strip, eu-strip, objcopy, debuginfo
elf(可执行和可链接格式–Executable and Linkable Format)文件由ELF Header、Sections、Section Header Table 和Program Header Table组成，其中section,又包括代码段(.text), 数据段(.data)，只读数据段(.rodata), .bss, 注释信息段(.comment), 堆栈提示段(.note.GNU-stack)等等.
查看efl文件，可以用到file，readelf，objdump，nm等工具。 写个文件测试下：
#include <stdio.h>
int g_value = 1;
int g_value_uninit;
const int g_const_value = 10;
void fun(int age)
{
printf("age = %d\n",age);
}
int main()
{
int value = 1;
static int s_value = 1;
char *name = "Win!xp!";
fun(value);
return 0;
}
编译生成可执行文件：
$ gcc -g hello.c -o hello
使用file，查看hello信息:
$ file hello
hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/l, for GNU/Linux 2.6.32, BuildID[sha1]=61469bbc6f06707b5c145ac3ba6d79892423e58f, not stripped
readelf可以查看更多elf文件的信息，比如文件头，段表等
$ readelf -h hello
ELF Header: Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 Class: ELF64 Data: 2's complement, little endian Version: 1 (current) OS/ABI: UNIX - System V ABI Version: 0 Type: EXEC (Executable file) Machine: Advanced Micro Devices X86-64 Version: 0x1 Entry point address: 0x400430 Start of program headers: 64 (bytes into file) Start of section headers: 7824 (bytes into file) Flags: 0x0 Size of this header: 64 (bytes) Size of program headers: 56 (bytes) Number of program headers: 9 Size of section headers: 64 (bytes) Number of section headers: 36 Section header string table index: 33
段表（Section Header Table）保存了efl中段的基本属性信息，比如每个段的段名，长度，在文件中的偏移，读写权限等。
编译器、链接器和装载器都是依据段表来定位和访问各个段的属性。可以使用readelf -S 查看段表信息：
$ readelf -S -W hello
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .interp PROGBITS 0000000000400238 000238 00001c 00 A 0 0 1
[ 2] .note.ABI-tag NOTE 0000000000400254 000254 000020 00 A 0 0 4
[ 3] .note.gnu.build-id NOTE 0000000000400274 000274 000024 00 A 0 0 4
[ 4] .gnu.hash GNU_HASH 0000000000400298 000298 00001c 00 A 5 0 8
[ 5] .dynsym DYNSYM 00000000004002b8 0002b8 000060 18 A 6 1 8
[ 6] .dynstr STRTAB 0000000000400318 000318 00003f 00 A 0 0 1
[ 7] .gnu.version VERSYM 0000000000400358 000358 000008 02 A 5 0 2
[ 8] .gnu.version_r VERNEED 0000000000400360 000360 000020 00 A 6 1 8
[ 9] .rela.dyn RELA 0000000000400380 000380 000018 18 A 5 0 8
[10] .rela.plt RELA 0000000000400398 000398 000030 18 AI 5 24 8
[11] .init PROGBITS 00000000004003c8 0003c8 00001a 00 AX 0 0 4
[12] .plt PROGBITS 00000000004003f0 0003f0 000030 10 AX 0 0 16
[13] .plt.got PROGBITS 0000000000400420 000420 000008 00 AX 0 0 8
[14] .text PROGBITS 0000000000400430 000430 0001b2 00 AX 0 0 16
[15] .fini PROGBITS 00000000004005e4 0005e4 000009 00 AX 0 0 4
[16] .rodata PROGBITS 00000000004005f0 0005f0 00001a 00 A 0 0 4
[17] .eh_frame_hdr PROGBITS 000000000040060c 00060c 00003c 00 A 0 0 4
[18] .eh_frame PROGBITS 0000000000400648 000648 000114 00 A 0 0 8
[19] .init_array INIT_ARRAY 0000000000600e10 000e10 000008 00 WA 0 0 8
[20] .fini_array FINI_ARRAY 0000000000600e18 000e18 000008 00 WA 0 0 8
[21] .jcr PROGBITS 0000000000600e20 000e20 000008 00 WA 0 0 8
[22] .dynamic DYNAMIC 0000000000600e28 000e28 0001d0 10 WA 6 0 8
[23] .got PROGBITS 0000000000600ff8 000ff8 000008 08 WA 0 0 8
[24] .got.plt PROGBITS 0000000000601000 001000 000028 08 WA 0 0 8
[25] .data PROGBITS 0000000000601028 001028 000018 00 WA 0 0 8
[26] .bss NOBITS 0000000000601040 001040 000008 00 WA 0 0 4
[27] .comment PROGBITS 0000000000000000 001040 000035 01 MS 0 0 1
[28] .debug_aranges PROGBITS 0000000000000000 001075 000030 00 0 0 1
