目的檔
目的檔格式
可執行檔格式(Executable)
Windows : PE (Portable Executable)
Linux : ELF (Executable Linkable Format)
Intel : OMF (Object Module Format)
Unix : .out
DOS : .COM
以上都是 COFF(Common object file format) 格式的變化
動態連結程式庫
Windows : .dll
Linux : .so
靜態連結程式庫
Windows : .lib
Linux : .a
動態/靜態連結程式庫也是按照可執行檔格式儲存
ELF 格式
Relocatable File : 程式碼和資料，可連結成可執行檔(.o/.obj)
Executable File : 可執行的程式(/bin/bash/.exe)
Shared Object File : 可變為目的檔或連結其他共用目的檔和可執行檔(.so/.dll)
Core Dump File : 當行程意外終止時，可將行程資訊 dump(core dump)
在 Linux 可用 file 指令看檔案格式
$ file hello.c
hello.c: C source, ASCII text
$ file hello.i
hello.i: C source, ASCII text
$ file hello.o //目的檔也是可執行檔格式(relocatable)
hello.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
$ file hello.s
hello.s: assembler source text
$ file /bin/bash //可執行檔格式(executable)
/bin/bash: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24
$ file /lib32/ld-2.19.so //動態連結程式庫(shared object)
/lib32/ld-2.19.so: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked
目的檔內容
目的檔不外乎儲存編譯後的機器碼指令
還有連結需要的資訊(符號表、除錯資訊)
這些不同資訊按照屬性存在 Section 或稱為 Segment 以下稱段
機器碼放在程式碼區段，此段稱為 .code 或 .text
全域變數和區域靜態變數在資料區段，稱為.data
int g_init_var = 84; // .data
int g_uninit_var; // .bss
void func1(int i) ---+
{ |
printf("%d\n",i); |--> .text
} |
|
int main(void) |
{ ---+
static int static_var1 = 85; // .data
static int static_var2; // .bss
int a = 1; ---+
int b; |
func1(static_var1 + |
static_var2 + a + b); |--> .text
|
return 0; |
} ---+
Executable File / Object File
// 分成 4 個區塊
File Header
.text
.data
.bss
目的檔可用 objdump 看
Usage: objdump <option(s)> <file(s)>
Display information from object <file(s)>.
At least one of the following switches must be given:
-a, --archive-headers Display archive header information
-f, --file-headers Display the contents of the overall file header
-p, --private-headers Display object format specific file header contents
-P, --private=OPT,OPT... Display object format specific contents
-h, --[section-]headers Display the contents of the section headers
-x, --all-headers Display the contents of all headers
-d, --disassemble Display assembler contents of executable sections
-D, --disassemble-all Display assembler contents of all sections
-S, --source Intermix source code with disassembly
-s, --full-contents Display the full contents of all sections requested
-g, --debugging Display debug information in object file
-e, --debugging-tags Display debug information using ctags style
-G, --stabs Display (in raw form) any STABS info in the file
-W[lLiaprmfFsoRt] or
--dwarf[=rawline,=decodedline,=info,=abbrev,=pubnames,=aranges,=macro,=frames,
=frames-interp,=str,=loc,=Ranges,=pubtypes,
=gdb_index,=trace_info,=trace_abbrev,=trace_aranges,
=addr,=cu_index]
Display DWARF info in the file
-t, --syms Display the contents of the symbol table(s)
-T, --dynamic-syms Display the contents of the dynamic symbol table
-r, --reloc Display the relocation entries in the file
-R, --dynamic-reloc Display the dynamic relocation entries in the file
@<file> Read options from <file>
-v, --version Display this program's version number
-i, --info List object formats and architectures supported
-H, --help Display this information
編譯不同階段檔案格式 script
#!/bin/sh
export file_name="objectTest"
gcc -E $file_name.c -o $file_name.i
gcc -S $file_name.i -o $file_name.s
gcc -c $file_name.s -o $file_name.o
不同程式碼編譯後存放的位址如下
機器碼 -> .text
全域變數/區域靜態變數(初始化) -> .data
全域變數/區域靜態變數(未初始化) -> .bss
bss 只是為初始化的全域變數/區域靜態變數預留位置，他沒有內容
所以在檔案中不佔空間
程式指令和資料分開放比較好，主要優點如下
1.程式載入後，指令和資料對應 2 個虛擬記憶體空間
資料區段是 R/W，指令區段是 R，所以這樣可以保護指令不會被惡意改寫
2.分開放可以加強 CPU 快取命中率
3.如果執行相同程式，因為指令是一樣的所以可以共用，當然資料區段是各自擁有的
挖掘 Section
// 使用 objdump 來看 headers，還有個工具 readelf 也可使用
objdump -h SimpleSection.o
SimpleSection.o: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000056 0000000000000000 0000000000000000 00000040 2**0
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .data 00000008 0000000000000000 0000000000000000 00000098 2**2
