哪些落实以Windows上运行Linux程序,附示例代码

微软于上年公布了Bash On Windows, 这项技艺允许在Windows上运行Linux程序,
我深信已经起多稿子解释过Bash On Windows的法则,
若果今天之及时篇稿子用见面讲课如何团结实现一个简练的原生Linux程序运行器,
这个运行器在用户层实现, 原理与Bash On
Windows不净平等,比较像样Linux上之Wine.

尝试用Markdown写一篇博客
3142: [Hnoi2013]数列
Description
小T最近以模拟在购买股票,他取内部消息:F公司的股票将会见疯涨。股票每天的价已经了解是刚整数,并且鉴于客观上的故,最多只能为N。在疯涨的K天中小T观察到:除第一龙他每天的股价都于前天高,且大出底价位(即当天的股价和前一天之股价的差)不见面超过M,M为正整数。并且这些参数满足M(K-1)<N。
小T忘记了这K天每天的切实可行股价了,他现纪念了解就K天的股价有小种可能。
Input
单纯出一行用空格隔开的季独数:N、K、M、P。对P的征参见后面“输出格式”中对P的说。
输入保证20%底数据M,N,K,P≤20000,保证100%的数据M,K,P≤10^9,N≤10^18 。
Output
就含一个勤,表示马上K天的股价的或种数对于P的模值。
Sample Input
7 3 2 997
Sample Output
16

以身作则程序完整的代码在github上, 地址是
https://github.com/303248153/HelloElfLoader

第一来言出口自己是怎么开(鬼)出当下道题的。
无错就是打表。
本着上次考从完表没看出1,2,6,24凡阶乘的政工耿耿于怀的自家操就此打表做出这道一样拘留就是自表题的写。
首先自己花费了20分钟碌碌无为,对于答案f(n,k,m)打了一个小表,什么还并未意识。
20分钟左右本身开始稳定k和m,移动n。
实验了几组k在2~4的数目后发觉于n到n+1,答案会增进m^(k-1)。
试试到30分钟,总结发生:规律是当n=m(k-1)处起之。

始于了解ELF格式

率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),[2] and later in the Tool Interface Standard,[1] 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,
也便是咱常常采取的exe文件的格式.

ELF格式的布局如下

图片 1

大概上得以分成这些部分

  • ELF头,在文书之顶开始,储存了型和版本等信息
  • 先后头, 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, 运行时莫待读节头
  • 省内容, 不同的节作用都不雷同
    • .text 代码节,保存了关键的程序代码
    • .rodata 保存了单独念之数码,例如字符串(const char*)
    • .data 保存了而是读写的多寡,例如全局变量
    • 还产生其它各种各样的节

深受咱们来其实圈一下Linux可执行程序的楷模
以下的编译环境是Ubuntu 16.04 x64 + gcc 5.4.0,
编译环境不等同或会见汲取不同之结果

率先创建hello.c,写副以下的代码

#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;
}

下一场下gcc编译就卖代码

gcc hello.c

编译完成后您可视hello.c一旁多了一个a.out,
这就是是linux的可执行文件了, 现在足在linux上运行它们

./a.out

汝可以看出以下输出

max is 321
test many arguments 1 2 3 a b c d e f

咱来探a.out饱含了呀,解析ELF文件可使用readelf命令

readelf -a ./a.out

可看看输出了以下的音讯

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             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .interp           PROGBITS         0000000000400238  00000238
       000000000000001c  0000000000000000   A       0     0     1
  [ 2] .note.ABI-tag     NOTE             0000000000400254  00000254
       0000000000000020  0000000000000000   A       0     0     4
  [ 3] .note.gnu.build-i NOTE             0000000000400274  00000274
       0000000000000024  0000000000000000   A       0     0     4
  [ 4] .gnu.hash         GNU_HASH         0000000000400298  00000298
       000000000000001c  0000000000000000   A       5     0     8
  [ 5] .dynsym           DYNSYM           00000000004002b8  000002b8
       0000000000000060  0000000000000018   A       6     1     8
  [ 6] .dynstr           STRTAB           0000000000400318  00000318
       000000000000003f  0000000000000000   A       0     0     1
  [ 7] .gnu.version      VERSYM           0000000000400358  00000358
       0000000000000008  0000000000000002   A       5     0     2
  [ 8] .gnu.version_r    VERNEED          0000000000400360  00000360
       0000000000000020  0000000000000000   A       6     1     8
  [ 9] .rela.dyn         RELA             0000000000400380  00000380
       0000000000000018  0000000000000018   A       5     0     8
  [10] .rela.plt         RELA             0000000000400398  00000398
       0000000000000030  0000000000000018  AI       5    24     8
  [11] .init             PROGBITS         00000000004003c8  000003c8
       000000000000001a  0000000000000000  AX       0     0     4
  [12] .plt              PROGBITS         00000000004003f0  000003f0
       0000000000000030  0000000000000010  AX       0     0     16
  [13] .plt.got          PROGBITS         0000000000400420  00000420
       0000000000000008  0000000000000000  AX       0     0     8
  [14] .text             PROGBITS         0000000000400430  00000430
       00000000000001f2  0000000000000000  AX       0     0     16
  [15] .fini             PROGBITS         0000000000400624  00000624
       0000000000000009  0000000000000000  AX       0     0     4
  [16] .rodata           PROGBITS         0000000000400630  00000630
       0000000000000050  0000000000000000   A       0     0     8
  [17] .eh_frame_hdr     PROGBITS         0000000000400680  00000680
       000000000000003c  0000000000000000   A       0     0     4
  [18] .eh_frame         PROGBITS         00000000004006c0  000006c0
       0000000000000114  0000000000000000   A       0     0     8
  [19] .init_array       INIT_ARRAY       0000000000600e10  00000e10
       0000000000000008  0000000000000000  WA       0     0     8
  [20] .fini_array       FINI_ARRAY       0000000000600e18  00000e18
       0000000000000008  0000000000000000  WA       0     0     8
  [21] .jcr              PROGBITS         0000000000600e20  00000e20
       0000000000000008  0000000000000000  WA       0     0     8
  [22] .dynamic          DYNAMIC          0000000000600e28  00000e28
       00000000000001d0  0000000000000010  WA       6     0     8
  [23] .got              PROGBITS         0000000000600ff8  00000ff8
       0000000000000008  0000000000000008  WA       0     0     8
  [24] .got.plt          PROGBITS         0000000000601000  00001000
       0000000000000028  0000000000000008  WA       0     0     8
  [25] .data             PROGBITS         0000000000601028  00001028
       0000000000000010  0000000000000000  WA       0     0     8
  [26] .bss              NOBITS           0000000000601038  00001038
       0000000000000008  0000000000000000  WA       0     0     1
  [27] .comment          PROGBITS         0000000000000000  00001038
       0000000000000034  0000000000000001  MS       0     0     1
  [28] .shstrtab         STRTAB           0000000000000000  000018ea
       000000000000010c  0000000000000000           0     0     1
  [29] .symtab           SYMTAB           0000000000000000  00001070
       0000000000000660  0000000000000018          30    47     8
  [30] .strtab           STRTAB           0000000000000000  000016d0
       000000000000021a  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)

