鸿蒙关键技术研究,鸿蒙内核源码分析(静态链接篇) | 完整小项目看透静态链接过程 | 百篇博客分析HarmonyOS源码

百篇博客系列篇.本篇为:

下图是一个可执行文件编译,链接的过程.

本篇将通过一个完整的小工程来阐述ELF编译,链接过程,并分析.o和bin文件中各区,符号表之间的关系.从一个崭新的视角去看中间过程,阅读之前建议先看

准备工作

先得有个小工程,麻雀虽小,但五脏俱全,标准的文件夹和Makefile结构,如下:

目录结构

root@5e3abe332c5a:/home/docker/test4harmony/54# tree

├── bin

│ └── weharmony

├── include

│ └── part.h

├── Makefile

├── obj

│ ├── main.o

│ └── part.o

└── src

├── main.c

└── part.c

看到 .c .h .o 就感觉特别的亲切 : ),项目很简单,但具有代表性,有全局变量/函数,extern,多文件链接,和动态链接库的printf,用cat命令看看三个文件内容.

cat .c .h

root@5e3abe332c5a:/home/docker/test4harmony/54# cat ./src/main.c

#include

#include "part.h"

extern int g_int;

extern char *g_str;

int main() {

int loc_int = 53;

char *loc_str = "harmony os";

printf("main 开始 - 全局 g_int = %d, 全局 g_str = %s.\n", g_int, g_str);

func_int(loc_int);

func_str(loc_str);

printf("main 结束 - 全局 g_int = %d, 全局 g_str = %s.\n", g_int, g_str);

return 0;

}

root@5e3abe332c5a:/home/docker/test4harmony/54# cat ./src/part.c

#include

#include "part.h"

int g_int = 51;

char *g_str = "hello world";

void func_int(int i) {

int tmp = i;

g_int = 2 * tmp ;

printf("func_int g_int = %d,tmp = %d.\n", g_int,tmp);

}

void func_str(char *str) {

g_str = str;

printf("func_str g_str = %s.\n", g_str);

}

root@5e3abe332c5a:/home/docker/test4harmony/54# cat ./include/part.h

#ifndef _PART_H_

#define _PART_H_

void func_int(int i);

void func_str(char *str);

#endif

cat Makefile

Makefile采用标准写法,关于makefile系列篇会在编译过程篇中详细说明,此处先看点简单的.

root@5e3abe332c5a:/home/docker/test4harmony/54# cat Makefile

DIR_INC = ./include

DIR_SRC = ./src

DIR_OBJ = ./obj

DIR_BIN = ./bin

SRC = $(wildcard ${DIR_SRC}/*.c)

OBJ = $(patsubst %.c,${DIR_OBJ}/%.o,$(notdir ${SRC}))

TARGET = weharmony

BIN_TARGET = ${DIR_BIN}/${TARGET}

CC = gcc

CFLAGS = -g -Wall -I${DIR_INC}

${BIN_TARGET}:${OBJ}

$(CC) $(OBJ) -o $@

${DIR_OBJ}/%.o:${DIR_SRC}/%.c

$(CC) $(CFLAGS) -c $< -o $@

.PHONY:clean

clean:

find ${DIR_OBJ} -name *.o -exec rm -rf {}

编译.链接.运行.看结果

root@5e3abe332c5a:/home/docker/test4harmony/54# make

gcc -g -Wall -I./include -c src/part.c -o obj/part.o

gcc -g -Wall -I./include -c src/main.c -o obj/main.o

gcc ./obj/part.o ./obj/main.o -o bin/weharmony

root@5e3abe332c5a:/home/docker/test4harmony/54# ./bin/weharmony

main 开始 - 全局 g_int = 51, 全局 g_str = hello world.

func_int g_int = 106,tmp = 53.

func_str g_str = harmony os.

main 结束 - 全局 g_int = 106, 全局 g_str = harmony os.

结果很简单,没什么好说的.