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000004 0000000000000000 0000000000000000 000000a0 2**2
ALLOC
3 .rodata 00000004 0000000000000000 0000000000000000 000000a0 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .comment 0000002c 0000000000000000 0000000000000000 000000a4 2**0
CONTENTS, READONLY
5 .note.GNU-stack 00000000 0000000000000000 0000000000000000 000000d0 2**0
CONTENTS, READONLY
6 .eh_frame 00000058 0000000000000000 0000000000000000 000000d0 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
// readelf Header
readelf -h SimpleSection.o
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: REL (Relocatable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 400 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 13
Section header string table index: 10
// readelf Section
readelf -S SimpleSection.o
There are 13 section headers, starting at offset 0x190:
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
0000000000000056 0000000000000000 AX 0 0 1
[ 2] .rela.text RELA 0000000000000000 000006b0
0000000000000078 0000000000000018 11 1 8
[ 3] .data PROGBITS 0000000000000000 00000098
0000000000000008 0000000000000000 WA 0 0 4
[ 4] .bss NOBITS 0000000000000000 000000a0
0000000000000004 0000000000000000 WA 0 0 4
[ 5] .rodata PROGBITS 0000000000000000 000000a0
0000000000000004 0000000000000000 A 0 0 1
[ 6] .comment PROGBITS 0000000000000000 000000a4
000000000000002c 0000000000000001 MS 0 0 1
[ 7] .note.GNU-stack PROGBITS 0000000000000000 000000d0
0000000000000000 0000000000000000 0 0 1
[ 8] .eh_frame PROGBITS 0000000000000000 000000d0
0000000000000058 0000000000000000 A 0 0 8
[ 9] .rela.eh_frame RELA 0000000000000000 00000728
0000000000000030 0000000000000018 11 8 8
[10] .shstrtab STRTAB 0000000000000000 00000128
0000000000000061 0000000000000000 0 0 1
[11] .symtab SYMTAB 0000000000000000 000004d0
0000000000000180 0000000000000018 12 11 8
[12] .strtab STRTAB 0000000000000000 00000650
000000000000005d 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
我們可以發現有許多不同的 Section
除了原本的 .text、.data、.bss，還多了 .rodata(唯讀資料區段)
.coment(註釋區段)、note.GUN-stack(堆疊提示區段)
再來看區段屬性，CONTENTS 表示此區段在檔案中存在
BSS 沒有 CONTENTS，表示他在 ELF 不存在內容
note.GUN-stack 雖然有 CONTENTS，但長度是 0
根據 Header 結構如下
$ ls -al
-rw-rw-r-- 1 jason.cc.chiu jason.cc.chiu 1880 9月 22 13:22 SimpleSection.o
$ size SimpleSection.o
text data bss dec hex filename
178 8 4 190 be SimpleSection.o
+----------------+ 0x0000_0758
0x688 | Other data |
+----------------+ 0x0000_00d0
0x2c | .commnt |
+----------------+ 0x0000_00a4
0x04 | .rodata |
+----------------+ 0x0000_00a0
0x08 | .data |
+----------------+ 0x0000_0098
0x56 | .text |
+----------------+ 0x0000_0040
0x40 | ELF Header |
+----------------+ 0x0000_0000
使用 objdump 來看區段內容
// -d 是將指令區段顯示出來
objdump -s -d SimpleSection.o
SimpleSection.o: file format elf64-x86-64
Contents of section .text: // Size = 0x56
0000 554889e5 4883ec10 897dfc8b 45fc89c6 UH..H....}..E...
0010 bf000000 00b80000 0000e800 000000c9 ................
0020 c3554889 e54883ec 10c745f8 01000000 .UH..H....E.....
0030 8b150000 00008b05 00000000 01c28b45 ...............E
0040 f801c28b 45fc01d0 89c7e800 000000b8 ....E...........
0050 00000000 c9c3 ......
Contents of section .data:
0000 54000000 55000000 T...U...
Contents of section .rodata:
0000 25640a00 %d..
Contents of section .comment:
0000 00474343 3a202855 62756e74 7520342e .GCC: (Ubuntu 4.
0010 382e342d 32756275 6e747531 7e31342e 8.4-2ubuntu1~14.
0020 30342e33 2920342e 382e3400 04.3) 4.8.4.
Contents of section .eh_frame:
0000 14000000 00000000 017a5200 01781001 .........zR..x..
0010 1b0c0708 90010000 1c000000 1c000000 ................
0020 00000000 21000000 00410e10 8602430d ....!....A....C.
0030 065c0c07 08000000 1c000000 3c000000 .\..........<...
0040 00000000 35000000 00410e10 8602430d ....5....A....C.
0050 06700c07 08000000 .p......