[29] .debug_info PROGBITS 0000000000000000 0010a5 00013d 00 0 0 1
[30] .debug_abbrev PROGBITS 0000000000000000 0011e2 00009d 00 0 0 1
[31] .debug_line PROGBITS 0000000000000000 00127f 000044 00 0 0 1
[32] .debug_str PROGBITS 0000000000000000 0012c3 0000fd 01 MS 0 0 1
[33] .shstrtab STRTAB 0000000000000000 001d44 00014c 00 0 0 1
[34] .symtab SYMTAB 0000000000000000 0013c0 000738 18 35 53 8
[35] .strtab STRTAB 0000000000000000 001af8 00024c 00 0 0 1
section
| 段名 | 说明 |
|---|---|
| .data | 保存已经初始化的全局变量和静态变量 |
| .rodata | Read only Data, 保存只读数据，比如字符串常量，全局const变量 |
| .comment | 存放编译器版本信息，比如：GCC: (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609 |
| .debug | 调试信息 |
| .strtab | 字符串表，存储ELF文件中用到的各种字符串 |
| .symtab | 符号表，存储ELF文件用到的所有符号 |
| .shstrtab | Section String Table,段表字符串表，保存所有段名 |
| .bss | 存放所有未初始化的全局变量和静态变量 |
| .text | 代码段 |
| .init | 程序初始化代码段 |
| .fini | 程序终结代码段 |
更多信息可以参考Executable and Linkable Format (ELF)
查看段信息：
readelf -p .comment hello
String dump of section '.comment': [ 0] GCC: (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609
readelf -p .strtab hello
String dump of section '.strtab': [ 1] crtstuff.c [ c] __JCR_LIST__ [ 19] deregister_tm_clones [ 2e] __do_global_dtors_aux [ 44] completed.7594 [ 53] __do_global_dtors_aux_fini_array_entry [ 7a] frame_dummy [ 86] __frame_dummy_init_array_entry [ a5] hello.c [ ad] s_value.2292 [ ba] __FRAME_END__ [ c8] __JCR_END__ [ d4] __init_array_end [ e5] _DYNAMIC [ ee] __init_array_start [ 101] __GNU_EH_FRAME_HDR [ 114] _GLOBAL_OFFSET_TABLE_ [ 12a] __libc_csu_fini [ 13a] g_value [ 142] _ITM_deregisterTMCloneTable [ 15e] _edata [ 165] fun [ 169] printf@@GLIBC_2.2.5 [ 17d] __libc_start_main@@GLIBC_2.2.5 [ 19c] __data_start [ 1a9] __gmon_start__ [ 1b8] __dso_handle [ 1c5] _IO_stdin_used [ 1d4] __libc_csu_init [ 1e4] g_const_value [ 1f2] __bss_start [ 1fe] main [ 203] _Jv_RegisterClasses [ 217] __TMC_END__ [ 223] _ITM_registerTMCloneTable [ 23d] g_value_uninitsymtab信息可以使用nm查看：
nm hello0000000000601040 B __bss_start 0000000000601040 b completed.7594 0000000000601028 D __data_start 0000000000601028 W data_start 0000000000400460 t deregister_tm_clones 00000000004004e0 t __do_global_dtors_aux 0000000000600e18 t __do_global_dtors_aux_fini_array_entry 0000000000601030 D __dso_handle 0000000000600e28 d _DYNAMIC 0000000000601040 D _edata 0000000000601048 B _end 00000000004005e4 T _fini 0000000000400500 t frame_dummy 0000000000600e10 t __frame_dummy_init_array_entry 0000000000400758 r __FRAME_END__ 0000000000400526 T fun 00000000004005f4 R g_const_value 0000000000601000 d _GLOBAL_OFFSET_TABLE_ w __gmon_start__ 000000000040060c r __GNU_EH_FRAME_HDR 0000000000601038 D g_value 0000000000601044 B g_value_uninit 00000000004003c8 T _init 0000000000600e18 t __init_array_end 0000000000600e10 t __init_array_start 00000000004005f0 R _IO_stdin_used w _ITM_deregisterTMCloneTable w _ITM_registerTMCloneTable 0000000000600e20 d __JCR_END__ 0000000000600e20 d __JCR_LIST__ w _Jv_RegisterClasses 00000000004005e0 T __libc_csu_fini 0000000000400570 T __libc_csu_init U __libc_start_main@@GLIBC_2.2.5 0000000000400548 T main U printf@@GLIBC_2.2.5 00000000004004a0 t register_tm_clones 0000000000400430 T _start 000000000060103c d s_value.2292 0000000000601040 D __TMC_END__
strip
实际发布的软件中，不会包含调试信息，否则安装包会很大，并且实际运行时，不需要这些调试信息。比如打包发布的rpm, 会将调试信息，符号表信息等放入到debuginfo中，如果发成了core dump再安装对应的debuginfo rpm，进行调试。对ELF进行瘦身，有很多工具可以用，strip，eu-strip，objcopy. 上面的hello文件，使用file查看时，可以看到not stripped.
