如何实现在Windows上运行Linux程序，附示例代码
微软在去年发布了Bash On Windows, 这项技术允许在Windows上运行Linux程序, 我相信已经有很多文章解释过Bash On Windows的原理,而今天的这篇文章将会讲解如何自己实现一个简单的原生Linux程序运行器, 这个运行器在用户层实现, 原理和Bash On Windows不完全一样，比较接近Linux上的Wine.
示例程序完整的代码在github上, 地址是 https://github.com/303248153/HelloElfLoader
初步了解ELF格式
首先让我们先了解什么是原生Linux程序, 以下说明摘自维基百科
In computing, the Executable and Linkable Format (ELF, formerly named Extensible Linking Format), is a common standard file format for executable files, object code, shared libraries, and core dumps. First published in the specification for the application binary interface (ABI) of the Unix operating system version named System V Release 4 (SVR4), and later in the Tool Interface Standard, it was quickly accepted among different vendors of Unix systems. In 1999, it was chosen as the standard binary file format for Unix and Unix-like systems on x86 processors by the 86open project.
By design, ELF is flexible, extensible, and cross-platform, not bound to any given central processing unit (CPU) or instruction set architecture. This has allowed it to be adopted by many different operating systems on many different hardware platforms.Linux的可执行文件格式采用了ELF格式, 而Windows采用了PE格式, 也就是我们经常使用的exe文件的格式.
ELF格式的结构如下
https://upload.wikimedia.org/wikipedia/commons/thumb/7/77/Elf-layout--en.svg/260px-Elf-layout--en.svg.png
大致上可以分为这些部分
[*]ELF头，在文件的最开头，储存了类型和版本等信息
[*]程序头, 供程序运行时解释器(interpreter)使用
[*]节头, 供程序编译时链接器(linker)使用, 运行时不需要读节头
[*]节内容, 不同的节作用都不一样
[*].text 代码节，保存了主要的程序代码
[*].rodata 保存了只读的数据，例如字符串(const char*)
[*].data 保存了可读写的数据，例如全局变量
[*]还有其他各种各样的节
让我们来实际看一下Linux可执行程序的样子
以下的编译环境是Ubuntu 16.04 x64 + gcc 5.4.0, 编译环境不一样可能会得出不同的结果
首先创建hello.c，写入以下的代码
#include <stdio.h>
int max(int x, int y) {
return x > y ? x : y;
}
int main() {
printf("max is %d\n", max(123, 321));
printf("test many arguments %d %d %d %s %s %s %s %s %s\n", 1, 2, 3, "a", "b", "c", "d", "e", "f");
return 100;
}然后使用gcc编译这份代码
gcc hello.c编译完成后你可以看到hello.c旁边多了一个a.out, 这就是linux的可执行文件了, 现在可以在linux上运行它
./a.out你可以看到以下输出
max is 321
test many arguments 1 2 3 a b c d e f我们来看看a.out包含了什么，解析ELF文件可以使用readelf命令
readelf -a ./a.out可以看到输出了以下的信息
ELF 头：
Magic： 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
类别: ELF64
数据: 2 补码，小端序 (little endian)
版本: 1 (current)
OS/ABI: UNIX - System V
ABI 版本: 0
类型: EXEC (可执行文件)
系统架构: Advanced Micro Devices X86-64
版本: 0x1
入口点地址： 0x400430
程序头起点： 64 (bytes into file)
Start of section headers: 6648 (bytes into file)
标志： 0x0
本头的大小： 64 (字节)
程序头大小： 56 (字节)
Number of program headers: 9
节头大小： 64 (字节)
节头数量： 31
字符串表索引节头： 28
节头：
[号] 名称 类型 地址 偏移量
大小 全体大小 旗标 链接 信息 对齐
[ 0] NULL 000000000000000000000000
00000000000000000000000000000000 0 0 0
[ 1] .interp PROGBITS 000000000040023800000238
000000000000001c0000000000000000 A 0 0 1
[ 2] .note.ABI-tag NOTE 000000000040025400000254
00000000000000200000000000000000 A 0 0 4
[ 3] .note.gnu.build-i NOTE 000000000040027400000274
00000000000000240000000000000000 A 0 0 4
[ 4] .gnu.hash GNU_HASH 000000000040029800000298
000000000000001c0000000000000000 A 5 0 8
[ 5] .dynsym DYNSYM 00000000004002b8000002b8
00000000000000600000000000000018 A 6 1 8
[ 6] .dynstr STRTAB 000000000040031800000318