There are no section groups in this file.

程序头:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000400040 0x0000000000400040
                 0x00000000000001f8 0x00000000000001f8  R E    8
  INTERP         0x0000000000000238 0x0000000000400238 0x0000000000400238
                 0x000000000000001c 0x000000000000001c  R      1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x00000000000007d4 0x00000000000007d4  R E    200000
  LOAD           0x0000000000000e10 0x0000000000600e10 0x0000000000600e10
                 0x0000000000000228 0x0000000000000230  RW     200000
  DYNAMIC        0x0000000000000e28 0x0000000000600e28 0x0000000000600e28
                 0x00000000000001d0 0x00000000000001d0  RW     8
  NOTE           0x0000000000000254 0x0000000000400254 0x0000000000400254
                 0x0000000000000044 0x0000000000000044  R      4
  GNU_EH_FRAME   0x0000000000000680 0x0000000000400680 0x0000000000400680
                 0x000000000000003c 0x000000000000003c  R      4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     10
  GNU_RELRO      0x0000000000000e10 0x0000000000600e10 0x0000000000600e10
                 0x00000000000001f0 0x00000000000001f0  R      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:[libc.so.6]
 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
000000600ff8  000300000006 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
000000601018  000100000007 R_X86_64_JUMP_SLO 0000000000000000 printf@GLIBC_2.2.5 + 0
000000601020  000200000007 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 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND printf@GLIBC_2.2.5 (2)
     2: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.2.5 (2)
     3: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__

