目标文件和ELF格式详解
Linux生成的目标文件是标准的ELF文件格式，使用objdump工具和readelf工具可以查看分析elf文件的格式
gcc -c 选项只编译不连接生成目标文件
# c语言源代码
[root@localhost simple-secion-linux-elf]# cat SimpleSection.c
#include <stdio.h>
int global_init_var = 84;
int global_uninit_var;
void func1(int i) {
printf("%d\n", i);
}
int main(int argc, char const *argv[])
{
static int static_var = 82;
static int static_var2;
int a = 1;
int b;
func1(static_var + static_var2 + a + b);
return 0;
}
[root@localhost simple-secion-linux-elf]# gcc -c SimpleSection.c
[root@localhost simple-secion-linux-elf]# ll
总用量 40
-rwxr-xr-x. 1 root root 6740 2月 23 18:20 a.out
-rw-r--r--. 1 root root 286 2月 23 18:17 SimpleSection.c
-rw-r--r--. 1 root root 16968 2月 23 18:17 SimpleSection.i
-rw-r--r--. 1 root root 1904 2月 23 18:20 SimpleSection.o
-rwxr-xr-x. 1 root root 1343 2月 23 18:19 SimpleSection.s
[root@localhost simple-secion-linux-elf]# ./a.out
初探目标文件
objdump工具可以查看ELF文件的段的信息，包含
- 查看目标文件各个段的信息(-h -x)
- 以十六进制的方式查看段的内容(-s)
- 查看反汇编之后的代码(-d)
ELF文件各个段的基本信息
objdump -h 选项查看ELF文件各个段的基本信息
[root@localhost simple-secion-linux-elf]# objdump -h SimpleSection.o
SimpleSection.o: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 0000005d 0000000000000000 0000000000000000 00000040 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .data 00000008 0000000000000000 0000000000000000 000000a0 2**2
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000004 0000000000000000 0000000000000000 000000a8 2**2
ALLOC
3 .rodata 00000004 0000000000000000 0000000000000000 000000a8 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .comment 0000002d 0000000000000000 0000000000000000 000000ac 2**0
CONTENTS, READONLY
5 .note.GNU-stack 00000000 0000000000000000 0000000000000000 000000d9 2**0
CONTENTS, READONLY
6 .eh_frame 00000058 0000000000000000 0000000000000000 000000e0 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
size工具可以查看ELF文件的各个段的大小，.data段和.bss段和objdump工具查看的一致，而.text段不一致，这个地方暂时还不懂为什么？？
[root@localhost simple-secion-linux-elf]# size SimpleSection.o
text data bss dec hex filename
185 8 4 197 c5 SimpleSection.o
目标文件代码段
objdump -s 选项把ELF文件的所有信息以十六进制的方式打印出来；-d选项把代码段反汇编，可以看到代码段的地址从0-5c一共为5d个字节，和使用objdump -h查看的一致
[root@localhost simple-secion-linux-elf]# objdump -s -d SimpleSection.o
SimpleSection.o: file format elf64-x86-64
Contents of section .text:
0000 554889e5 4883ec10 897dfcb8 00000000 UH..H....}......
0010 8b55fc89 d64889c7 b8000000 00e80000 .U...H..........
0020 0000c9c3 554889e5 4883ec20 897dec48 ....UH..H.. .}.H
0030 8975e0c7 45f80100 00008b15 00000000 .u..E...........
0040 8b050000 00008d04 020345f8 0345fc89 ..........E..E..
0050 c7e80000 0000b800 000000c9 c3 .............
Contents of section .data:
0000 54000000 52000000 T...R...
Contents of section .rodata:
0000 25640a00 %d..
Contents of section .comment:
0000 00474343 3a202847 4e552920 342e342e .GCC: (GNU) 4.4.
0010 37203230 31323033 31332028 52656420 7 20120313 (Red
0020 48617420 342e342e 372d3429 00 Hat 4.4.7-4).
Contents of section .eh_frame:
0000 14000000 00000000 017a5200 01781001 .........zR..x..
0010 1b0c0708 90010000 1c000000 1c000000 ................
0020 00000000 24000000 00410e10 8602430d ....$....A....C.
0030 065f0c07 08000000 1c000000 3c000000 ._..........<...
0040 00000000 39000000 00410e10 8602430d ....9....A....C.
0050 06740c07 08000000 .t......