000000000000003f0000000000000000 A 0 0 1
[ 7] .gnu.version VERSYM 000000000040035800000358
00000000000000080000000000000002 A 5 0 2
[ 8] .gnu.version_r VERNEED 000000000040036000000360
00000000000000200000000000000000 A 6 1 8
[ 9] .rela.dyn RELA 000000000040038000000380
00000000000000180000000000000018 A 5 0 8
.rela.plt RELA 000000000040039800000398
00000000000000300000000000000018AI 5 24 8
.init PROGBITS 00000000004003c8000003c8
000000000000001a0000000000000000AX 0 0 4
.plt PROGBITS 00000000004003f0000003f0
00000000000000300000000000000010AX 0 0 16
.plt.got PROGBITS 000000000040042000000420
00000000000000080000000000000000AX 0 0 8
.text PROGBITS 000000000040043000000430
00000000000001f20000000000000000AX 0 0 16
.fini PROGBITS 000000000040062400000624
00000000000000090000000000000000AX 0 0 4
.rodata PROGBITS 000000000040063000000630
00000000000000500000000000000000 A 0 0 8
.eh_frame_hdr PROGBITS 000000000040068000000680
000000000000003c0000000000000000 A 0 0 4
.eh_frame PROGBITS 00000000004006c0000006c0
00000000000001140000000000000000 A 0 0 8
.init_array INIT_ARRAY 0000000000600e1000000e10
00000000000000080000000000000000WA 0 0 8
.fini_array FINI_ARRAY 0000000000600e1800000e18
00000000000000080000000000000000WA 0 0 8
.jcr PROGBITS 0000000000600e2000000e20
00000000000000080000000000000000WA 0 0 8
.dynamic DYNAMIC 0000000000600e2800000e28
00000000000001d00000000000000010WA 6 0 8
.got PROGBITS 0000000000600ff800000ff8
00000000000000080000000000000008WA 0 0 8
.got.plt PROGBITS 000000000060100000001000
00000000000000280000000000000008WA 0 0 8
.data PROGBITS 000000000060102800001028
00000000000000100000000000000000WA 0 0 8
.bss NOBITS 000000000060103800001038
00000000000000080000000000000000WA 0 0 1
.comment PROGBITS 000000000000000000001038
00000000000000340000000000000001MS 0 0 1
.shstrtab STRTAB 0000000000000000000018ea
000000000000010c0000000000000000 0 0 1
.symtab SYMTAB 000000000000000000001070
00000000000006600000000000000018 30 47 8
.strtab STRTAB 0000000000000000000016d0
000000000000021a0000000000000000 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)
There are no section groups in this file.
程序头：
Type Offset VirtAddr PhysAddr
FileSiz MemSiz FlagsAlign
PHDR 0x0000000000000040 0x0000000000400040 0x0000000000400040
0x00000000000001f8 0x00000000000001f8R E 8
INTERP 0x0000000000000238 0x0000000000400238 0x0000000000400238
0x000000000000001c 0x000000000000001cR 1
LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000
0x00000000000007d4 0x00000000000007d4R E 200000
LOAD 0x0000000000000e10 0x0000000000600e10 0x0000000000600e10
0x0000000000000228 0x0000000000000230RW 200000
DYNAMIC 0x0000000000000e28 0x0000000000600e28 0x0000000000600e28
0x00000000000001d0 0x00000000000001d0RW 8
NOTE 0x0000000000000254 0x0000000000400254 0x0000000000400254
0x0000000000000044 0x0000000000000044R 4
GNU_EH_FRAME 0x0000000000000680 0x0000000000400680 0x0000000000400680
0x000000000000003c 0x000000000000003cR 4
GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000RW 10
GNU_RELRO 0x0000000000000e10 0x0000000000600e10 0x0000000000600e10
0x00000000000001f0 0x00000000000001f0R 1
Section to Segment mapping:
段节...
