MIN - 返回两个给定表达式中的最大、最小值；

• NEXT - 返回一个是当前表达式倍数的未分配地址；

• SIZEOF - 返回给定名字的段以字节计数的大小。

That's all.

以上就是全部了。

总结

This is the end of the post about linkers. We learned many things about linkers in this post, such as what is a linker and why it is needed, how to use it, etc..

这是关于链接器文章的结尾。在这篇文章中，我们已经学习了很多关于链接器的知识，比如什么是链接器、为什么需要它、如何使用它等等...

If you have any questions or suggestions, write me an email or ping me on twitter.

如果你发现文中描述有任何问题，请提交一个 PR 到 linux-insides-zh 。

Please note that English is not my first language, and I am really sorry for any inconvenience. If you find any mistakes please let me know via email or send a PR.

相关链接

• Book about Linux kernel insides
• linker
• object files
• glibc
• opcode
• ELF
• GNU linker
• My posts about assembly programming for x86_64
• readelf