Disassembly of section .text:
0000000000000000 <func1>:
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: b8 00 00 00 00 mov $0x0,%eax
10: 8b 55 fc mov -0x4(%rbp),%edx
13: 89 d6 mov %edx,%esi
15: 48 89 c7 mov %rax,%rdi
18: b8 00 00 00 00 mov $0x0,%eax
1d: e8 00 00 00 00 callq 22 <func1+0x22>
22: c9 leaveq
23: c3 retq
0000000000000024 <main>:
24: 55 push %rbp
25: 48 89 e5 mov %rsp,%rbp
28: 48 83 ec 20 sub $0x20,%rsp
2c: 89 7d ec mov %edi,-0x14(%rbp)
2f: 48 89 75 e0 mov %rsi,-0x20(%rbp)
33: c7 45 f8 01 00 00 00 movl $0x1,-0x8(%rbp)
3a: 8b 15 00 00 00 00 mov 0x0(%rip),%edx # 40 <main+0x1c>
40: 8b 05 00 00 00 00 mov 0x0(%rip),%eax # 46 <main+0x22>
46: 8d 04 02 lea (%rdx,%rax,1),%eax
49: 03 45 f8 add -0x8(%rbp),%eax
4c: 03 45 fc add -0x4(%rbp),%eax
4f: 89 c7 mov %eax,%edi
51: e8 00 00 00 00 callq 56 <main+0x32>
56: b8 00 00 00 00 mov $0x0,%eax
5b: c9 leaveq
5c: c3 retq
目标文件数据段
数据段包含了读写数据段.data和只读数据段.rodata，objdump工具提供的-x选项可以查看段的内容
[root@localhost simple-secion-linux-elf]# objdump -s -x SimpleSection.o
...
Contents of section .data:
0000 54000000 52000000 T...R...
Contents of section .rodata:
0000 25640a00 %d..
...
-
.data段保存已初始化的全局变量和局部静态变量
54000000 对应的是int global_init_var = 84;
52000000 对应的是static int static_var = 82;; -
.rodata段保存的是只读数据，一般是程序中的只读变量（const）和字符串常亮
25640a00 %d..保存的是printf函数用到的字符串常量%d\n(25640a)
目标文件BSS段
BSS段保存的是未初始化的全局变量和局部静态变量
[root@localhost simple-secion-linux-elf]# objdump -h SimpleSection.o
...
2 .bss 00000004 0000000000000000 0000000000000000 000000a8 2**2
ALLOC
...
ELF文件结构解析
ELF文件结构
主要包含：
- ELF文件头（ELF Header）包含了ELF文件版本，目标机器型号、程序入口地址
- ELF文件各个段（代码段，数据段、BSS段）
- 段表（Section Header Table），ELF文件中所有段的信息，比如段名、段长度、段在文件中的偏移、段的读写权限和其他属性
- 其他的辅助结构，比如字符串表、符号表等
readelf工具提供的以下选项可以查看elf文件的信息：
Usage: readelf <option(s)> elf-file(s)
Display information about the contents of ELF format files
Options are:
-a --all Equivalent to: -h -l -S -s -r -d -V -A -I
-h --file-header Display the ELF file header
-l --program-headers Display the program headers
--segments An alias for --program-headers
-S --section-headers Display the sections' header
--sections An alias for --section-headers
-g --section-groups Display the section groups
-t --section-details Display the section details
-e --headers Equivalent to: -h -l -S
-s --syms Display the symbol table
--symbols An alias for --syms
-n --notes Display the core notes (if present)
-r --relocs Display the relocations (if present)
-u --unwind Display the unwind info (if present)
-d --dynamic Display the dynamic section (if present)
-V --version-info Display the version sections (if present)
-A --arch-specific Display architecture specific information (if any).
-c --archive-index Display the symbol/file index in an archive
-D --use-dynamic Use the dynamic section info when displaying symbols
-x --hex-dump=<number|name>
Dump the contents of section <number|name> as bytes
-p --string-dump=<number|name>
Dump the contents of section <number|name> as strings
-R --relocated-dump=<number|name>
Dump the contents of section <number|name> as relocated bytes
-w[lLiaprmfFsoR] or
--debug-dump[=rawline,=decodedline,=info,=abbrev,=pubnames,=aranges,=macro,=frames,=str,=loc,=Ranges]
Display the contents of DWARF2 debug sections
-I --histogram Display histogram of bucket list lengths
-W --wide Allow output width to exceed 80 characters
@<file> Read options from <file>
-H --help Display this information
-v --version Display the version number of readelf
ELF 文件头
使用readelf工具的-h选项查看ELF文件头部信息
[root@localhost simple-secion-linux-elf]# readelf -h SimpleSection.o
ELF Header:
Magic（ELF魔数）: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class（文件机器字节长度）: ELF64
Data（数据存储方式）: 2's complement, little endian
Version（版本）: 1 (current)
OS/ABI（运行平台）: UNIX - System V
ABI Version（ABI版本）: 0
Type（ELF重定位类型）: REL (Relocatable file)
Machine（硬件平台）: Advanced Micro Devices X86-64
Version（硬件平台版本）: 0x1
Entry point address（入口地址）: 0x0
Start of program headers（程序头入口）: 0 (bytes into file)
Start of section headers（段表入口）: 416 (bytes into file)
Flags: 0x0
Size of this header（ELF头自身的大小）: 64 (bytes)
Size of program headers（程序头长度）: 0 (bytes)
Number of program headers: 0
Size of section headers（段表的长度）: 64 (bytes)
Number of section headers（段的数量）: 13