00
01 .interp
02 .interp .note.ABI-tag .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .plt.got .text .fini .rodata .eh_frame_hdr .eh_frame
03 .init_array .fini_array .jcr .dynamic .got .got.plt .data .bss
04 .dynamic
05 .note.ABI-tag .note.gnu.build-id
06 .eh_frame_hdr
07
08 .init_array .fini_array .jcr .dynamic .got
Dynamic section at offset 0xe28 contains 24 entries:
标记 类型 名称/值
0x0000000000000001 (NEEDED) 共享库：
0x000000000000000c (INIT) 0x4003c8
0x000000000000000d (FINI) 0x400624
0x0000000000000019 (INIT_ARRAY) 0x600e10
0x000000000000001b (INIT_ARRAYSZ) 8 (bytes)
0x000000000000001a (FINI_ARRAY) 0x600e18
0x000000000000001c (FINI_ARRAYSZ) 8 (bytes)
0x000000006ffffef5 (GNU_HASH) 0x400298
0x0000000000000005 (STRTAB) 0x400318
0x0000000000000006 (SYMTAB) 0x4002b8
0x000000000000000a (STRSZ) 63 (bytes)
0x000000000000000b (SYMENT) 24 (bytes)
0x0000000000000015 (DEBUG) 0x0
0x0000000000000003 (PLTGOT) 0x601000
0x0000000000000002 (PLTRELSZ) 48 (bytes)
0x0000000000000014 (PLTREL) RELA
0x0000000000000017 (JMPREL) 0x400398
0x0000000000000007 (RELA) 0x400380
0x0000000000000008 (RELASZ) 24 (bytes)
0x0000000000000009 (RELAENT) 24 (bytes)
0x000000006ffffffe (VERNEED) 0x400360
0x000000006fffffff (VERNEEDNUM) 1
0x000000006ffffff0 (VERSYM) 0x400358
0x0000000000000000 (NULL) 0x0
重定位节 '.rela.dyn' 位于偏移量 0x380 含有 1 个条目：
偏移量 信息 类型 符号值 符号名称 + 加数
000000600ff8000300000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0
重定位节 '.rela.plt' 位于偏移量 0x398 含有 2 个条目：
偏移量 信息 类型 符号值 符号名称 + 加数
000000601018000100000007 R_X86_64_JUMP_SLO 0000000000000000 printf@GLIBC_2.2.5 + 0
000000601020000200000007 R_X86_64_JUMP_SLO 0000000000000000 __libc_start_main@GLIBC_2.2.5 + 0
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
Symbol table '.dynsym' contains 4 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPELOCALDEFAULTUND
1: 0000000000000000 0 FUNC GLOBAL DEFAULTUND printf@GLIBC_2.2.5 (2)
2: 0000000000000000 0 FUNC GLOBAL DEFAULTUND __libc_start_main@GLIBC_2.2.5 (2)
3: 0000000000000000 0 NOTYPEWEAK DEFAULTUND __gmon_start__
Symbol table '.symtab' contains 68 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPELOCALDEFAULTUND
1: 0000000000400238 0 SECTION LOCALDEFAULT 1
2: 0000000000400254 0 SECTION LOCALDEFAULT 2
3: 0000000000400274 0 SECTION LOCALDEFAULT 3
4: 0000000000400298 0 SECTION LOCALDEFAULT 4
5: 00000000004002b8 0 SECTION LOCALDEFAULT 5
6: 0000000000400318 0 SECTION LOCALDEFAULT 6
7: 0000000000400358 0 SECTION LOCALDEFAULT 7
8: 0000000000400360 0 SECTION LOCALDEFAULT 8
9: 0000000000400380 0 SECTION LOCALDEFAULT 9
10: 0000000000400398 0 SECTION LOCALDEFAULT 10
11: 00000000004003c8 0 SECTION LOCALDEFAULT 11
12: 00000000004003f0 0 SECTION LOCALDEFAULT 12
13: 0000000000400420 0 SECTION LOCALDEFAULT 13