Symbol table '.symtab' contains 68 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000400238     0 SECTION LOCAL  DEFAULT    1 
     2: 0000000000400254     0 SECTION LOCAL  DEFAULT    2 
     3: 0000000000400274     0 SECTION LOCAL  DEFAULT    3 
     4: 0000000000400298     0 SECTION LOCAL  DEFAULT    4 
     5: 00000000004002b8     0 SECTION LOCAL  DEFAULT    5 
     6: 0000000000400318     0 SECTION LOCAL  DEFAULT    6 
     7: 0000000000400358     0 SECTION LOCAL  DEFAULT    7 
     8: 0000000000400360     0 SECTION LOCAL  DEFAULT    8 
     9: 0000000000400380     0 SECTION LOCAL  DEFAULT    9 
    10: 0000000000400398     0 SECTION LOCAL  DEFAULT   10 
    11: 00000000004003c8     0 SECTION LOCAL  DEFAULT   11 
    12: 00000000004003f0     0 SECTION LOCAL  DEFAULT   12 
    13: 0000000000400420     0 SECTION LOCAL  DEFAULT   13 
    14: 0000000000400430     0 SECTION LOCAL  DEFAULT   14 
    15: 0000000000400624     0 SECTION LOCAL  DEFAULT   15 
    16: 0000000000400630     0 SECTION LOCAL  DEFAULT   16 
    17: 0000000000400680     0 SECTION LOCAL  DEFAULT   17 
    18: 00000000004006c0     0 SECTION LOCAL  DEFAULT   18 
    19: 0000000000600e10     0 SECTION LOCAL  DEFAULT   19 
    20: 0000000000600e18     0 SECTION LOCAL  DEFAULT   20 
    21: 0000000000600e20     0 SECTION LOCAL  DEFAULT   21 
    22: 0000000000600e28     0 SECTION LOCAL  DEFAULT   22 
    23: 0000000000600ff8     0 SECTION LOCAL  DEFAULT   23 
    24: 0000000000601000     0 SECTION LOCAL  DEFAULT   24 
    25: 0000000000601028     0 SECTION LOCAL  DEFAULT   25 
    26: 0000000000601038     0 SECTION LOCAL  DEFAULT   26 
    27: 0000000000000000     0 SECTION LOCAL  DEFAULT   27 
    28: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    29: 0000000000600e20     0 OBJECT  LOCAL  DEFAULT   21 __JCR_LIST__
    30: 0000000000400460     0 FUNC    LOCAL  DEFAULT   14 deregister_tm_clones
    31: 00000000004004a0     0 FUNC    LOCAL  DEFAULT   14 register_tm_clones
    32: 00000000004004e0     0 FUNC    LOCAL  DEFAULT   14 __do_global_dtors_aux
    33: 0000000000601038     1 OBJECT  LOCAL  DEFAULT   26 completed.7585
    34: 0000000000600e18     0 OBJECT  LOCAL  DEFAULT   20 __do_global_dtors_aux_fin
    35: 0000000000400500     0 FUNC    LOCAL  DEFAULT   14 frame_dummy
    36: 0000000000600e10     0 OBJECT  LOCAL  DEFAULT   19 __frame_dummy_init_array_
    37: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS hello.c
    38: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    39: 00000000004007d0     0 OBJECT  LOCAL  DEFAULT   18 __FRAME_END__
    40: 0000000000600e20     0 OBJECT  LOCAL  DEFAULT   21 __JCR_END__
    41: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS 
    42: 0000000000600e18     0 NOTYPE  LOCAL  DEFAULT   19 __init_array_end
    43: 0000000000600e28     0 OBJECT  LOCAL  DEFAULT   22 _DYNAMIC
    44: 0000000000600e10     0 NOTYPE  LOCAL  DEFAULT   19 __init_array_start
    45: 0000000000400680     0 NOTYPE  LOCAL  DEFAULT   17 __GNU_EH_FRAME_HDR
    46: 0000000000601000     0 OBJECT  LOCAL  DEFAULT   24 _GLOBAL_OFFSET_TABLE_
    47: 0000000000400620     2 FUNC    GLOBAL DEFAULT   14 __libc_csu_fini
    48: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTab
    49: 0000000000601028     0 NOTYPE  WEAK   DEFAULT   25 data_start
    50: 0000000000601038     0 NOTYPE  GLOBAL DEFAULT   25 _edata
    51: 0000000000400624     0 FUNC    GLOBAL DEFAULT   15 _fini
    52: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND printf@@GLIBC_2.2.5
    53: 0000000000400526    22 FUNC    GLOBAL DEFAULT   14 max
    54: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@@GLIBC_
    55: 0000000000601028     0 NOTYPE  GLOBAL DEFAULT   25 __data_start
    56: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
    57: 0000000000601030     0 OBJECT  GLOBAL HIDDEN    25 __dso_handle
    58: 0000000000400630     4 OBJECT  GLOBAL DEFAULT   16 _IO_stdin_used
    59: 00000000004005b0   101 FUNC    GLOBAL DEFAULT   14 __libc_csu_init
    60: 0000000000601040     0 NOTYPE  GLOBAL DEFAULT   26 _end
    61: 0000000000400430    42 FUNC    GLOBAL DEFAULT   14 _start
    62: 0000000000601038     0 NOTYPE  GLOBAL DEFAULT   26 __bss_start
    63: 000000000040053c   109 FUNC    GLOBAL DEFAULT   14 main
    64: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
    65: 0000000000601038     0 OBJECT  GLOBAL HIDDEN    25 __TMC_END__
    66: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
    67: 00000000004003c8     0 FUNC    GLOBAL DEFAULT   11 _init

Version symbols section '.gnu.version' contains 4 entries:
 地址: 0000000000400358  Offset: 0x000358  Link: 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:0x0000000000400360  Offset: 0x000360  Link: 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

从今点的信息遭我们得以领略是文件的类是ELF64,
也就是64各项之可执行程序, 并且有9个程序头和31单节头,
各个节的用意大家可以于网上找到资料, 这首文章被独涉嫌到以下的省

  • .init 程序初始化的代码
  • .rela.dyn 需要再次一贯的变量列表
  • .rela.plt 需要再行一贯的函数列表
  • .plt 调用动态链接函数的代码
  • .text 保存了重要的程序代码
  • .init 保存了次的初始化代码, 用于初始化全局变量等
  • .fini 保存了先后的告一段落代码, 用于析构全局变量等
  • .rodata 保存了仅读的多寡,例如字符串(const char*)
  • .data 保存了可是读写的数量,例如全局变量
  • .dynsym 动态链接的符号表
  • .dynstr 动态链接的号子名称字符串
  • .dynamic 动态链接所待的信,供程序运行时使用(不需看节头)

针对!因为问题保证了n>m(k-1),所以是规律可放心大胆用。

下一场我起了关于k,m的f(m*(k-1),k,m)的表,即临界表。
横长这个样子:

k\m     2     3     4     5
2       1     3     6     10
3       4     18    48    100
4       12    81    288   750
5       32    324   1536  5000

率先当即过去没什么规律?
滥整到40分钟,发现第k尽的都能够吃(k-1)整除,除掉又看:

k\m     2     3     4     5
2       1     3     6     10
3       2     9     24    50
4       4     27    96    250
5       8     81    384   1250