Section header string table index（段表字符串表在段中的位置）: 10
ELF 文件头结构对应的结构体，位于/usr/include/elf.h里，包含了32位和64位的版本，下面是64位版本的头信息结构体
/* The ELF file header. This appears at the start of every ELF file. */
#define EI_NIDENT (16)
typedef struct
{
unsigned char e_ident[EI_NIDENT]; /* Magic number and other info */
Elf64_Half e_type; /* Object file type */
Elf64_Half e_machine; /* Architecture */
Elf64_Word e_version; /* Object file version */
Elf64_Addr e_entry; /* Entry point virtual address */
Elf64_Off e_phoff; /* Program header table file offset */
Elf64_Off e_shoff; /* Section header table file offset */
Elf64_Word e_flags; /* Processor-specific flags */
Elf64_Half e_ehsize; /* ELF header size in bytes */
Elf64_Half e_phentsize; /* Program header table entry size */
Elf64_Half e_phnum; /* Program header table entry count */
Elf64_Half e_shentsize; /* Section header table entry size */
Elf64_Half e_shnum; /* Section header table entry count */
Elf64_Half e_shstrndx; /* Section header string table index */
} Elf64_Ehdr;
类型描述以及长度信息如下所示：
类型描述以及长度信息
对应关系如下表所示：
ELF文件头结构成员含义
ELF魔数
标识ELF文件的平台属性，比如ELF字长、字节序、ELF文件版本，如下图所示。
ELF魔数
ELF 文件类型
ELF 文件类型
ELF机器类型
ELF机器类型
ELF 段表
ELF文件中有很多段，段表（Section Header Table）就是保存这些段的基本信息的结构，包括了段名、段长度、段在文件中的偏移位置、读写权限和其他段属性。
objdump工具可以查看ELF文件基本的段结构
[root@localhost simple-secion-linux-elf]# objdump -h SimpleSection.o
SimpleSection.o: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 0000005d 0000000000000000 0000000000000000 00000040 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .data 00000008 0000000000000000 0000000000000000 000000a0 2**2
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000004 0000000000000000 0000000000000000 000000a8 2**2
ALLOC
3 .rodata 00000004 0000000000000000 0000000000000000 000000a8 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .comment 0000002d 0000000000000000 0000000000000000 000000ac 2**0
CONTENTS, READONLY
5 .note.GNU-stack 00000000 0000000000000000 0000000000000000 000000d9 2**0
CONTENTS, READONLY
6 .eh_frame 00000058 0000000000000000 0000000000000000 000000e0 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
使用readelf可以看到ELF文件全部的段结构
[root@localhost simple-secion-linux-elf]# readelf -S SimpleSection.o
There are 13 section headers, starting at offset 0x1a0:
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
000000000000005d 0000000000000000 AX 0 0 4
[ 2] .rela.text RELA 0000000000000000 000006c8
0000000000000078 0000000000000018 11 1 8
[ 3] .data PROGBITS 0000000000000000 000000a0
0000000000000008 0000000000000000 WA 0 0 4
[ 4] .bss NOBITS 0000000000000000 000000a8
0000000000000004 0000000000000000 WA 0 0 4
[ 5] .rodata PROGBITS 0000000000000000 000000a8
0000000000000004 0000000000000000 A 0 0 1
[ 6] .comment PROGBITS 0000000000000000 000000ac
000000000000002d 0000000000000001 MS 0 0 1
[ 7] .note.GNU-stack PROGBITS 0000000000000000 000000d9
0000000000000000 0000000000000000 0 0 1
[ 8] .eh_frame PROGBITS 0000000000000000 000000e0
0000000000000058 0000000000000000 A 0 0 8
[ 9] .rela.eh_frame RELA 0000000000000000 00000740
0000000000000030 0000000000000018 11 8 8
[10] .shstrtab STRTAB 0000000000000000 00000138
0000000000000061 0000000000000000 0 0 1
[11] .symtab SYMTAB 0000000000000000 000004e0
0000000000000180 0000000000000018 12 11 8
[12] .strtab STRTAB 0000000000000000 00000660
0000000000000066 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
可以看到一共有13个段，每个段对应一个Section Header数据结构，下面是64位的Section Header数据结构。
Elf64_Shdr的定义位于“/usr/include/elf.h”文件
typedef struct
{
Elf64_Word sh_name; /* Section name (string tbl index) */
Elf64_Word sh_type; /* Section type */
Elf64_Xword sh_flags; /* Section flags */
Elf64_Addr sh_addr; /* Section virtual addr at execution */
Elf64_Off sh_offset; /* Section file offset */
Elf64_Xword sh_size; /* Section size in bytes */
Elf64_Word sh_link; /* Link to another section */
Elf64_Word sh_info; /* Additional section information */
Elf64_Xword sh_addralign; /* Section alignment */
Elf64_Xword sh_entsize; /* Entry size if section holds table */
} Elf64_Shdr;
段表成员含义描述：
段表成员含义描述
Section Table及所有段的长度
段类型（sh_type）
段类型（sh_type）
段类型（sh_type）
段的标志位（sh_flag）
段的标志位（sh_flag）
** 段的链接信息（sh_link、sh_info）**
段的链接信息（sh_link、sh_info）
``
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