14: 0000000000400430 0 SECTION LOCALDEFAULT 14
15: 0000000000400624 0 SECTION LOCALDEFAULT 15
16: 0000000000400630 0 SECTION LOCALDEFAULT 16
17: 0000000000400680 0 SECTION LOCALDEFAULT 17
18: 00000000004006c0 0 SECTION LOCALDEFAULT 18
19: 0000000000600e10 0 SECTION LOCALDEFAULT 19
20: 0000000000600e18 0 SECTION LOCALDEFAULT 20
21: 0000000000600e20 0 SECTION LOCALDEFAULT 21
22: 0000000000600e28 0 SECTION LOCALDEFAULT 22
23: 0000000000600ff8 0 SECTION LOCALDEFAULT 23
24: 0000000000601000 0 SECTION LOCALDEFAULT 24
25: 0000000000601028 0 SECTION LOCALDEFAULT 25
26: 0000000000601038 0 SECTION LOCALDEFAULT 26
27: 0000000000000000 0 SECTION LOCALDEFAULT 27
28: 0000000000000000 0 FILE LOCALDEFAULTABS crtstuff.c
29: 0000000000600e20 0 OBJECTLOCALDEFAULT 21 __JCR_LIST__
30: 0000000000400460 0 FUNC LOCALDEFAULT 14 deregister_tm_clones
31: 00000000004004a0 0 FUNC LOCALDEFAULT 14 register_tm_clones
32: 00000000004004e0 0 FUNC LOCALDEFAULT 14 __do_global_dtors_aux
33: 0000000000601038 1 OBJECTLOCALDEFAULT 26 completed.7585
34: 0000000000600e18 0 OBJECTLOCALDEFAULT 20 __do_global_dtors_aux_fin
35: 0000000000400500 0 FUNC LOCALDEFAULT 14 frame_dummy
36: 0000000000600e10 0 OBJECTLOCALDEFAULT 19 __frame_dummy_init_array_
37: 0000000000000000 0 FILE LOCALDEFAULTABS hello.c
38: 0000000000000000 0 FILE LOCALDEFAULTABS crtstuff.c
39: 00000000004007d0 0 OBJECTLOCALDEFAULT 18 __FRAME_END__
40: 0000000000600e20 0 OBJECTLOCALDEFAULT 21 __JCR_END__
41: 0000000000000000 0 FILE LOCALDEFAULTABS
42: 0000000000600e18 0 NOTYPELOCALDEFAULT 19 __init_array_end
43: 0000000000600e28 0 OBJECTLOCALDEFAULT 22 _DYNAMIC
44: 0000000000600e10 0 NOTYPELOCALDEFAULT 19 __init_array_start
45: 0000000000400680 0 NOTYPELOCALDEFAULT 17 __GNU_EH_FRAME_HDR
46: 0000000000601000 0 OBJECTLOCALDEFAULT 24 _GLOBAL_OFFSET_TABLE_
47: 0000000000400620 2 FUNC GLOBAL DEFAULT 14 __libc_csu_fini
48: 0000000000000000 0 NOTYPEWEAK DEFAULTUND _ITM_deregisterTMCloneTab
49: 0000000000601028 0 NOTYPEWEAK DEFAULT 25 data_start
50: 0000000000601038 0 NOTYPEGLOBAL DEFAULT 25 _edata
51: 0000000000400624 0 FUNC GLOBAL DEFAULT 15 _fini
52: 0000000000000000 0 FUNC GLOBAL DEFAULTUND printf@@GLIBC_2.2.5
53: 0000000000400526 22 FUNC GLOBAL DEFAULT 14 max
54: 0000000000000000 0 FUNC GLOBAL DEFAULTUND __libc_start_main@@GLIBC_
55: 0000000000601028 0 NOTYPEGLOBAL DEFAULT 25 __data_start
56: 0000000000000000 0 NOTYPEWEAK DEFAULTUND __gmon_start__
57: 0000000000601030 0 OBJECTGLOBAL HIDDEN 25 __dso_handle
58: 0000000000400630 4 OBJECTGLOBAL DEFAULT 16 _IO_stdin_used
59: 00000000004005b0 101 FUNC GLOBAL DEFAULT 14 __libc_csu_init
60: 0000000000601040 0 NOTYPEGLOBAL DEFAULT 26 _end
61: 0000000000400430 42 FUNC GLOBAL DEFAULT 14 _start