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: 8b 45 fc mov -0x4(%rbp),%eax
e: 89 c6 mov %eax,%esi
10: bf 00 00 00 00 mov $0x0,%edi
15: b8 00 00 00 00 mov $0x0,%eax
1a: e8 00 00 00 00 callq 1f <func1+0x1f>
1f: c9 leaveq
20: c3 retq
0000000000000021 <main>:
21: 55 push %rbp
22: 48 89 e5 mov %rsp,%rbp
25: 48 83 ec 10 sub $0x10,%rsp
29: c7 45 f8 01 00 00 00 movl $0x1,-0x8(%rbp)
30: 8b 15 00 00 00 00 mov 0x0(%rip),%edx # 36 <main+0x15>
36: 8b 05 00 00 00 00 mov 0x0(%rip),%eax # 3c <main+0x1b>
3c: 01 c2 add %eax,%edx
3e: 8b 45 f8 mov -0x8(%rbp),%eax
41: 01 c2 add %eax,%edx
43: 8b 45 fc mov -0x4(%rbp),%eax
46: 01 d0 add %edx,%eax
48: 89 c7 mov %eax,%edi
4a: e8 00 00 00 00 callq 4f <main+0x2e>
4f: b8 00 00 00 00 mov $0x0,%eax
54: c9 leaveq
55: c3 retq
/*
0000 55 ......
0050 ...... c3
可以看出 55 是 func1 第 1 個 byte
最後面 c3 是 main 最後 1 個 byte
*/
資料區段和唯讀資料區段
資料區段 .data 儲存初始化的全域變數和靜態區域變數 前面程式碼中有兩個存在此區段
int 類型大小為 4 byte，所以 2 個變數為 8 byte
所以 .data 的 size 為 8 byte
printf 用到 "%d\n" 為字串常數，是唯讀資料
放在 .rodata，4 個 byte
.rodata 放唯讀資料，如 const 變數和字串常數
Contents of section .rodata:
0000 25640a00 %d..
0x25 = %
0x64 = d
0x0a
0x00 = 0
再來用 objdump 看看
Sections:
Idx Name Size VMA LMA File off Algn
1 .data 00000008 0000000000000000 0000000000000000 00000098 2**2
CONTENTS, ALLOC, LOAD, DATA
3 .rodata 00000004 0000000000000000 0000000000000000 000000a0 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
Contents of section .data:
0000 54000000 55000000 T...U...
Contents of section .rodata:
0000 25640a00 %d..
.data 前 4 byte = 54 00 00 00 = 84(10進位)
對應 int g_init_var = 84;
.data 後 4 byte = 55 00 00 00 = 85(10進位)
對應 static int static_var1 = 85;
而為什麼順序是 54 00 00 00 不是 00 00 00 54 ?
是關係到 CPU 的 byte order
BSS
BSS 存放位初始化的全域變數和靜態區域變數
int g_uninit_var;
static int static_var2;
兩個變數理論要 8 byte size 但 bss 只有 4 byte
2 .bss 00000004
我們可以透過符號表看到，只有 static_var2 有存入 bss
g_uninit_var 為未定義的 COMMON 符號
SYMBOL TABLE:
......
0000000000000004 l O .data 0000000000000004 static_var1.2184
0000000000000000 l O .bss 0000000000000004 static_var2.2185
0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack
......
0000000000000004 O *COM* 0000000000000004 g_uninit_var
......