开始分析

准备工作完成,开始了真正的分析. 因为命令输出内容太多,本篇做了精简,去除了干扰项.对这些命令还不行清楚的请翻看系列篇其他文章,此处不做介绍,阅读本篇需要一定的基础.

readelf 大S小s ./obj/main.o

root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -S ./obj/main.o

There are 22 section headers, starting at offset 0x1498:

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

000000000000007b 0000000000000000 AX 0 0 1

[ 2] .rela.text RELA 0000000000000000 00000c80

0000000000000108 0000000000000018 I 19 1 8

[ 3] .data PROGBITS 0000000000000000 000000bb

0000000000000000 0000000000000000 WA 0 0 1

[ 4] .bss NOBITS 0000000000000000 000000bb

0000000000000000 0000000000000000 WA 0 0 1

[ 5] .rodata PROGBITS 0000000000000000 000000c0

000000000000007d 0000000000000000 A 0 0 8

......

root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -s ./obj/main.o

Symbol table '.symtab' contains 22 entries:

Num: Value Size Type Bind Vis Ndx Name

0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND

1: 0000000000000000 0 FILE LOCAL DEFAULT ABS main.c

2: 0000000000000000 0 SECTION LOCAL DEFAULT 1

...

15: 0000000000000000 123 FUNC GLOBAL DEFAULT 1 main

16: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND g_str

17: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND g_int

18: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _GLOBAL_OFFSET_TABLE_

19: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND printf

20: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND func_int

21: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND func_str

解读

编译 main.c 后 main.o 告诉了链接器以下信息

有一个文件叫 main.c (Type=FILE)

文件中有个函数叫 main (Type=FUNC),并且这是一个全局函数,(Bind = GLOBAL , Vis = DEFAULT,全局的意思就是可以被外部文件所引用.

剩下的g_str,printf,func_int,....,都是需要外部提供,并未在本文件中定义的符号 (Ndx = UND , Type = NOTYPE),至于怎么顺藤摸瓜找到这些符号那我不管,.o文件是独立存在,它只是告诉你我用了哪些东西,但我也不知道在哪里.

printf和func_int对它来说一视同仁,都是外部链接符号,没有特殊对待.

readelf 大S小s ./obj/part.o

root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -S ./obj/part.o

[ 1] .text PROGBITS 0000000000000000 00000040

0000000000000078 0000000000000000 AX 0 0 1

[ 2] .rela.text RELA 0000000000000000 00000cf0

00000000000000c0 0000000000000018 I 21 1 8

[ 3] .data PROGBITS 0000000000000000 000000b8

0000000000000004 0000000000000000 WA 0 0 4

[ 4] .bss NOBITS 0000000000000000 000000bc

0000000000000000 0000000000000000 WA 0 0 1

[ 5] .rodata PROGBITS 0000000000000000 000000c0

0000000000000045 0000000000000000 A 0 0 8

[ 6] .data.rel.local PROGBITS 0000000000000000 00000108

0000000000000008 0000000000000000 WA 0 0 8

......

root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -s ./obj/part.o

Symbol table '.symtab' contains 22 entries:

Num: Value Size Type Bind Vis Ndx Name

0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND

1: 0000000000000000 0 FILE LOCAL DEFAULT ABS part.c

2: 0000000000000000 0 SECTION LOCAL DEFAULT 1

...

16: 0000000000000000 4 OBJECT GLOBAL DEFAULT 3 g_int

17: 0000000000000000 8 OBJECT GLOBAL DEFAULT 6 g_str

18: 0000000000000000 52 FUNC GLOBAL DEFAULT 1 func_int

19: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _GLOBAL_OFFSET_TABLE_

20: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND printf

21: 0000000000000034 57 FUNC GLOBAL DEFAULT 1 func_str

解读

编译 part.c 后part.o告诉了链接器以下信息

有一个文件叫 part.c (Type=FILE)

文件中有两个函数叫 func_int,func_str (Type=FUNC),并且都是全局函数,(Bind = GLOBAL , Vis = DEFAULT,全局的意思就是可以被外部文件所引用.