发现每一样排列下都是乘以m?所以要是看率先列。

m     2     3     4     5
      1     3     6     10

离开是只相当差数列,那便是独次坏多项式了。
此刻规律就是比明显了:(m-1)*m/2。
接下来又理一下就算会获得答案:

啊是动态链接

点的程序中调用了printf函数, 然而以此函数的贯彻并无在./a.out中,
那么printf函数在乌, 又是怎受调用的?

printf函数的兑现以glibc库中,
也就是/lib/x86_64-linux-gnu/libc.so.6中,
在执行./a.out的时会当glibc库房中找到这函数并进行调用,
我们来看望这段代码

实行以下命令反编译./a.out

objdump -c -S ./a.out

咱得以视以下的代码

00000000004003f0 <printf@plt-0x10>:
  4003f0:   ff 35 12 0c 20 00       pushq  0x200c12(%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          callq  400526 <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          callq  400400 <printf@plt>

当即时无异截代码中,我们好观看调用printf会晤首先调用0x400400printf@plt
printf@plt会面负担在运行时找到实际的printf函数并跨越反至拖欠函数
当此处其实的printf函数会管存在0x400406 + 0x200c12 = 0x601018

内需专注的凡0x601018同一开始连无会见对实际的printf函数,而是会指向0x400406,
为什么会这么?
因为Linux的可执行程序为了考虑性能,不会见于平等开始就化解有动态连接的函数,而是选择了延解决.
以地方第一涂鸦jmpq *0x200c12(%rip)会晤跨反至下一样长长的指令0x400406,
又会持续过反至0x4003f0, 再超反到0x601010对的地址,
0x601010本着的地方便是缓解决之落实, 第一浅推迟解决成功后,
0x601018便会指向实际的printf,
以后调用就会见一直跨越反至实际的printf上.

Ans=(k-1)×(m-1)×m/2×m^(k-2)+[n-m(k-1)]×m^(k-1)

50分钟未至开打,一个小时不至即开了了。
在省选里面是时空是可接受的(NOIPT2为是1h左右咔嚓?)。

其一时咱们无能够满足是吧?要明白正解是啊。
率先步:将本来数组差分,得到k-1独[1,m]外之正整数a[1…k-1]。
次步:当前方案往往就为n-sum(a[1] to a[k-1])。
之所以究竟的方案往往就是sum(n-sum(a[1] to a[k-1]))。
把n提出来,为n×m^(k-1)。
下一场后面那个东西,网上的解我推不出去,是只要于每个东西单独考虑?不会见。

先后入口点

Linux程序运行首先会见起_start函数开始,
上面readelf倍受之入口点地址0x400430就是_start函数的地点,

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          callq  400410 <__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大凡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函数还当安装返回值和剥离过程,
可以看来地方调用__libc_start_main继的下令是hlt,
这个令永远不见面让执行.

实现Linux程序运行器

于所有上述的学问后我们好事先构想以下的运行器需要举行什么.

因x64的Windows和Linux程序行使的cpu指令集还是平的,我们得一直实施汇编而未欲一个命模拟器,
还要这次自己打算于用户层实现, 所以不克像Bash On Windows一样模拟syscall,
这个运行器会像下图一律模拟libc库的函数

图片 2

如此运行器需要开的事务时有发生:

  • 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
  • 供动态链接的函数实现
  • 尽加载的程序代码

这些工作会当以下的演示程序中逐条实现, 完整的源代码可以拘留文章顶部的链接

率先我们用拿ELF文件格式对应的代码从binutils吃复制过来,
它富含了ELF头, 程序头和连锁的数据结构,
里面用unsigned char[]大凡为着防范alignment,
这样结构体可以一直由文本内容中改换过来

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[16];        /* ELF "magic number" */
        unsigned char   e_type[2];      /* Identifies object file type */
        unsigned char   e_machine[2];       /* Specifies required architecture */
        unsigned char   e_version[4];       /* Identifies object file version */
        unsigned char   e_entry[8];     /* Entry point virtual address */
        unsigned char   e_phoff[8];     /* Program header table file offset */
        unsigned char   e_shoff[8];     /* Section header table file offset */
        unsigned char   e_flags[4];     /* Processor-specific flags */
        unsigned char   e_ehsize[2];        /* ELF header size in bytes */
        unsigned char   e_phentsize[2];     /* Program header table entry size */
        unsigned char   e_phnum[2];     /* Program header table entry count */
        unsigned char   e_shentsize[2];     /* Section header table entry size */
        unsigned char   e_shnum[2];     /* Section header table entry count */
        unsigned char   e_shstrndx[2];      /* Section header string table index */
    } Elf64_External_Ehdr;

    // 程序头的定义
    typedef struct {
        unsigned char   p_type[4];      /* Identifies program segment type */
        unsigned char   p_flags[4];     /* Segment flags */
        unsigned char   p_offset[8];        /* Segment file offset */
        unsigned char   p_vaddr[8];     /* Segment virtual address */
        unsigned char   p_paddr[8];     /* Segment physical address */
        unsigned char   p_filesz[8];        /* Segment size in file */
        unsigned char   p_memsz[8];     /* Segment size in memory */
        unsigned char   p_align[8];     /* 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[8];       /* entry tag value */
        union {
            unsigned char   d_val[8];
            unsigned char   d_ptr[8];
        } 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[8];  /* Location at which to apply the action */
        unsigned char   r_info[8];  /* index and type of relocation */
        unsigned char   r_addend[8];    /* Constant addend used to compute value */
    } Elf64_External_Rela;