要注意的是初始化指派為 0 的變數還是會放在 BSS ，例如
sataic int x1 = 0;
sataic int x2 = 1;
x1 放 .bss 而 x2 放 .data，因為指派 0 會被認定還未初始化
最佳化後會放在 .bss，這樣可以節省空間
其他區段
.rodatel : 唯讀資料
.comment : 編譯器資訊
.debug : 除錯資訊
.dynamic : 動態連結資訊
.hash : 符號雜湊表
.line : 除錯行號表
.note : 額外編譯器資訊
.strtab
.symtab
.shstrtab
.plt
.got
.init
.fini
將檔案作為目的檔區段
我們要將一個 size 為 0x48D 的 image.jpg 作為目的檔中的一個區段
# 將 image.jpg 轉為 image.o
$ objcopy -I binary -O elf64-x86-64 -B i386 image.jpg image.o
$ objdump -ht image.o
image.o: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .data 0000048d 0000000000000000 0000000000000000 00000040 2**0
CONTENTS, ALLOC, LOAD, DATA
SYMBOL TABLE:
0000000000000000 l d .data 0000000000000000 .data
0000000000000000 g .data 0000000000000000 _binary_image_jpg_start
000000000000048d g .data 0000000000000000 _binary_image_jpg_end
000000000000048d g *ABS* 0000000000000000 _binary_image_jpg_size
現在可以在程式中直接宣告並使用
一般 GCC 編譯原始檔後，會把程式碼放到 .text，全域變數和靜態變數放 .data 和 .bss
GCC 提供一中方法可以讓你將變數、方法放到指定的區段
__attribute__((section("SectionName")))
#include <stdio.h>
int g_init_var = 84;
int g_uninit_var;
__attribute__((section("FOO"))) int g_foo = 42;
void func1(int i)
{
printf("%d\n",i);
}
__attribute__((section("BAR"))) void func_bar()
{
printf("func_bar\n");
}
int main(void)
{
static int static_var1 = 85; // .data
static int static_var2; // .bss
int a = 1;
int b;
func1(static_var1 + static_var2 + a + b);
return 0;
}
objdump -ht SimpleSection.o
SimpleSection.o: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
3 FOO 00000004 0000000000000000 0000000000000000 000000a0 2**2
CONTENTS, ALLOC, LOAD, DATA
5 BAR 00000010 0000000000000000 0000000000000000 000000b1 2**0
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
0000000000000000 l d FOO 0000000000000000 FOO
0000000000000000 l d BAR 0000000000000000 BAR
ELF 結構描述
目前 ELF 結構如下
+---------------------+
| ELF Header |
+---------------------+
| .text |
+---------------------+
| .data |
+---------------------+
| .bss |
+---------------------+
| .... |
+---------------------+
| Section Header TLB |
+---------------------+
| String TLB |
+---------------------+
| Symbol TLB |
+---------------------+
先看ELF Header
$ readelf -h SimpleSection.o
ELF Header:
# 魔數
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
# 檔案機器位元長度
Class: ELF64
# 資料儲存方式
Data: 2's complement, little endian
# 版本
Version: 1 (current)
# 執行平臺
OS/ABI: UNIX - System V
# ABI 版本
ABI Version: 0
# ELF 重定類型
Type: REL (Relocatable file)
# 硬體平臺
Machine: Advanced Micro Devices X86-64
# 硬體平臺版本
Version: 0x1
# 入口位址
Entry point address: 0x0
# 程式標頭入口和長度、段頭表的位址和長度、區段數量
Start of program headers: 0 (bytes into file)
Start of section headers: 400 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 13
Section header string table index: 10
ELF 檔案在各平臺下通用，ELF 分為 32 和 64 位元兩種版本
可參考/usr/include/elf.h
例如如果是 32 位元，Elf32_Addr = uint32_t = 4 byte
30 /* Type for a 16-bit quantity. */
31 typedef uint16_t Elf32_Half;
32 typedef uint16_t Elf64_Half;
33
34 /* Types for signed and unsigned 32-bit quantities. */
35 typedef uint32_t Elf32_Word;
36 typedef int32_t Elf32_Sword;
37 typedef uint32_t Elf64_Word;
38 typedef int32_t Elf64_Sword;
39
40 /* Types for signed and unsigned 64-bit quantities. */
41 typedef uint64_t Elf32_Xword;
42 typedef int64_t Elf32_Sxword;
43 typedef uint64_t Elf64_Xword;
44 typedef int64_t Elf64_Sxword;
45
46 /* Type of addresses. */
47 typedef uint32_t Elf32_Addr;
48 typedef uint64_t Elf64_Addr;
49
50 /* Type of file offsets. */
51 typedef uint32_t Elf32_Off;
52 typedef uint64_t Elf64_Off;
53
54 /* Type for section indices, which are 16-bit quantities. */
55 typedef uint16_t Elf32_Section;
56 typedef uint16_t Elf64_Section;
57
58 /* Type for version symbol information. */
59 typedef Elf32_Half Elf32_Versym;
60 typedef Elf64_Half Elf64_Versym;
67 typedef struct
68 {
69 unsigned char e_ident[EI_NIDENT]; /* Magic number and other info */
70 Elf32_Half e_type; /* Object file type */
71 Elf32_Half e_machine; /* Architecture */
72 Elf32_Word e_version; /* Object file version */
73 Elf32_Addr e_entry; /* Entry point virtual address */
74 Elf32_Off e_phoff; /* Program header table file offset */
75 Elf32_Off e_shoff; /* Section header table file offset */
76 Elf32_Word e_flags; /* Processor-specific flags */
77 Elf32_Half e_ehsize; /* ELF header size in bytes */
78 Elf32_Half e_phentsize; /* Program header table entry size */
79 Elf32_Half e_phnum; /* Program header table entry count */
80 Elf32_Half e_shentsize; /* Section header table entry size */