62: 0000000000601038 0 NOTYPEGLOBAL DEFAULT 26 __bss_start
63: 000000000040053c 109 FUNC GLOBAL DEFAULT 14 main
64: 0000000000000000 0 NOTYPEWEAK DEFAULTUND _Jv_RegisterClasses
65: 0000000000601038 0 OBJECTGLOBAL HIDDEN 25 __TMC_END__
66: 0000000000000000 0 NOTYPEWEAK DEFAULTUND _ITM_registerTMCloneTable
67: 00000000004003c8 0 FUNC GLOBAL DEFAULT 11 _init
Version symbols section '.gnu.version' contains 4 entries:
地址： 0000000000400358Offset: 0x000358Link: 5 (.dynsym)
000: 0 (*本地*) 2 (GLIBC_2.2.5) 2 (GLIBC_2.2.5) 0 (*本地*)
Version needs section '.gnu.version_r' contains 1 entries:
地址：0x0000000000400360Offset: 0x000360Link: 6 (.dynstr)
000000: 版本: 1文件：libc.so.6计数：1
0x0010：名称：GLIBC_2.2.5标志：无版本：2
Displaying notes found at file offset 0x00000254 with length 0x00000020:
Owner Data size Description
GNU 0x00000010 NT_GNU_ABI_TAG (ABI version tag)
OS: Linux, ABI: 2.6.32
Displaying notes found at file offset 0x00000274 with length 0x00000024:
Owner Data size Description
GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
Build ID: debd3d7912be860a432b5c685a6cff7fd9418528从上面的信息中我们可以知道这个文件的类型是ELF64, 也就是64位的可执行程序, 并且有9个程序头和31个节头, 各个节的作用大家可以在网上找到资料, 这篇文章中只涉及到以下的节
[*].init 程序初始化的代码
[*].rela.dyn 需要重定位的变量列表
[*].rela.plt 需要重定位的函数列表
[*].plt 调用动态链接函数的代码
[*].text 保存了主要的程序代码
[*].init 保存了程序的初始化代码, 用于初始化全局变量等
[*].fini 保存了程序的终止代码, 用于析构全局变量等
[*].rodata 保存了只读的数据，例如字符串(const char*)
[*].data 保存了可读写的数据，例如全局变量
[*].dynsym 动态链接的符号表
[*].dynstr 动态链接的符号名称字符串
[*].dynamic 动态链接所需要的信息，供程序运行时使用(不需要访问节头)
什么是动态链接
上面的程序中调用了printf函数, 然而这个函数的实现并不在./a.out中, 那么printf函数在哪里, 又是怎么被调用的?
printf函数的实现在glibc库中, 也就是/lib/x86_64-linux-gnu/libc.so.6中, 在执行./a.out的时候会在glibc库中找到这个函数并进行调用, 我们来看看这段代码
执行以下命令反编译./a.out
objdump -c -S ./a.out我们可以看到以下的代码
00000000004003f0 <printf@plt-0x10>:
4003f0: ff 35 12 0c 20 00 pushq0x200c12(%rip) # 601008 <_GLOBAL_OFFSET_TABLE_+0x8>
4003f6: ff 25 14 0c 20 00 jmpq *0x200c14(%rip) # 601010 <_GLOBAL_OFFSET_TABLE_+0x10>
4003fc: 0f 1f 40 00 nopl 0x0(%rax)
0000000000400400 <printf@plt>:
400400: ff 25 12 0c 20 00 jmpq *0x200c12(%rip) # 601018 <_GLOBAL_OFFSET_TABLE_+0x18>
400406: 68 00 00 00 00 pushq$0x0
40040b: e9 e0 ff ff ff jmpq 4003f0 <_init+0x28>
000000000040053c <main>:
40053c: 55 push %rbp
40053d: 48 89 e5 mov %rsp,%rbp
400540: be 41 01 00 00 mov $0x141,%esi
400545: bf 7b 00 00 00 mov $0x7b,%edi
40054a: e8 d7 ff ff ff callq400526 <max>
40054f: 89 c6 mov %eax,%esi
400551: bf 38 06 40 00 mov $0x400638,%edi
400556: b8 00 00 00 00 mov $0x0,%eax
40055b: e8 a0 fe ff ff callq400400 <printf@plt>在这一段代码中，我们可以看到调用printf会首先调用0x400400的printf@plt
printf@plt会负责在运行时找到实际的printf函数并跳转到该函数
在这里实际的printf函数会保存在0x400406 + 0x200c12 = 0x601018中
需要注意的是0x601018一开始并不会指向实际的printf函数，而是会指向0x400406, 为什么会这样? 因为Linux的可执行程序为了考虑性能，不会在一开始就解决所有动态连接的函数，而是选择了延迟解决.