文件中有两个对象叫 g_int,g_str (Type=OBJECT),并且都是全局对象,同样可以被外部使用.

剩下的printf,_GLOBAL_OFFSET_TABLE_,都是需要外部提供,并未在本文件中定义的符号 (Ndx = UND , Type = NOTYPE)

另外 part.c的局部变量tmp并没有出现在符号表中.因为符号表相当于外交部,只有对外的内容.

func_int,func_str在1区代码区.text.

g_int 在3区.data数据区, 打开3区,发现了 0x33 就是源码中 int g_int = 51;的值 root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -x 3 ./obj/part.o

Hex dump of section '.data':

0x00000000 33000000 3...

g_str 在6区,.data.rel.local数据区,打开6区看结果 root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -x 6 ./obj/part.o

Hex dump of section '.data.rel.local':

NOTE: This section has relocations against it, but these have NOT been applied to this dump.

0x00000000 00000000 00000000 ........ 并未发现 char *g_str = "hello world";的身影,反而抛下一句话 NOTE: This section has relocations against it, but these have NOT been applied to this dump.翻译过来是注意：此部分已针对它进行重定位，但是尚未将其应用于此转储. 最后在5区 '.rodata'找到了 hello world root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -x 5 ./obj/part.o

Hex dump of section '.rodata':

0x00000000 68656c6c 6f20776f 726c6400 00000000 hello world.....

0x00000010 66756e63 5f696e74 20675f69 6e74203d func_int g_int =

0x00000020 2025642c 746d7020 3d202564 2e0a0066 %d,tmp = %d...f

0x00000030 756e635f 73747220 675f7374 72203d20 unc_str g_str =

0x00000040 25732e0a 00 %s.. 至于重定向是如何实现的,在系列篇重定向篇中已有详细说明,不再此展开说.

看完两个符号表总结下来就是三句话

我是谁,我在哪

我能提供什么给别人用

我需要别人提供什么给我用.

readelf 大S小s ./bin/weharmony

weharmony是将 main.o,part.o和库文件链接完成后的可执行文件.

root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -S ./bin/weharmony

There are 36 section headers, starting at offset 0x4908:

Section Headers:

[Nr] Name Type Address Offset

Size EntSize Flags Link Info Align

......

[16] .text PROGBITS 0000000000001060 00001060

0000000000000255 0000000000000000 AX 0 0 16

[17] .fini PROGBITS 00000000000012b8 000012b8

000000000000000d 0000000000000000 AX 0 0 4

[18] .rodata PROGBITS 0000000000002000 00002000

00000000000000cd 0000000000000000 A 0 0 8

......

[25] .data PROGBITS 0000000000004000 00003000

0000000000000020 0000000000000000 WA 0 0 8

[26] .bss NOBITS 0000000000004020 00003020

0000000000000008 0000000000000000 WA 0 0 1

root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -s ./bin/weharmony

Symbol table '.dynsym' contains 7 entries:

Num: Value Size Type Bind Vis Ndx Name

0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND

1: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterTMCloneTab

2: 0000000000000000 0 FUNC GLOBAL DEFAULT UND printf@GLIBC_2.2.5 (2)

3: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.2.5 (2)

4: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__

5: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMCloneTable

6: 0000000000000000 0 FUNC WEAK DEFAULT UND __cxa_finalize@GLIBC_2.2.5 (2)

Symbol table '.symtab' contains 75 entries:

Num: Value Size Type Bind Vis Ndx Name

0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND

1: 0000000000000318 0 SECTION LOCAL DEFAULT 1

2: 0000000000000338 0 SECTION LOCAL DEFAULT 2

3: 0000000000000358 0 SECTION LOCAL DEFAULT 3

....

