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

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.

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  4 directories， 7 files

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 <stdio.h>
#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 <stdio.h>
#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 = ./binSRC = $(wildcard ${DIR_SRC}/*.c)
OBJ = $(patsubst %.c，${DIR_OBJ}/%.o，$(notdir ${SRC}))TARGET = weharmonyBIN_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           OffsetSize              EntSize          Flags  Link  Info  Align[ 0]                   NULL             0000000000000000  000000000000000000000000  0000000000000000           0     0     0[ 1] .text             PROGBITS         0000000000000000  00000040000000000000007b  0000000000000000  AX       0     0     1[ 2] .rela.text        RELA             0000000000000000  00000c800000000000000108  0000000000000018   I      19     1     8[ 3] .data             PROGBITS         0000000000000000  000000bb0000000000000000  0000000000000000  WA       0     0     1[ 4] .bss              NOBITS           0000000000000000  000000bb0000000000000000  0000000000000000  WA       0     0     1[ 5] .rodata           PROGBITS         0000000000000000  000000c0000000000000007d  0000000000000000   A       0     0     8......
root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -s ./obj/main.oSymbol 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.c2: 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 printf20: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND func_int21: 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  000000400000000000000078  0000000000000000  AX       0     0     1[ 2] .rela.text        RELA             0000000000000000  00000cf000000000000000c0  0000000000000018   I      21     1     8[ 3] .data             PROGBITS         0000000000000000  000000b80000000000000004  0000000000000000  WA       0     0     4[ 4] .bss              NOBITS           0000000000000000  000000bc0000000000000000  0000000000000000  WA       0     0     1[ 5] .rodata           PROGBITS         0000000000000000  000000c00000000000000045  0000000000000000   A       0     0     8[ 6] .data.rel.local   PROGBITS         0000000000000000  000001080000000000000008  0000000000000000  WA       0     0     8......
root@5e3abe332c5a:/home/docker/test4harmony/54# readelf -s ./obj/part.oSymbol table '.symtab' contains 22 entries:Num:    Value          Size Type    Bind   Vis      Ndx Name0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS part.c2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1...16: 0000000000000000     4 OBJECT  GLOBAL DEFAULT    3 g_int17: 0000000000000000     8 OBJECT  GLOBAL DEFAULT    6 g_str18: 0000000000000000    52 FUNC    GLOBAL DEFAULT    1 func_int19: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _GLOBAL_OFFSET_TABLE_20: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND printf21: 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.oHex 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.oHex 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.oHex 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...f0x00000030 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           OffsetSize              EntSize          Flags  Link  Info  Align......[16] .text             PROGBITS         0000000000001060  000010600000000000000255  0000000000000000  AX       0     0     16[17] .fini             PROGBITS         00000000000012b8  000012b8000000000000000d  0000000000000000  AX       0     0     4[18] .rodata           PROGBITS         0000000000002000  0000200000000000000000cd  0000000000000000   A       0     0     8       ......[25] .data             PROGBITS         0000000000004000  000030000000000000000020  0000000000000000  WA       0     0     8[26] .bss              NOBITS           0000000000004020  000030200000000000000008  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 Name0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND1: 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 Name0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND1: 0000000000000318     0 SECTION LOCAL  DEFAULT    12: 0000000000000338     0 SECTION LOCAL  DEFAULT    23: 0000000000000358     0 SECTION LOCAL  DEFAULT    3....33: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c34: 0000000000001090     0 FUNC    LOCAL  DEFAULT   16 deregister_tm_clones35: 00000000000010c0     0 FUNC    LOCAL  DEFAULT   16 register_tm_clones36: 0000000000001100     0 FUNC    LOCAL  DEFAULT   16 __do_global_dtors_aux37: 0000000000004020     1 OBJECT  LOCAL  DEFAULT   26 completed.806038: 0000000000003dc0     0 OBJECT  LOCAL  DEFAULT   22 __do_global_dtors_aux_fin39: 0000000000001140     0 FUNC    LOCAL  DEFAULT   16 frame_dummy40: 0000000000003db8     0 OBJECT  LOCAL  DEFAULT   21 __frame_dummy_init_array_41: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS part.c42: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS main.c43: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c44: 000000000000225c     0 OBJECT  LOCAL  DEFAULT   20 __FRAME_END__45: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS46: 0000000000003dc0     0 NOTYPE  LOCAL  DEFAULT   21 __init_array_end47: 0000000000003dc8     0 OBJECT  LOCAL  DEFAULT   23 _DYNAMIC48: 0000000000003db8     0 NOTYPE  LOCAL  DEFAULT   21 __init_array_start49: 00000000000020c0     0 NOTYPE  LOCAL  DEFAULT   19 __GNU_EH_FRAME_HDR50: 0000000000003fb8     0 OBJECT  LOCAL  DEFAULT   24 _GLOBAL_OFFSET_TABLE_51: 0000000000001000     0 FUNC    LOCAL  DEFAULT   12 _init52: 00000000000012b0     5 FUNC    GLOBAL DEFAULT   16 __libc_csu_fini53: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTab54: 0000000000004000     0 NOTYPE  WEAK   DEFAULT   25 data_start55: 0000000000004020     0 NOTYPE  GLOBAL DEFAULT   25 _edata56: 00000000000012b8     0 FUNC    GLOBAL HIDDEN    17 _fini57: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND printf@@GLIBC_2.2.558: 0000000000004010     4 OBJECT  GLOBAL DEFAULT   25 g_int59: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@@GLIBC_60: 0000000000004000     0 NOTYPE  GLOBAL DEFAULT   25 __data_start61: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__62: 0000000000004008     0 OBJECT  GLOBAL HIDDEN    25 __dso_handle63: 0000000000004018     8 OBJECT  GLOBAL DEFAULT   25 g_str64: 0000000000002000     4 OBJECT  GLOBAL DEFAULT   18 _IO_stdin_used65: 0000000000001240   101 FUNC    GLOBAL DEFAULT   16 __libc_csu_init66: 0000000000001149    52 FUNC    GLOBAL DEFAULT   16 func_int67: 0000000000004028     0 NOTYPE  GLOBAL DEFAULT   26 _end68: 0000000000001060    47 FUNC    GLOBAL DEFAULT   16 _start69: 000000000000117d    57 FUNC    GLOBAL DEFAULT   16 func_str70: 0000000000004020     0 NOTYPE  GLOBAL DEFAULT   26 __bss_start71: 00000000000011b6   123 FUNC    GLOBAL DEFAULT   16 main72: 0000000000004020     0 OBJECT  GLOBAL HIDDEN    25 __TMC_END__73: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable74: 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/weharmonyHex dump of section '.rodata':0x00002000 01000200 00000000 68656c6c 6f20776f ........hello wo0x00002010 726c6400 00000000 66756e63 5f696e74 rld.....func_int0x00002020 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_i0x000020b0 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 Name0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND1: 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)