    // 动态链接的符号信息
    typedef struct {
        unsigned char   st_name[4];     /* Symbol name, index in string tbl */
        unsigned char   st_info[1];     /* Type and binding attributes */
        unsigned char   st_other[1];        /* No defined meaning, 0 */
        unsigned char   st_shndx[2];        /* Associated section index */
        unsigned char   st_value[8];        /* Value of the symbol */
        unsigned char   st_size[8];     /* Associated symbol size */
    } Elf64_External_Sym;
}

连通下我们定义一个读取和执行ELF文件的好像,
这个类会在初始化时拿公文加载到fileStream_, execute函数会负责履行

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, 也尽管是正经的c++打开文件之代码

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");
    }
}

连片下去将实现地方所说之步调, 首先是解析ELF文件

void Loader::execute() {
    std::cout << "====== start loading elf ======" << std::endl;

    // 检查当前运行程序是否64位
    if (sizeof(intptr_t) != sizeof(std::int64_t)) {
        throw std::runtime_error("please use x64 compile and run this program");
    }

    // 读取ELF头
    Elf64_External_Ehdr elfHeader = {};
    fileStream_.seekg(0);
    fileStream_.read(reinterpret_cast<char*>(&elfHeader), sizeof(elfHeader));

    // 检查ELF头,只支持64位且byte order是little endian的程序
    if (std::string(reinterpret_cast<char*>(elfHeader.e_ident), 4) != "\x7f\x45\x4c\x46") {
        throw std::runtime_error("magic not match");
    }
    else if (elfHeader.e_ident[EI_CLASS] != ELFCLASS64) {
        throw std::runtime_error("only support ELF64");
    }
    else if (elfHeader.e_ident[EI_DATA] != ELFDATA2LSB) {
        throw std::runtime_error("only support little endian");
    }

    // 获取program table的信息
    std::uint32_t programTableOffset = *reinterpret_cast<std::uint32_t*>(elfHeader.e_phoff);
    std::uint16_t programTableEntrySize = *reinterpret_cast<std::uint16_t*>(elfHeader.e_phentsize);
    std::uint16_t programTableEntryNum = *reinterpret_cast<std::uint16_t*>(elfHeader.e_phnum);
    std::cout << "program table at: " << programTableOffset << ", "
        << programTableEntryNum << " x " << programTableEntrySize << std::endl;

    // 获取section table的信息
    // section table只给linker用,loader中其实不需要访问section table
    std::uint32_t sectionTableOffset = *reinterpret_cast<std::uint32_t*>(elfHeader.e_shoff);
    std::uint16_t sectionTableEntrySize = *reinterpret_cast<std::uint16_t*>(elfHeader.e_shentsize);
    std::uint16_t sectionTableEntryNum = *reinterpret_cast<std::uint16_t*>(elfHeader.e_shentsize);
    std::cout << "section table at: " << sectionTableOffset << ", "
        << sectionTableEntryNum << " x " << sectionTableEntrySize << std::endl;

ELF文件的的开端有就是ELF头,和Elf64_External_Ehdr结构体的结构同样,
我们好读到Elf64_External_Ehdr结构体中,
然后ELF头包含了程序头和节头的偏移值, 我们得以先获取到这些参数

节头在运作时未欲采用, 运行时要全历程序头

    // 准备动态链接的信息
    std::uint64_t jmpRelAddr = 0; // 重定位记录的开始地址
    std::uint64_t pltRelType = 0; // 重定位记录的类型 RELA或REL
    std::uint64_t pltRelSize = 0; // 重定位记录的总大小
    std::uint64_t symTabAddr = 0; // 动态符号表的开始地址
    std::uint64_t strTabAddr = 0; // 动态符号名称表的开始地址
    std::uint64_t strTabSize = 0; // 动态符号名称表的总大小