81 Elf32_Half e_shnum; /* Section header table entry count */
82 Elf32_Half e_shstrndx; /* Section header string table index */
83 } Elf32_Ehdr;
84
85 typedef struct
86 {
87 unsigned char e_ident[EI_NIDENT]; /* Magic number and other info */
88 Elf64_Half e_type; /* Object file type */
89 Elf64_Half e_machine; /* Architecture */
90 Elf64_Word e_version; /* Object file version */
91 Elf64_Addr e_entry; /* Entry point virtual address */
92 Elf64_Off e_phoff; /* Program header table file offset */
93 Elf64_Off e_shoff; /* Section header table file offset */
94 Elf64_Word e_flags; /* Processor-specific flags */
95 Elf64_Half e_ehsize; /* ELF header size in bytes */
96 Elf64_Half e_phentsize; /* Program header table entry size */
97 Elf64_Half e_phnum; /* Program header table entry count */
98 Elf64_Half e_shentsize; /* Section header table entry size */
99 Elf64_Half e_shnum; /* Section header table entry count */
100 Elf64_Half e_shstrndx; /* Section header string table index */
101 } Elf64_Ehdr;
可以看出 ELF Header 符合 Elf(32/64)_Ehdr 結構的定義
# EI_NIDENT (16)
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 /* e_ident[EI_NIDENT];*/
Class: ELF64 /* e_ident[EI_NIDENT];*/
Data: 2's complement, little endian /* e_ident[EI_NIDENT];*/
Version: 1 (current) /* e_ident[EI_NIDENT];*/
OS/ABI: UNIX - System V /* e_ident[EI_NIDENT];*/
ABI Version: 0 /* e_ident[EI_NIDENT];*/
Type: REL (Relocatable file) /* e_type */
Machine: Advanced Micro Devices X86-64 /* e_machine */
Version: 0x1 /* e_version */
Entry point address: 0x0 /* e_entry */
Start of program headers: 0 (bytes into file) /* e_phoff */
Start of section headers: 400 (bytes into file) /* e_shoff */
Flags: 0x0 /* e_flags */
Size of this header: 64 (bytes) /* e_ehsize */
Size of program headers: 0 (bytes) /* e_phentsize */
Number of program headers: 0 /* e_phnum */
Size of section headers: 64 (bytes) /* e_shentsize */
Number of section headers: 13 /* e_shnum */
Section header string table index: 10 /* e_shstrndx */
魔數(Magic)
Magic 為 16 byte，用來標示 ELF 平臺屬性
# 一般可執行檔前面幾個 byte 為魔數
# OS 在載入執行檔前會先檢查魔數是否正確
7f = DEL
45 = E
4c = L
46 = F
# ELF 檔案格式
# 0 = 無效
# 1 = 32 位元
# 2 = 64 位元
02
# Byte Order
# 0 = 無效
# 1 = 小端
# 2 = 大端
01
# ELF 版本
01
# 後面目前無定義
00 00 00 00 00 00 00 00 00
Byte Order 為位元組順序，Endian 源自格列佛遊記
小人國對吃水煮蛋分兩派，一派為要從大頭先敲開(Big Endian)
一派為要從小頭先敲開(Little Endian)，為此常常發生內戰
MSB 是 Most Significant Bit/Byte
代表最重要的位元/位元組，他表示在一個 bit/byte 中對順序取值影響最大的那個 bit/byte
LSB 是 Least Significant Bit/Byte 和 MSB 相反
也就是在一個 bit/byte 中對順序取值影響最小的那個 bit/byte
例如: 0x12345678
0x12 0x34 0x56 0x78
0x12 = MSB
0X78 = LSB
如果單看 0x78 = 0111 1000
最左邊 bit = 0 = MSB
最又邊 bit = 0 = LSB
bit-endian 規定
MSB 儲存時放在低位址，傳輸時放在資料流開始
LSB 儲存時放在高位址，傳輸時放在資料流結尾
Big Little
0 byte 0x12 0x78
1 byte 0x34 0x56
2 byte 0x56 0x34
3 byte 0x78 0x21
檔案類型(Type)
OS 透過此欄位判斷檔案類型，而不是透過副檔名，
檔案類型為常數，以ET開頭
/* Legal values for e_type (object file type). */
162 #define ET_NONE 0 /* No file type */
163 #define ET_REL 1 /* Relocatable file */
164 #define ET_EXEC 2 /* Executable file */
165 #define ET_DYN 3 /* Shared object file */
166 #define ET_CORE 4 /* Core file */
167 #define ET_NUM 5 /* Number of defined types */
168 #define ET_LOOS 0xfe00 /* OS-specific range start */
169 #define ET_HIOS 0xfeff /* OS-specific range end */
170 #define ET_LOPROC 0xff00 /* Processor-specific range start */
171 #define ET_HIPROC 0xffff /* Processor-specific range end */
機器類型(Machine)
此欄位表示可在哪個平臺使用，例如 Advanced Micro Devices X86-64 就是能在 x86 下使用
相關常數以EM 開頭
173 /* Legal values for e_machine (architecture). */
174
175 #define EM_NONE 0 /* No machine */
176 #define EM_M32 1 /* AT&T WE 32100 */
177 #define EM_SPARC 2 /* SUN SPARC */
178 #define EM_386 3 /* Intel 80386 */
179 #define EM_68K 4 /* Motorola m68k family */
180 #define EM_88K 5 /* Motorola m88k family */
181 #define EM_860 7 /* Intel 80860 */
182 #define EM_MIPS 8 /* MIPS R3000 big-endian */
183 #define EM_S370 9 /* IBM System/370 */
184 #define EM_MIPS_RS3_LE 10 /* MIPS R3000 little-endian */
186 #define EM_PARISC 15 /* HPPA */
187 #define EM_VPP500 17 /* Fujitsu VPP500 */
188 #define EM_SPARC32PLUS 18 /* Sun's "v8plus" */
189 #define EM_960 19 /* Intel 80960 */
190 #define EM_PPC 20 /* PowerPC */
191 #define EM_PPC64 21 /* PowerPC 64-bit */
192 #define EM_S390 22 /* IBM S390 */
194 #define EM_V800 36 /* NEC V800 series */
195 #define EM_FR20 37 /* Fujitsu FR20 */
196 #define EM_RH32 38 /* TRW RH-32 */
197 #define EM_RCE 39 /* Motorola RCE */
198 #define EM_ARM 40 /* ARM */
省略......