eu-strip
eu-strip在elfutils工具集中，elfutils包含了一系列读取，修改，创建elf文件的工具，包括：
- eu-ld (a linker)
- eu-nm (for listing symbols from object files)
- eu-size (for listing the section sizes of an object or archive file)
- eu-strip (for discarding symbols)
- eu-readelf (to see the raw ELF file structures)
- eu-elflint (to check for well-formed ELF files)
安装eu-strip
sudo apt-get install elfutils
eu-strip说明
Output selection:
-f FILE Extract the removed sections into FILE
-F FILE Embed name FILE instead of -f argument
-o, --output=FILE Place stripped output into FILE
Output options:
-g, -d, -S, --strip-debug Remove all debugging symbols
-p, --preserve-dates Copy modified/access timestamps to the output
--permissive Relax a few rules to handle slightly broken ELF
files
--reloc-debug-sections Resolve all trivial relocations between debug
sections if the removed sections are placed in a
debug file (only relevant for ET_REL files,
operation is not reversable, needs -f)
--remove-comment Remove .comment section
--strip-sections Remove section headers (not recommended)
使用时，发现–remove-comment选项，并非只是去除.comment段，而是会去除.debug, .comment, .strtab, .symtab.
$ eu-strip –remove-comment hello $ readelf -S -W hello
Section Headers: [Nr] Name Type Address Off Size ES Flg Lk Inf Al [ 0] NULL 0000000000000000 000000 000000 00 0 0 0 [ 1] .interp PROGBITS 0000000000400238 000238 00001c 00 A 0 0 1 [ 2] .note.ABI-tag NOTE 0000000000400254 000254 000020 00 A 0 0 4 [ 3] .note.gnu.build-id NOTE 0000000000400274 000274 000024 00 A 0 0 4 [ 4] .gnu.hash GNU_HASH 0000000000400298 000298 00001c 00 A 5 0 8 [ 5] .dynsym DYNSYM 00000000004002b8 0002b8 000060 18 A 6 1 8 [ 6] .dynstr STRTAB 0000000000400318 000318 00003f 00 A 0 0 1 [ 7] .gnu.version VERSYM 0000000000400358 000358 000008 02 A 5 0 2 [ 8] .gnu.version_r VERNEED 0000000000400360 000360 000020 00 A 6 1 8 [ 9] .rela.dyn RELA 0000000000400380 000380 000018 18 A 5 0 8 [10] .rela.plt RELA 0000000000400398 000398 000030 18 AI 5 24 8 [11] .init PROGBITS 00000000004003c8 0003c8 00001a 00 AX 0 0 4 [12] .plt PROGBITS 00000000004003f0 0003f0 000030 10 AX 0 0 16 [13] .plt.got PROGBITS 0000000000400420 000420 000008 00 AX 0 0 8 [14] .text PROGBITS 0000000000400430 000430 0001b2 00 AX 0 0 16 [15] .fini PROGBITS 00000000004005e4 0005e4 000009 00 AX 0 0 4 [16] .rodata PROGBITS 00000000004005f0 0005f0 00001a 00 A 0 0 4 [17] .eh_frame_hdr PROGBITS 000000000040060c 00060c 00003c 00 A 0 0 4 [18] .eh_frame PROGBITS 0000000000400648 000648 000114 00 A 0 0 8 [19] .init_array INIT_ARRAY 0000000000600e10 000e10 000008 00 WA 0 0 8 [20] .fini_array FINI_ARRAY 0000000000600e18 000e18 000008 00 WA 0 0 8 [21] .jcr PROGBITS 0000000000600e20 000e20 000008 00 WA 0 0 8 [22] .dynamic DYNAMIC 0000000000600e28 000e28 0001d0 10 WA 6 0 8 [23] .got PROGBITS 0000000000600ff8 000ff8 000008 08 WA 0 0 8 [24] .got.plt PROGBITS 0000000000601000 001000 000028 08 WA 0 0 8 [25] .data PROGBITS 0000000000601028 001028 000018 00 WA 0 0 8 [26] .bss NOBITS 0000000000601040 001040 000008 00 WA 0 0 4 [27] .shstrtab STRTAB 0000000000000000 001040 0000f3 00 0 0
objcopy
Linux Objcopy Command Examples to Copy and Translate Object Files