33: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c

34: 0000000000001090 0 FUNC LOCAL DEFAULT 16 deregister_tm_clones

35: 00000000000010c0 0 FUNC LOCAL DEFAULT 16 register_tm_clones

36: 0000000000001100 0 FUNC LOCAL DEFAULT 16 __do_global_dtors_aux

37: 0000000000004020 1 OBJECT LOCAL DEFAULT 26 completed.8060

38: 0000000000003dc0 0 OBJECT LOCAL DEFAULT 22 __do_global_dtors_aux_fin

39: 0000000000001140 0 FUNC LOCAL DEFAULT 16 frame_dummy

40: 0000000000003db8 0 OBJECT LOCAL DEFAULT 21 __frame_dummy_init_array_

41: 0000000000000000 0 FILE LOCAL DEFAULT ABS part.c

42: 0000000000000000 0 FILE LOCAL DEFAULT ABS main.c

43: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c

44: 000000000000225c 0 OBJECT LOCAL DEFAULT 20 __FRAME_END__

45: 0000000000000000 0 FILE LOCAL DEFAULT ABS

46: 0000000000003dc0 0 NOTYPE LOCAL DEFAULT 21 __init_array_end

47: 0000000000003dc8 0 OBJECT LOCAL DEFAULT 23 _DYNAMIC

48: 0000000000003db8 0 NOTYPE LOCAL DEFAULT 21 __init_array_start

49: 00000000000020c0 0 NOTYPE LOCAL DEFAULT 19 __GNU_EH_FRAME_HDR

50: 0000000000003fb8 0 OBJECT LOCAL DEFAULT 24 _GLOBAL_OFFSET_TABLE_

51: 0000000000001000 0 FUNC LOCAL DEFAULT 12 _init

52: 00000000000012b0 5 FUNC GLOBAL DEFAULT 16 __libc_csu_fini

53: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterTMCloneTab

54: 0000000000004000 0 NOTYPE WEAK DEFAULT 25 data_start

55: 0000000000004020 0 NOTYPE GLOBAL DEFAULT 25 _edata

56: 00000000000012b8 0 FUNC GLOBAL HIDDEN 17 _fini

57: 0000000000000000 0 FUNC GLOBAL DEFAULT UND printf@@GLIBC_2.2.5

58: 0000000000004010 4 OBJECT GLOBAL DEFAULT 25 g_int

59: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@@GLIBC_

60: 0000000000004000 0 NOTYPE GLOBAL DEFAULT 25 __data_start

61: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__

62: 0000000000004008 0 OBJECT GLOBAL HIDDEN 25 __dso_handle

63: 0000000000004018 8 OBJECT GLOBAL DEFAULT 25 g_str

64: 0000000000002000 4 OBJECT GLOBAL DEFAULT 18 _IO_stdin_used

65: 0000000000001240 101 FUNC GLOBAL DEFAULT 16 __libc_csu_init

66: 0000000000001149 52 FUNC GLOBAL DEFAULT 16 func_int

67: 0000000000004028 0 NOTYPE GLOBAL DEFAULT 26 _end

68: 0000000000001060 47 FUNC GLOBAL DEFAULT 16 _start

69: 000000000000117d 57 FUNC GLOBAL DEFAULT 16 func_str

70: 0000000000004020 0 NOTYPE GLOBAL DEFAULT 26 __bss_start

71: 00000000000011b6 123 FUNC GLOBAL DEFAULT 16 main

72: 0000000000004020 0 OBJECT GLOBAL HIDDEN 25 __TMC_END__

73: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMCloneTable

74: 0000000000000000 0 FUNC WEAK DEFAULT UND __cxa_finalize@@GLIBC_2.2

解读

链接后的可执行文件 weharmony将告诉加载器以下信息

涉及文件有哪些 Type = FILE

涉及函数有哪些 Type = FUNC func_str,func_int,_start,main

涉及对象有哪些 Type = OBJECT g_int,g_str,....它将这些数据统一归到了25区. 前往25区查看下数据,同样只发现了 int g_int = 51; 的数据. root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -x 25 ./bin/weharmony

Hex dump of section '.data':

0x00004000 00000000 00000000 08400000 00000000 .........@......