    // 遍历program hedaer
    std::vector<Elf64_External_Phdr> programHeaders;
    programHeaders.resize(programTableEntryNum);
    fileStream_.read(reinterpret_cast<char*>(programHeaders.data()), programTableEntryNum * programTableEntrySize);
    std::vector<std::shared_ptr<void>> loadedSegments;
    for (const auto& programHeader : programHeaders) {
        std::uint32_t type = *reinterpret_cast<const std::uint32_t*>(programHeader.p_type);
        if (type == PT_LOAD) {
            // 把文件内容(包含程序代码和数据)加载到虚拟内存,这个示例不考虑地址冲突
            std::uint64_t fileOffset = *reinterpret_cast<const std::uint64_t*>(programHeader.p_offset);
            std::uint64_t fileSize = *reinterpret_cast<const std::uint64_t*>(programHeader.p_filesz);
            std::uint64_t virtAddr = *reinterpret_cast<const std::uint64_t*>(programHeader.p_vaddr);
            std::uint64_t memSize = *reinterpret_cast<const std::uint64_t*>(programHeader.p_memsz);
            if (memSize < fileSize) {
                throw std::runtime_error("invalid memsz in program header, it shouldn't less than filesz");
            }
            // 在指定的虚拟地址分配内存
            std::cout << std::hex << "allocate address at: 0x" << virtAddr <<
                " size: 0x" << memSize << std::dec << std::endl;
            void* addr = ::VirtualAlloc((void*)virtAddr, memSize, MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE);
            if (addr == nullptr) {
                throw std::runtime_error("allocate memory at specific address failed");
            }
            loadedSegments.emplace_back(addr, [](void* ptr) { ::VirtualFree(ptr, 0, MEM_RELEASE); });
            // 复制文件内容到虚拟内存
            fileStream_.seekg(fileOffset);
            if (!fileStream_.read(reinterpret_cast<char*>(addr), fileSize)) {
                throw std::runtime_error("read contents into memory from LOAD program header failed");
            }
        }
        else if (type == PT_DYNAMIC) {
            // 遍历动态节
            std::uint64_t fileOffset = *reinterpret_cast<const std::uint64_t*>(programHeader.p_offset);
            fileStream_.seekg(fileOffset);
            Elf64_External_Dyn dynSection = {};
            std::uint64_t dynSectionTag = 0;
            std::uint64_t dynSectionVal = 0;
            do {
                if (!fileStream_.read(reinterpret_cast<char*>(&dynSection), sizeof(dynSection))) {
                    throw std::runtime_error("read dynamic section failed");
                }
                dynSectionTag = *reinterpret_cast<const std::uint64_t*>(dynSection.d_tag);
                dynSectionVal = *reinterpret_cast<const std::uint64_t*>(dynSection.d_un.d_val);
                if (dynSectionTag == DT_JMPREL) {
                    jmpRelAddr = dynSectionVal;
                }
                else if (dynSectionTag == DT_PLTREL) {
                    pltRelType = dynSectionVal;
                }
                else if (dynSectionTag == DT_PLTRELSZ) {
                    pltRelSize = dynSectionVal;
                }
                else if (dynSectionTag == DT_SYMTAB) {
                    symTabAddr = dynSectionVal;
                }
                else if (dynSectionTag == DT_STRTAB) {
                    strTabAddr = dynSectionVal;
                }
                else if (dynSectionTag == DT_STRSZ) {
                    strTabSize = dynSectionVal;
                }
            } while (dynSectionTag != 0);
        }
    }

还记得我们地方运用readelf读取到之音为?

程序头:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000400040 0x0000000000400040
                 0x00000000000001f8 0x00000000000001f8  R E    8
  INTERP         0x0000000000000238 0x0000000000400238 0x0000000000400238
                 0x000000000000001c 0x000000000000001c  R      1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x00000000000007d4 0x00000000000007d4  R E    200000
  LOAD           0x0000000000000e10 0x0000000000600e10 0x0000000000600e10
                 0x0000000000000228 0x0000000000000230  RW     200000
  DYNAMIC        0x0000000000000e28 0x0000000000600e28 0x0000000000600e28
                 0x00000000000001d0 0x00000000000001d0  RW     8
  NOTE           0x0000000000000254 0x0000000000400254 0x0000000000400254
                 0x0000000000000044 0x0000000000000044  R      4
  GNU_EH_FRAME   0x0000000000000680 0x0000000000400680 0x0000000000400680
                 0x000000000000003c 0x000000000000003c  R      4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     10
  GNU_RELRO      0x0000000000000e10 0x0000000000600e10 0x0000000000600e10
                 0x00000000000001f0 0x00000000000001f0  R      1

立刻个中种是LOAD的条代表待加载文件的内容到内存,
Offset凡是文本之偏移值, VirtAddr凡虚拟内存地址,
FileSiz是索要加载的文件大小, MemSiz大凡亟需分配的内存大小,
Flags举凡内存的看权限,
本条示例不考虑访问权限(统一用PAGE_EXECUTE_READWRITE).

此顺序来一定量独LOAD头, 第一独带有了代码和特念数据(.data, .init,
.rodata等节之情节), 第二只包含了但写多少(.init_array,
.fini_array等省之始末).

LOAD头对应之情节加载到指定的内存地址后我们就是完事了构想中之第2个第3单步骤,
现在代码和数据都于内存中了.

连片下去我们尚亟需处理动态链接的函数,
处理所欲的信息可以打DYNAMIC头得到
DYNAMIC头包含的信发生

Dynamic section at offset 0xe28 contains 24 entries:
  标记        类型                         名称/值
 0x0000000000000001 (NEEDED)             共享库:[libc.so.6]
 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

一个个看上面代码中关系到之档次

  • DT_JMPREL: 重定位记录之开端地址,
    指向.rela.plt节在内存中保存的地方
  • DT_PLTREL: 重定位记录之门类 RELA或RE, 这里是RELAL
  • DT_PLTRELSZ: 重定位记录之到底大小, 这里是24 * 2 = 48

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
000000601018  000100000007 R_X86_64_JUMP_SLO 0000000000000000 printf@GLIBC_2.2.5 + 0
000000601020  000200000007 R_X86_64_JUMP_SLO 0000000000000000 __libc_start_main@GLIBC_2.2.5 + 0
  • DT_SYMTAB: 动态符号表的开地址,
    指向.dynsym节在内存中保存之地点
  • DT_STRTAB: 动态符号名称表的上马地址,
    指向.dynstr节在内存中保留之地方
  • DT_STRSZ: 动态符号名称表的总大小

Symbol table '.dynsym' contains 4 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND printf@GLIBC_2.2.5 (2)
     2: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.2.5 (2)
     3: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__