段頭表(Start of section headers)
段頭表描述 ELF 個區段的資訊，如區段的區段名稱、區段長度
檔案中的偏移、R/W 權限、其他屬性
編譯器、連結器、載入器都是依靠此表決定定位和存取各區段屬性
範例位於 400 = 0x190 的偏移位址
# objdump 只會顯示關鍵的區段
$ objdump -h SimpleSection.o
SimpleSection.o: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000056 0000000000000000 0000000000000000 00000040 2**0
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .data 00000008 0000000000000000 0000000000000000 00000098 2**2
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000004 0000000000000000 0000000000000000 000000a0 2**2
ALLOC
3 .rodata 00000004 0000000000000000 0000000000000000 000000a0 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .comment 0000002c 0000000000000000 0000000000000000 000000a4 2**0
CONTENTS, READONLY
5 .note.GNU-stack 00000000 0000000000000000 0000000000000000 000000d0 2**0
CONTENTS, READONLY
6 .eh_frame 00000058 0000000000000000 0000000000000000 000000d0 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
# readelf 可以顯示全部的區段
$ readelf -S SimpleSection.o
There are 13 section headers, starting at offset 0x190:
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 0000000000000056 0000000000000000 AX 0 0 1
[ 2] .rela.text RELA 0000000000000000 000006b0 0000000000000078 0000000000000018 11 1 8
[ 3] .data PROGBITS 0000000000000000 00000098 0000000000000008 0000000000000000 WA 0 0 4
[ 4] .bss NOBITS 0000000000000000 000000a0 0000000000000004 0000000000000000 WA 0 0 4
[ 5] .rodata PROGBITS 0000000000000000 000000a0 0000000000000004 0000000000000000 A 0 0 1
[ 6] .comment PROGBITS 0000000000000000 000000a4 000000000000002c 0000000000000001 MS 0 0 1
[ 7] .note.GNU-stack PROGBITS 0000000000000000 000000d0 0000000000000000 0000000000000000 0 0 1
[ 8] .eh_frame PROGBITS 0000000000000000 000000d0 0000000000000058 0000000000000000 A 0 0 8
[ 9] .rela.eh_frame RELA 0000000000000000 00000728 0000000000000030 0000000000000018 11 8 8
[10] .shstrtab STRTAB 0000000000000000 00000128 0000000000000061 0000000000000000 0 0 1
[11] .symtab SYMTAB 0000000000000000 000004d0 0000000000000180 0000000000000018 12 11 8
[12] .strtab STRTAB 0000000000000000 00000650 000000000000005d 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
區段表是以Elf(32/64)_Shdr結構的陣列
所以陣列內元素個數 = 區段總個數 = There are 13 section headers，共 13 個
但是第一個是無效的，所以有效的有 12 個
270 /* Section header. */
271
272 typedef struct
273 {
274 Elf32_Word sh_name; /* Section name (string tbl index) */
275 Elf32_Word sh_type; /* Section type */
276 Elf32_Word sh_flags; /* Section flags */
277 Elf32_Addr sh_addr; /* Section virtual addr at execution */
278 Elf32_Off sh_offset; /* Section file offset */
279 Elf32_Word sh_size; /* Section size in bytes */
280 Elf32_Word sh_link; /* Link to another section */
281 Elf32_Word sh_info; /* Additional section information */
282 Elf32_Word sh_addralign; /* Section alignment */
283 Elf32_Word sh_entsize; /* Entry size if section holds table */
284 } Elf32_Shdr;
285
286 typedef struct
287 {
288 Elf64_Word sh_name; /* Section name (string tbl index) */
289 Elf64_Word sh_type; /* Section type */
290 Elf64_Xword sh_flags; /* Section flags */
291 Elf64_Addr sh_addr; /* Section virtual addr at execution */
292 Elf64_Off sh_offset; /* Section file offset */
293 Elf64_Xword sh_size; /* Section size in bytes */
294 Elf64_Word sh_link; /* Link to another section */