在上面第一次jmpq *0x200c12(%rip)会跳转到下一条指令0x400406, 又会继续跳转到0x4003f0, 再跳转到0x601010指向的地址, 0x601010指向的地址就是延迟解决的实现, 第一次延迟解决成功后, 0x601018就会指向实际的printf, 以后调用就会直接跳转到实际的printf上.
程序入口点
Linux程序运行首先会从_start函数开始, 上面readelf中的入口点地址0x400430就是_start函数的地址,
0000000000400430 <_start>:
400430: 31 ed xor %ebp,%ebp
400432: 49 89 d1 mov %rdx,%r9
400435: 5e pop %rsi
400436: 48 89 e2 mov %rsp,%rdx
400439: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp
40043d: 50 push %rax
40043e: 54 push %rsp
40043f: 49 c7 c0 20 06 40 00 mov $0x400620,%r8
400446: 48 c7 c1 b0 05 40 00 mov $0x4005b0,%rcx
40044d: 48 c7 c7 3c 05 40 00 mov $0x40053c,%rdi
400454: e8 b7 ff ff ff callq400410 <__libc_start_main@plt>
400459: f4 hlt
40045a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)接下来_start函数会调用__libc_start_main函数, __libc_start_main是libc库中定义的初始化函数, 负责初始化全局变量和调用main函数等工作.
__libc_start_main函数还负责设置返回值和退出进程, 可以看到上面调用__libc_start_main后的指令是hlt, 这个指令永远不会被执行.
实现Linux程序运行器
在拥有以上的知识后我们可以先构想以下的运行器需要做什么.
因为x64的Windows和Linux程序使用的cpu指令集都是一样的，我们可以直接执行汇编而不需要一个指令模拟器,
而且这次我打算在用户层实现, 所以不能像Bash On Windows一样模拟syscall, 这个运行器会像下图一样模拟libc库的函数
这样运行器需要做的事情有:
[*]解析ELF文件
[*]加载程序代码到指定的内存地址
[*]加载数据到指定的内存地址
[*]提供动态链接的函数实现
[*]执行加载的程序代码
这些工作会在以下的示例程序中一一实现, 完整的源代码可以看文章顶部的链接
首先我们需要把ELF文件格式对应的代码从binutils中复制过来, 它包含了ELF头, 程序头和相关的数据结构, 里面用unsigned char[]是为了防止alignment, 这样结构体可以直接从文件内容中转换过来
ELFDefine.h:
#pragma once
namespace HelloElfLoader {
// 以下内容复制自
// https://github.com/aeste/binutils/blob/develop/elfcpp/elfcpp.h
// https://github.com/aeste/binutils/blob/develop/include/elf/external.h
// e_ident中各项的偏移值
const int EI_MAG0 = 0;
const int EI_MAG1 = 1;
const int EI_MAG2 = 2;
const int EI_MAG3 = 3;
const int EI_CLASS = 4;
const int EI_DATA = 5;
const int EI_VERSION = 6;
const int EI_OSABI = 7;
const int EI_ABIVERSION = 8;
const int EI_PAD = 9;
const int EI_NIDENT = 16;
// ELF文件类型
enum {
ELFCLASSNONE = 0,
ELFCLASS32 = 1,
ELFCLASS64 = 2
};
// ByteOrder
enum {
ELFDATANONE = 0,
ELFDATA2LSB = 1,
ELFDATA2MSB = 2
};
// 程序头类型
enum PT
{
PT_NULL = 0,
PT_LOAD = 1,
PT_DYNAMIC = 2,
PT_INTERP = 3,
PT_NOTE = 4,
PT_SHLIB = 5,
PT_PHDR = 6,
PT_TLS = 7,
PT_LOOS = 0x60000000,
PT_HIOS = 0x6fffffff,
PT_LOPROC = 0x70000000,
PT_HIPROC = 0x7fffffff,
// The remaining values are not in the standard.