在遍历完程序头以后, 我们得以知道出星星点点个动态链接的函数需要更一贯,
它们分别是__libc_start_mainprintf,
其中__libc_start_main顶住调用main函数
紧接下吃我们用安装这些函数的地址

    // 读取动态链接符号表
    std::string dynamicSymbolNames(reinterpret_cast<char*>(strTabAddr), strTabSize);
    Elf64_External_Sym* dynamicSymbols = reinterpret_cast<Elf64_External_Sym*>(symTabAddr);

    // 设置动态链接的函数地址
    std::cout << std::hex << "read dynamic entires at: 0x" << jmpRelAddr <<
        " size: 0x" << pltRelSize << std::dec << std::endl;
    if (jmpRelAddr == 0 || pltRelType != DT_RELA || pltRelSize % sizeof(Elf64_External_Rela) != 0) {
        throw std::runtime_error("invalid dynamic entry info, rel type should be rela");
    }
    std::vector<std::shared_ptr<void>> libraryFuncs;
    for (std::uint64_t offset = 0; offset < pltRelSize; offset += sizeof(Elf64_External_Rela)) {
        Elf64_External_Rela* rela = (Elf64_External_Rela*)(jmpRelAddr + offset);
        std::uint64_t relaOffset = *reinterpret_cast<const std::uint64_t*>(rela->r_offset);
        std::uint64_t relaInfo = *reinterpret_cast<const std::uint64_t*>(rela->r_info);
        std::uint64_t relaSym = relaInfo >> 32; // ELF64_R_SYM
        std::uint64_t relaType = relaInfo & 0xffffffff; // ELF64_R_TYPE
        // 获取符号
        Elf64_External_Sym* symbol = dynamicSymbols + relaSym;
        std::uint32_t symbolNameOffset = *reinterpret_cast<std::uint32_t*>(symbol->st_name);
        std::string symbolName(dynamicSymbolNames.data() + symbolNameOffset);
        std::cout << "relocate symbol: " << symbolName << std::endl;
        // 替换函数地址
        // 原本应该延迟解决,这里图简单就直接覆盖了
        void** relaPtr = reinterpret_cast<void**>(relaOffset);
        std::shared_ptr<void> func = resolveLibraryFunc(symbolName);
        if (func == nullptr) {
            throw std::runtime_error("unsupport symbol name");
        }
        libraryFuncs.emplace_back(func);
        *relaPtr = func.get();
    }

地方的代码遍历了DT_JMPREL重定位记录,
并且在加载时设置了这些函数的地址,
实际当经过延迟解决实现的, 但是此处以简单就径直调换成最终的地方了.

点得到函数实际地址的逻辑本身形容到了resolveLibraryFunc中,这个函数的贯彻在另外一个文书,
如下

namespace HelloElfLoader {
    namespace {
        // 原始的返回地址
        thread_local void* originalReturnAddress = nullptr;

        void* getOriginalReturnAddress() {
            return originalReturnAddress;
        }

        void setOriginalReturnAddress(void* address) {
            originalReturnAddress = address;
        }

        // 模拟libc调用main的函数,目前不支持传入argc和argv
        void __libc_start_main(int(*main)()) {
            std::cout << "call main: " << main << std::endl;
            int ret = main();
            std::cout << "result: " << ret << std::endl;
            std::exit(0);
        }

        // 模拟printf函数
        int printf(const char* fmt, ...) {
            int ret;
            va_list myargs;
            va_start(myargs, fmt);
            ret = ::vprintf(fmt, myargs);
            va_end(myargs);
            return ret;
        }

        // 把System V AMD64 ABI转换为Microsoft x64 calling convention
        // 因为vc++不支持inline asm,只能直接写hex
        // 这个函数支持任意长度的参数,但是性能会有损耗,如果参数数量已知可以编写更快的loader代码   
        const char generic_func_loader[]{
            // 让参数连续排列在栈上
            // [第一个参数] [第二个参数] [第三个参数] ...
            0x58, // pop %rax 暂存原返回地址
            0x41, 0x51, // push %r9 入栈第六个参数,之后的参数都在后续的栈上
            0x41, 0x50, // push %r8 入栈第五个参数
            0x51, // push %rcx 入栈第四个参数
            0x52, // push %rdx 入栈第三个参数
            0x56, // push %rsi 入栈第二个参数
            0x57, // push %rdi 入栈第一个参数

            // 调用setOriginalReturnAddress保存原返回地址
            0x48, 0x89, 0xc1, // mov %rax, %rcx 第一个参数是原返回地址
            0x48, 0x83, 0xec, 0x20, // sub $0x20, %rsp 预留32位的影子空间
            0x48, 0xb8, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // movabs $0, %rax
            0xff, 0xd0, // callq *%rax 调用setOriginalReturnAddress
            0x48, 0x83, 0xc4, 0x20, // add %0x20, %rsp 释放影子空间

            // 转换到Microsoft x64 calling convention
            0x59, // pop %rcx 出栈第一个参数
            0x5a, // pop %rdx 出栈第二个参数
            0x41, 0x58, // pop %r8 // 出栈第三个参数
            0x41, 0x59, // pop %r9 // 出栈第四个参数