295 Elf64_Word sh_info; /* Additional section information */
296 Elf64_Xword sh_addralign; /* Section alignment */
297 Elf64_Xword sh_entsize; /* Entry size if section holds table */
298 } Elf64_Shdr;
# 各區段成員作用
sh_name # 區段名稱，位於 .shstrtab 的字串表中
sh_type # 區段類型
sh_flags # 區段旗標
sh_addr # 區段虛擬位址
sh_offset # 區段偏移
sh_size # 區段長度
sh_link # 區段連結
sh_info # 區段資訊
sh_addralign# 區段位址對其
sh_entsize # 區段項目長度
所以 ELF 結構如下
+---------------------+
| ELF Header |
+---------------------+
| .text |
+---------------------+
| .data |
+---------------------+
| .bss |
+---------------------+
| .... |
+---------------------+
| Section TLB |
| 用來描述 Section |
+---------------------+
| .symtab |
+---------------------+
| .rel.text |
+---------------------+
再來看 Type(sh_type) 和 Flags(sh_flags)
317 /* Legal values for sh_type (section type). */
318
319 #define SHT_NULL 0 /* Section header table entry unused */
320 #define SHT_PROGBITS 1 /* Program data 程式碼區段 */
321 #define SHT_SYMTAB 2 /* Symbol table 符號表 */
322 #define SHT_STRTAB 3 /* String table 字串表 */
323 #define SHT_RELA 4 /* Relocation entries with addends 重定表 */
324 #define SHT_HASH 5 /* Symbol hash table 符號表的雜湊 */
325 #define SHT_DYNAMIC 6 /* Dynamic linking information 動態連結資訊 */
326 #define SHT_NOTE 7 /* Notes 提示資訊*/
327 #define SHT_NOBITS 8 /* Program space with no data (bss) 沒有內容*/
328 #define SHT_REL 9 /* Relocation entries, no addends 重定資訊 */
329 #define SHT_SHLIB 10 /* Reserved 保留*/
330 #define SHT_DYNSYM 11 /* Dynamic linker symbol table 動態連結符號表*/
331 #define SHT_INIT_ARRAY 14 /* Array of constructors */
332 #define SHT_FINI_ARRAY 15 /* Array of destructors */
333 #define SHT_PREINIT_ARRAY 16 /* Array of pre-constructors */
334 #define SHT_GROUP 17 /* Section group */
335 #define SHT_SYMTAB_SHNDX 18 /* Extended section indeces */
336 #define SHT_NUM 19 /* Number of defined types. */
337 #define SHT_LOOS 0x60000000 /* Start OS-specific. */
338 #define SHT_GNU_ATTRIBUTES 0x6ffffff5 /* Object attributes. */
339 #define SHT_GNU_HASH 0x6ffffff6 /* GNU-style hash table. */
340 #define SHT_GNU_LIBLIST 0x6ffffff7 /* Prelink library list */
341 #define SHT_CHECKSUM 0x6ffffff8 /* Checksum for DSO content. */
342 #define SHT_LOSUNW 0x6ffffffa /* Sun-specific low bound. */
343 #define SHT_SUNW_move 0x6ffffffa
344 #define SHT_SUNW_COMDAT 0x6ffffffb
345 #define SHT_SUNW_syminfo 0x6ffffffc
346 #define SHT_GNU_verdef 0x6ffffffd /* Version definition section. */
347 #define SHT_GNU_verneed 0x6ffffffe /* Version needs section. */
348 #define SHT_GNU_versym 0x6fffffff /* Version symbol table. */
349 #define SHT_HISUNW 0x6fffffff /* Sun-specific high bound. */
350 #define SHT_HIOS 0x6fffffff /* End OS-specific type */
351 #define SHT_LOPROC 0x70000000 /* Start of processor-specific */
352 #define SHT_HIPROC 0x7fffffff /* End of processor-specific */
353 #define SHT_LOUSER 0x80000000 /* Start of application-specific */
354 #define SHT_HIUSER 0x8fffffff /* End of application-specific */
356 /* Legal values for sh_flags (section flags). */
358 #define SHF_WRITE (1 << 0) /* Writable 此區段可寫*/
359 #define SHF_ALLOC (1 << 1) /* Occupies memory during execution 此區段在 process 需要分配空間*/
360 #define SHF_EXECINSTR (1 << 2) /* Executable 此區段可執行 */
361 #define SHF_MERGE (1 << 4) /* Might be merged */
362 #define SHF_STRINGS (1 << 5) /* Contains nul-terminated strings */