// Frame unwind information.
PT_GNU_EH_FRAME = 0x6474e550,
PT_SUNW_EH_FRAME = 0x6474e550,
// Stack flags.
PT_GNU_STACK = 0x6474e551,
// Read only after relocation.
PT_GNU_RELRO = 0x6474e552,
// Platform architecture compatibility information
PT_ARM_ARCHEXT = 0x70000000,
// Exception unwind tables
PT_ARM_EXIDX = 0x70000001
};
// 动态节类型
enum DT
{
DT_NULL = 0,
DT_NEEDED = 1,
DT_PLTRELSZ = 2,
DT_PLTGOT = 3,
DT_HASH = 4,
DT_STRTAB = 5,
DT_SYMTAB = 6,
DT_RELA = 7,
DT_RELASZ = 8,
DT_RELAENT = 9,
DT_STRSZ = 10,
DT_SYMENT = 11,
DT_INIT = 12,
DT_FINI = 13,
DT_SONAME = 14,
DT_RPATH = 15,
DT_SYMBOLIC = 16,
DT_REL = 17,
DT_RELSZ = 18,
DT_RELENT = 19,
DT_PLTREL = 20,
DT_DEBUG = 21,
DT_TEXTREL = 22,
DT_JMPREL = 23,
DT_BIND_NOW = 24,
DT_INIT_ARRAY = 25,
DT_FINI_ARRAY = 26,
DT_INIT_ARRAYSZ = 27,
DT_FINI_ARRAYSZ = 28,
DT_RUNPATH = 29,
DT_FLAGS = 30,
// This is used to mark a range of dynamic tags.It is not really
// a tag value.
DT_ENCODING = 32,
DT_PREINIT_ARRAY = 32,
DT_PREINIT_ARRAYSZ = 33,
DT_LOOS = 0x6000000d,
DT_HIOS = 0x6ffff000,
DT_LOPROC = 0x70000000,
DT_HIPROC = 0x7fffffff,
// The remaining values are extensions used by GNU or Solaris.
DT_VALRNGLO = 0x6ffffd00,
DT_GNU_PRELINKED = 0x6ffffdf5,
DT_GNU_CONFLICTSZ = 0x6ffffdf6,
DT_GNU_LIBLISTSZ = 0x6ffffdf7,
DT_CHECKSUM = 0x6ffffdf8,
DT_PLTPADSZ = 0x6ffffdf9,
DT_MOVEENT = 0x6ffffdfa,
DT_MOVESZ = 0x6ffffdfb,
DT_FEATURE = 0x6ffffdfc,
DT_POSFLAG_1 = 0x6ffffdfd,
DT_SYMINSZ = 0x6ffffdfe,
DT_SYMINENT = 0x6ffffdff,
DT_VALRNGHI = 0x6ffffdff,
DT_ADDRRNGLO = 0x6ffffe00,
DT_GNU_HASH = 0x6ffffef5,
DT_TLSDESC_PLT = 0x6ffffef6,
DT_TLSDESC_GOT = 0x6ffffef7,
DT_GNU_CONFLICT = 0x6ffffef8,
DT_GNU_LIBLIST = 0x6ffffef9,
DT_CONFIG = 0x6ffffefa,
DT_DEPAUDIT = 0x6ffffefb,
DT_AUDIT = 0x6ffffefc,
DT_PLTPAD = 0x6ffffefd,
DT_MOVETAB = 0x6ffffefe,
DT_SYMINFO = 0x6ffffeff,
DT_ADDRRNGHI = 0x6ffffeff,
DT_RELACOUNT = 0x6ffffff9,
DT_RELCOUNT = 0x6ffffffa,
DT_FLAGS_1 = 0x6ffffffb,
DT_VERDEF = 0x6ffffffc,
DT_VERDEFNUM = 0x6ffffffd,
DT_VERNEED = 0x6ffffffe,
DT_VERNEEDNUM = 0x6fffffff,
DT_VERSYM = 0x6ffffff0,
// Specify the value of _GLOBAL_OFFSET_TABLE_.