            // 调用目标函数
            0x48, 0x83, 0xec, 0x20, // sub $0x20, %esp 预留32位的影子空间
            0x48, 0xb8, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // movabs 0, %rax
            0xff, 0xd0, // callq *%rax 调用模拟的函数
            0x48, 0x83, 0xc4, 0x30, // add $0x30, %rsp 释放影子空间和参数(影子空间32 + 参数8*2)
            0x50, // push %rax 保存返回值

            // 调用getOriginalReturnAddress获取原返回地址
            0x48, 0xb8, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // movabs $0, %rax
            0xff, 0xd0, // callq *%rax 调用getOriginalReturnAddress
            0x48, 0x89, 0xc1, // mov %rax, %rcx 原返回地址存到rcx
            0x58, // 恢复返回值
            0x51, // 原返回地址入栈顶
            0xc3 // 返回
        };
        const int generic_func_loader_set_addr_offset = 18;
        const int generic_func_loader_target_offset = 44;
        const int generic_func_loader_get_addr_offset = 61;
    }

    // 获取动态链接函数的调用地址
    std::shared_ptr<void> resolveLibraryFunc(const std::string& name) {
        void* funcPtr = nullptr;
        if (name == "__libc_start_main") {
            funcPtr = __libc_start_main;
        }
        else if (name == "printf") {
            funcPtr = printf;
        }
        else {
            return nullptr;
        }
        void* addr = ::VirtualAlloc(nullptr,
            sizeof(generic_func_loader), MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE);
        if (addr == nullptr) {
            throw std::runtime_error("allocate memory for _libc_start_main_loader failed");
        }
        std::shared_ptr<void> result(addr, [](void* ptr) { ::VirtualFree(ptr, 0, MEM_RELEASE); });
        std::memcpy(addr, generic_func_loader, sizeof(generic_func_loader));
        char* addr_c = reinterpret_cast<char*>(addr);
        *reinterpret_cast<void**>(addr_c + generic_func_loader_set_addr_offset) = setOriginalReturnAddress;
        *reinterpret_cast<void**>(addr_c + generic_func_loader_target_offset) = funcPtr;
        *reinterpret_cast<void**>(addr_c + generic_func_loader_get_addr_offset) = getOriginalReturnAddress;
        return result;
    }
}

略知一二这段代码需要先了解什么是x86 calling
conventions,
在汇编中传递函数参数的法子由特别多种,
cdecl大凡把有参数还坐落栈中从低至高排列,
fastcall凡是管第一独参数放ecx, 第二个参数放edx, 其余参数放栈中.

俺们要效法的64各Linux程序,它传参使用了System V AMD64 ABI正式,
先把参数按RDI, RSI, RDX, RCX, R8, R9的依次设置,如果产生还多参数就厕栈中.
比方64号的Windows传参使用了Microsoft x64 calling convention业内,
先把参数按RCX, RDX, R8, R9的一一设置,如果出双重多参数就厕栈中,
除此之外还亟需留一个32字节的影空间.
倘我们用让Linux程序调用Windows程序中之函数,
需要针对参数的逐一进行转换, 这就是地方的汇编代码所开的事情.

换前之堆栈结构如下

[原返回地址 8bytes] [第七个参数] [第八个参数] ...

易后的库结构如下

[返回地址 8bytes] [影子空间 32 bytes] [第五个参数] [第六个参数] [第七个参数] ...

为急需支持不肯定个数的参数,
上面的代码用了一个thread local变量来保存原归地址,
这样的处理会潜移默化性, 如果函数的参数个数已掌握可以变换成重速之转换代码.

当安好动态链接的函数地址后, 我们完成了构想中之第4步,
接下来就足以运行主程序了

    // 获取入口点
    std::uint64_t entryPointAddress = *reinterpret_cast<const std::uint64_t*>(elfHeader.e_entry);
    void(*entryPointFunc)() = reinterpret_cast<void(*)()>(entryPointAddress);
    std::cout << "entry point: " << entryPointFunc << std::endl;
    std::cout << "====== finish loading elf ======" << std::endl;

    // 执行主程序
    // 会先调用__libc_start_main, 然后再调用main
    // 调用__libc_start_main后的指令是hlt,所以必须在__libc_start_main中退出执行
    entryPointFunc();

入口点的地点以ELF头中得收获到,这个地址便是_start函数的地方,
我们管其换成为一个void()种的函数指针再实践即可,
由来示例程序完成了构想中之有着功能.

施行效果使下图

图片 3

立卖演示程序还有许多欠缺, 例如非支持32员Linux程序,
不支持加载其他Linux动态链接库(so), 不支持命令执行参数等等.
以这卖演示程序和Bash On Windows的规律有所出入,
因为以用户层是心有余而力不足模拟syscall.
自家愿意她可以为你针对如何运行其他系统的可执行文件有一个方始的问询,
如果你望再深刻之垂询什么模拟syscall,
可以搜索rdmsrwrmsr命令相关的资料.

末段附上自在编辑这卖演示程序中查看的链接:

  • https://en.wikipedia.org/wiki/Executable_and_Linkable_Format
  • https://en.wikipedia.org/wiki/X86_calling_conventions
  • http://refspecs.linuxbase.org/elf/elf.pdf
  • https://github.com/aeste/binutils/blob/develop/elfcpp/elfcpp.h
  • https://github.com/aeste/binutils/blob/develop/include/elf/external.h

纠错(2017-10-28), 用户层通过vsyscall机制是可以如法炮制syscall的.