02_ELF文件结构_浅析内部文件结构
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ELF文件结构
1. 文件结构
1.1. Scope
通用的ELF文件，我们可以分为四大类：
- Header: 描述基本属性的
- Sections：各个段，包括.text .data .bss等
- Section header tables：ELF中所有段的段名、段长度、文件偏移、读写权限等
- Helper tables：辅助结构，字符串表、符号表。
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ELF Header（描述基本属性）
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.text （段1）
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.data （段2）
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.bss （段3）
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...other
sections...
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Section header tables （段表）
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String Tables （字符串表）
Symbol Tables （符号表）
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1.2. Header
aarch64-none-linux-gnu-gcc main.c -o main
readelf -h main
➜ readelf -h main
ELF Header:
// e_ident members
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 // ELF 魔数
Class: ELF64 // 有ELF32 / ELF64
Data: 2's complement, little endian
Version: 1 (current) // always 1
OS/ABI: UNIX - System V
ABI Version: 0
// e_type members
Type: EXEC (Executable file)
// e_machine members
Machine: AArch64
// e_version members
Version: 0x1
// e_entry members: 规定ELF程序的入口的虚拟地址，操作系统在加载完程序后从这个地址开始执行进程指令
Entry point address: 0x4004c0
// e_phoff members：ELF链接视图和执行视图相关
Start of program headers: 64 (bytes into file)
// e_shoff member：段表在文件中的偏移，从13521字节开始
Start of section headers: 13520 (bytes into file)
// e_word
Flags: 0x0
// e_ehsize： ELF文件头的大小
Size of this header: 64 (bytes)
// e_phentsize： ELF链接视图和执行视图相关
Size of program headers: 56 (bytes)
// e_phnum： ELF链接视图和执行视图相关
Number of program headers: 9
// e_shentsize： 段表描述符的大小 一般为sizeof(Elf32_Shdr)
Size of section headers: 64 (bytes)
// e_shnum: 段表描述符的数量。
Number of section headers: 38
// e_shstrndx： 段表字符串表所在的段在段表中的下标
Section header string table index: 37ELF的类型可以参考：elf(5) - Linux manual page (man7.org)
1.3. Section Header Table（段表）
段表的作用是，在ELF中记录每个段的基本属性（段名、段的长度、在文件中的偏移、读写权限及段的其他属性）。编译器和链接器还有装载器都需要依靠段表来定位和访问各个段的属性。在elf文件头中e_shoff 决定段表的存储位置。
readelf -S main
➜ work-temp readelf -S main
There are 38 section headers, starting at offset 0x34d0:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .interp PROGBITS 0000000000400238 00000238
000000000000001b 0000000000000000 A 0 0 1
[ 2] .note.ABI-tag NOTE 0000000000400254 00000254
0000000000000020 0000000000000000 A 0 0 4
[ 3] .hash HASH 0000000000400278 00000278
0000000000000028 0000000000000004 A 5 0 8
[ 4] .gnu.hash GNU_HASH 00000000004002a0 000002a0
000000000000001c 0000000000000000 A 5 0 8
[ 5] .dynsym DYNSYM 00000000004002c0 000002c0
0000000000000078 0000000000000018 A 6 1 8
[ 6] .dynstr STRTAB 0000000000400338 00000338
0000000000000044 0000000000000000 A 0 0 1
[ 7] .gnu.version VERSYM 000000000040037c 0000037c
000000000000000a 0000000000000002 A 5 0 2
[ 8] .gnu.version_r VERNEED 0000000000400388 00000388
0000000000000020 0000000000000000 A 6 1 8
[ 9] .rela.dyn RELA 00000000004003a8 000003a8
0000000000000018 0000000000000018 A 5 0 8
[10] .rela.plt RELA 00000000004003c0 000003c0
0000000000000060 0000000000000018 AI 5 23 8
[11] .init PROGBITS 0000000000400420 00000420
0000000000000018 0000000000000000 AX 0 0 4
[12] .plt PROGBITS 0000000000400440 00000440