DT_PPC_GOT = 0x70000000,
// Specify the start of the .glink section.
DT_PPC64_GLINK = 0x70000000,
// Specify the start and size of the .opd section.
DT_PPC64_OPD = 0x70000001,
DT_PPC64_OPDSZ = 0x70000002,
// The index of an STT_SPARC_REGISTER symbol within the DT_SYMTAB
// symbol table.One dynamic entry exists for every STT_SPARC_REGISTER
// symbol in the symbol table.
DT_SPARC_REGISTER = 0x70000001,
DT_AUXILIARY = 0x7ffffffd,
DT_USED = 0x7ffffffe,
DT_FILTER = 0x7fffffff
};;
// ELF头的定义
typedef struct {
unsigned char e_ident; /* ELF "magic number" */
unsigned char e_type; /* Identifies object file type */
unsigned char e_machine; /* Specifies required architecture */
unsigned char e_version; /* Identifies object file version */
unsigned char e_entry; /* Entry point virtual address */
unsigned char e_phoff; /* Program header table file offset */
unsigned char e_shoff; /* Section header table file offset */
unsigned char e_flags; /* Processor-specific flags */
unsigned char e_ehsize; /* ELF header size in bytes */
unsigned char e_phentsize; /* Program header table entry size */
unsigned char e_phnum; /* Program header table entry count */
unsigned char e_shentsize; /* Section header table entry size */
unsigned char e_shnum; /* Section header table entry count */
unsigned char e_shstrndx; /* Section header string table index */
} Elf64_External_Ehdr;
// 程序头的定义
typedef struct {
unsigned char p_type; /* Identifies program segment type */
unsigned char p_flags; /* Segment flags */
unsigned char p_offset; /* Segment file offset */
unsigned char p_vaddr; /* Segment virtual address */
unsigned char p_paddr; /* Segment physical address */
unsigned char p_filesz; /* Segment size in file */
unsigned char p_memsz; /* Segment size in memory */
unsigned char p_align; /* Segment alignment, file & memory */
} Elf64_External_Phdr;
// DYNAMIC类型的程序头的内容定义
typedef struct {
unsigned char d_tag; /* entry tag value */
union {
unsigned char d_val;
unsigned char d_ptr;
} d_un;
} Elf64_External_Dyn;
// 动态链接的重定位记录，部分系统会用Elf64_External_Rel
typedef struct {
unsigned char r_offset; /* Location at which to apply the action */
unsigned char r_info; /* index and type of relocation */
unsigned char r_addend; /* Constant addend used to compute value */
} Elf64_External_Rela;
// 动态链接的符号信息
typedef struct {
unsigned char st_name; /* Symbol name, index in string tbl */
unsigned char st_info; /* Type and binding attributes */
unsigned char st_other; /* No defined meaning, 0 */
unsigned char st_shndx; /* Associated section index */
unsigned char st_value; /* Value of the symbol */
unsigned char st_size; /* Associated symbol size */
} Elf64_External_Sym;
}接下来我们定义一个读取和执行ELF文件的类, 这个类会在初始化时把文件加载到fileStream_, execute函数会负责执行
HelloElfLoader.h:
#pragma once
#include <string>
#include <fstream>
namespace HelloElfLoader {
class Loader {
std::ifstream fileStream_;
public:
Loader(const std::string& path);
Loader(std::ifstream&& fileStream);
void execute();
};
}构造函数如下, 也就是标准的c++打开文件的代码
HelloElfLoader.cpp:
Loader::Loader(const std::string& path) :
Loader(std::ifstream(path, std::ios::in | std::ios::binary)) {}
Loader::Loader(std::ifstream&& fileStream) :
fileStream_(std::move(fileStream)) {
if (!fileStream_) {
throw std::runtime_error("open file failed");
}
}接下来将实现上面所说的步骤, 首先是解析ELF文件
void Loader::execute() { std::cout 这个有用。 感谢分享 感谢发布原创作品，程序园因你更精彩
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