363 #define SHF_INFO_LINK (1 << 6) /* `sh_info' contains SHT index */
364 #define SHF_LINK_ORDER (1 << 7) /* Preserve order after combining */
365 #define SHF_OS_NONCONFORMING (1 << 8) /* Non-standard OS specific handling required */
367 #define SHF_GROUP (1 << 9) /* Section is member of a group. */
368 #define SHF_TLS (1 << 10) /* Section hold thread-local data. */
369 #define SHF_MASKOS 0x0ff00000 /* OS-specific. */
370 #define SHF_MASKPROC 0xf0000000 /* Processor-specific */
371 #define SHF_ORDERED (1 << 30) /* Special ordering requirement (Solaris). */
373 #define SHF_EXCLUDE (1 << 31) /* Section is excluded unless referenced or allocated (Solaris).*/
重定表
有一個區段叫做.rela.text，類型為sh_type，代表對 .text 的重定
連結的介面(符號)
兩個目的檔要連結實際上之間對位址的引用
假設 B 要用 A 中的函式，則 A 定義此函式
B 引用此函式，函式和變數為符號，函式名稱和變數名稱為符號名稱
每一個目的檔都有對應的符號表，紀錄目的檔用的所有符號，符號分類如下
#include <stdio.h>
int g_init_var = 84;
int g_uninit_var;
void func1(int i)
{
printf("%d\n",i);
}
int main(void)
{
static int static_var1 = 85; // .data
static int static_var2; // .bss
int a = 1;
int b;
func1(static_var1 + static_var2 + a + b);
return 0;
}
# 全域符號
func1、main、g_init_var
# 外部符號
printf
# 區段名稱(由編譯器產生)
.text、.data
# 區域符號
static_var1、static_var2
# 行號資訊
符號表可以使用nm或readelf
連結過程只關心全域符號和外部符號
$ nm SimpleSection.o
0000000000000000 T func1
0000000000000000 D g_init_var
0000000000000004 C g_uninit_var
0000000000000021 T main
U printf
0000000000000004 d static_var1.2184
0000000000000000 b static_var2.2185
$ readelf -s SimpleSection.o
Symbol table '.symtab' contains 16 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS SimpleSection.c
2: 0000000000000000 0 SECTION LOCAL DEFAULT 1
3: 0000000000000000 0 SECTION LOCAL DEFAULT 3
4: 0000000000000000 0 SECTION LOCAL DEFAULT 4
5: 0000000000000000 0 SECTION LOCAL DEFAULT 5
6: 0000000000000004 4 OBJECT LOCAL DEFAULT 3 static_var1.2184
7: 0000000000000000 4 OBJECT LOCAL DEFAULT 4 static_var2.2185
8: 0000000000000000 0 SECTION LOCAL DEFAULT 7
9: 0000000000000000 0 SECTION LOCAL DEFAULT 8
10: 0000000000000000 0 SECTION LOCAL DEFAULT 6
11: 0000000000000000 4 OBJECT GLOBAL DEFAULT 3 g_init_var
12: 0000000000000004 4 OBJECT GLOBAL DEFAULT COM g_uninit_var
13: 0000000000000000 33 FUNC GLOBAL DEFAULT 1 func1
14: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND printf
15: 0000000000000021 53 FUNC GLOBAL DEFAULT 1 main
符號表也是區段之一，一般叫做.symtab
Section Headers:
[Nr] Name Type Address Offset Size EntSize Flags Link Info Align
[11] .symtab SYMTAB 0000000000000000 000004d0 0000000000000180 0000000000000018 12 11 8
符號表的結構如下，對應上面的 Num、Value、Size、Type、Bind、Vis、Ndx Name
379 /* Symbol table entry. */
380
381 typedef struct
382 {
383 Elf32_Word st_name; /* Symbol name (string tbl index) */
384 Elf32_Addr st_value; /* Symbol value */
385 Elf32_Word st_size; /* Symbol size */
386 unsigned char st_info; /* Symbol type and binding */
387 unsigned char st_other; /* Symbol visibility */
388 Elf32_Section st_shndx; /* Section index */
389 } Elf32_Sym;
390
391 typedef struct
392 {
393 Elf64_Word st_name; /* Symbol name (string tbl index) */
394 unsigned char st_info; /* Symbol type and binding */
395 unsigned char st_other; /* Symbol visibility */
396 Elf64_Section st_shndx; /* Section index */
397 Elf64_Addr st_value; /* Symbol value */
398 Elf64_Xword st_size; /* Symbol size */
399 } Elf64_Sym;