0000000000000060 0000000000000000 AX 0 0 16
[13] .text PROGBITS 00000000004004c0 000004c0
0000000000000214 0000000000000000 AX 0 0 64
[14] CARLOS_FUNC PROGBITS 00000000004006d4 000006d4
0000000000000030 0000000000000000 AX 0 0 4
[15] .fini PROGBITS 0000000000400704 00000704
0000000000000014 0000000000000000 AX 0 0 4
[16] .rodata PROGBITS 0000000000400718 00000718
000000000000002f 0000000000000000 A 0 0 8
[17] .eh_frame_hdr PROGBITS 0000000000400748 00000748
000000000000005c 0000000000000000 A 0 0 4
[18] .eh_frame PROGBITS 00000000004007a8 000007a8
000000000000012c 0000000000000000 A 0 0 8
[19] .init_array INIT_ARRAY 0000000000410de8 00000de8
0000000000000008 0000000000000008 WA 0 0 8
[20] .fini_array FINI_ARRAY 0000000000410df0 00000df0
0000000000000008 0000000000000008 WA 0 0 8
[21] .dynamic DYNAMIC 0000000000410df8 00000df8
00000000000001e0 0000000000000010 WA 6 0 8
[22] .got PROGBITS 0000000000410fd8 00000fd8
0000000000000010 0000000000000008 WA 0 0 8
[23] .got.plt PROGBITS 0000000000410fe8 00000fe8
0000000000000038 0000000000000008 WA 0 0 8
[24] .data PROGBITS 0000000000411020 00001020
0000000000000018 0000000000000000 WA 0 0 8
[25] CARLOS_DATA PROGBITS 0000000000411038 00001038
0000000000000004 0000000000000000 WA 0 0 4
[26] .bss NOBITS 000000000041103c 0000103c
000000000000000c 0000000000000000 WA 0 0 4
[27] .comment PROGBITS 0000000000000000 0000103c
000000000000005d 0000000000000001 MS 0 0 1
[28] .debug_aranges PROGBITS 0000000000000000 000010a0
0000000000000130 0000000000000000 0 0 16
[29] .debug_info PROGBITS 0000000000000000 000011d0
0000000000000715 0000000000000000 0 0 1
[30] .debug_abbrev PROGBITS 0000000000000000 000018e5
00000000000002a1 0000000000000000 0 0 1
[31] .debug_line PROGBITS 0000000000000000 00001b86
000000000000037d 0000000000000000 0 0 1
[32] .debug_str PROGBITS 0000000000000000 00001f03
00000000000004ea 0000000000000001 MS 0 0 1
[33] .debug_loc PROGBITS 0000000000000000 000023ed
0000000000000182 0000000000000000 0 0 1
[34] .debug_ranges PROGBITS 0000000000000000 00002570
0000000000000090 0000000000000000 0 0 16
[35] .symtab SYMTAB 0000000000000000 00002600
0000000000000b10 0000000000000018 36 90 8
[36] .strtab STRTAB 0000000000000000 00003110
0000000000000258 0000000000000000 0 0 1
[37] .shstrtab STRTAB 0000000000000000 00003368
0000000000000161 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
p (processor specific)
以上表为下列结构体的数组
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 如果该段可以被加载，显示为进程空间的虚拟地址，否则为0 */
Elf64_Off sh_offset; /* Section file offset 这段对.bbs没有意义 */
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 有的段有对齐要求，0/1表示没有对其要求 */
Elf64_Xword sh_entsize; /* Entry size if section holds table */
} Elf64_Shdr;重定位表 Relocation Table
在其他段里面引用了一些段的地址，这些地址单独存成一个重定位表，例如printf("hello world"), 里面的字符串就要被重定位到一个单独的区域。
字符串表 String Table
段名、变量名长度不固定，单独放在一个连续的区域里面，使用偏移来索引字符串。.strtab,.shstrtab字符串表， 在header里面 e_shstrndx： 段表字符串表所在的段在段表中的下标 表示
附录 I： 原始C文件
// main.c
#include <stdio.h>
int a = 84;
int b;
const int g = 0xAA;
void func(int i)
{
printf("helloworld!%d\n", i);
}
__attribute((section("CARLOS_DATA"))) int name = 4;
__attribute((section("CARLOS_FUNC"))) int func2 (void){
int m = 9, n = 10;
int q;
q = m+n;
return q;
}
int main(void)
{
static int var_1 = 85;
static int var_2;
int c = 6;
int d;
func(var_1 + var_2 + c + d);
return c;
}