0x00004010 33000000 00000000 08200000 00000000 3........ ...... 是不是和part.o一样也被放在了.rodata区,再反查 18区,果然发了 main.c和part.c的数据都放在了这里. root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -x 18 ./bin/weharmony

Hex dump of section '.rodata':

0x00002000 01000200 00000000 68656c6c 6f20776f ........hello wo

0x00002010 726c6400 00000000 66756e63 5f696e74 rld.....func_int

0x00002020 20675f69 6e74203d 2025642c 746d7020 g_int = %d,tmp

0x00002030 3d202564 2e0a0066 756e635f 73747220 = %d...func_str

0x00002040 675f7374 72203d20 25732e0a 00000000 g_str = %s......

0x00002050 6861726d 6f6e7920 6f730000 00000000 harmony os......

0x00002060 6d61696e 20e5bc80 e5a78b20 2d20e585 main ...... - ..

0x00002070 a8e5b180 20675f69 6e74203d 2025642c .... g_int = %d,

0x00002080 20e585a8 e5b18020 675f7374 72203d20 ...... g_str =

0x00002090 25732e0a 00000000 6d61696e 20e7bb93 %s......main ...

0x000020a0 e69d9f20 2d20e585 a8e5b180 20675f69 ... - ...... g_i

0x000020b0 6e74203d 2025642c 20e585a8 e5b18020 nt = %d, ......

0x000020c0 675f7374 72203d20 25732e0a 00 g_str = %s...

另外还有注意printf的变化,从Type = NOTYPE 变成了Type = FUNC,告诉了后续的动态链接这是个函数 57: 0000000000000000 0 FUNC GLOBAL DEFAULT UND printf@@GLIBC_2.2.5 但是内容依然是Ndx=UND,weharmony也提供不了,内容需要运行时环境提供.并在需要动态链接表中也已经注明了内容清单,运行环境必须提供以下内容才能真正跑起来weharmony. Symbol table '.dynsym' contains 7 entries:

Num: Value Size Type Bind Vis Ndx Name

0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND

1: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterTMCloneTab

2: 0000000000000000 0 FUNC GLOBAL DEFAULT UND printf@GLIBC_2.2.5 (2)

3: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.2.5 (2)

4: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__

5: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMCloneTable

6: 0000000000000000 0 FUNC WEAK DEFAULT UND __cxa_finalize@GLIBC_2.2.5 (2) 本例在windows环境中一般是跑不起来的.除非提供对应的运行时环境.

百篇博客.往期回顾

在加注过程中,整理出以下文章.内容立足源码,常以生活场景打比方尽可能多的将内核知识点置入某种场景,具有画面感,容易理解记忆.说别人能听得懂的话很重要! 百篇博客绝不是百度教条式的在说一堆诘屈聱牙的概念,那没什么意思.更希望让内核变得栩栩如生,倍感亲切.确实有难度,自不量力,但已经出发,回头已是不可能的了.:P 与代码有bug需不断debug一样,文章和注解内容会存在不少错漏之处,但会反复修正,持续更新,.xx代表修改的次数,精雕细琢,言简意赅,力求打造精品内容.

关于 51 .c .h .o

看系列篇文章会常看到 51 .c .h .o,希望这对大家阅读不会造成影响. 分别对应以下四个站点的首个字符,感谢这些站点一直以来对系列篇的支持和推荐,尤其是 oschina gitee ,很喜欢它的界面风格,简洁大方,让人感觉到开源的伟大!

而巧合的是.c .h .o是C语言的头/源/目标文件,这就很有意思了,冥冥之中似有天数,将这四个宝贝以这种方式融合在一起. 51 .c .h .o , 我要CHO ,嗯嗯,hin 顺口 : )

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