Linux系统的机器上查看MBR数据 (续)

来源：互联网发布：数控机床与编程教程pdf 编辑：程序博客网时间：2024/06/09 20:37

1. 概述

在 Linux系统的机器上查看MBR数据一文中，通过命令的方式给出了Linux下面MBR的数据示例，其中对MBR中每个数据结构的读取&解析也是人工方式的。本文继续该内容，但通过代码自动分析MBR各个字段的内容。

本文编码所在的环境和上篇不同，因此MBR数据和上文存在差异。

2. 迭代1：读MBR所在第一个扇区的数据

本文采用敏捷开发的模式，一步步给出问题的最终解决方案。

作为第一步，要能够读取到硬盘的第一个扇区，即MBR数据。至于数据解析，我们留到下一个迭代。

2.1 代码

下面是读取MBR的代码：

#include <stdio.h>#include <sys/types.h>#include <sys/stat.h>#include <fcntl.h>#include <unistd.h>int main() {int fd = -1;const unsigned int SECTOR_SIZE = 512;unsigned char buf[SECTOR_SIZE];ssize_t read_count;const char* DEV_NAME = "/dev/sda";fd = open(DEV_NAME, O_RDONLY);if (fd == -1) {printf("Open file failed.\n");return -1;}read_count = read(fd, buf, SECTOR_SIZE);if (read_count != SECTOR_SIZE) {printf("Read error: %d\n", read_count);close(fd);return -1;}printf("\n\n Addr  00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F\n");for (ssize_t i = 0; i < SECTOR_SIZE; i++) {if (i % 16 == 0) {printf("\n0x%04X ", i);}printf("%02X ", buf[i]);}printf("\n");close(fd);return 0;}

2.2 运行

flying-bird@flyingbird:~/examples/cpp/read_mbr$ gcc -std=c99 main.cflying-bird@flyingbird:~/examples/cpp/read_mbr$ sudo ./a.out  Addr  00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F0x0000 EB 63 90 00 02 8E D7 BC 00 7A BB A0 07 8B CE 8E 0x0010 DB 8E C3 F3 A4 EA 5D 00 A0 07 10 00 01 00 00 7A 0x0020 00 00 00 00 00 00 00 00 00 00 07 66 8B 55 08 B4 0x0030 42 C6 06 1F 00 7C 32 C0 66 89 16 22 00 BE 1A 00 0x0040 B2 80 CD 13 0F 82 CD 00 81 3E FE 03 55 AA C3 AC 0x0050 0A C0 74 FA B4 0E BB 07 00 CD 00 80 01 00 00 00 0x0060 00 00 00 00 FF FA 90 90 F6 C2 80 74 05 F6 C2 70 0x0070 74 02 B2 80 EA 79 7C 00 00 31 C0 8E D8 8E D0 BC 0x0080 00 20 FB A0 64 7C 3C FF 74 02 88 C2 52 BB 17 04 0x0090 80 27 03 74 06 BE 88 7D E8 17 01 BE 05 7C B4 41 0x00A0 BB AA 55 CD 13 5A 52 72 3D 81 FB 55 AA 75 37 83 0x00B0 E1 01 74 32 31 C0 89 44 04 40 88 44 FF 89 44 02 0x00C0 C7 04 10 00 66 8B 1E 5C 7C 66 89 5C 08 66 8B 1E 0x00D0 60 7C 66 89 5C 0C C7 44 06 00 70 B4 42 CD 13 72 0x00E0 05 BB 00 70 EB 76 B4 08 CD 13 73 0D F6 C2 80 0F 0x00F0 84 D0 00 BE 93 7D E9 82 00 66 0F B6 C6 88 64 FF 0x0100 40 66 89 44 04 0F B6 D1 C1 E2 02 88 E8 88 F4 40 0x0110 89 44 08 0F B6 C2 C0 E8 02 66 89 04 66 A1 60 7C 0x0120 66 09 C0 75 4E 66 A1 5C 7C 66 31 D2 66 F7 34 88 0x0130 D1 31 D2 66 F7 74 04 3B 44 08 7D 37 FE C1 88 C5 0x0140 30 C0 C1 E8 02 08 C1 88 D0 5A 88 C6 BB 00 70 8E 0x0150 C3 31 DB B8 01 02 CD 13 72 1E 8C C3 60 1E B9 00 0x0160 01 8E DB 31 F6 BF 00 80 8E C6 FC F3 A5 1F 61 FF 0x0170 26 5A 7C BE 8E 7D EB 03 BE 9D 7D E8 34 00 BE A2 0x0180 7D E8 2E 00 CD 18 EB FE 47 52 55 42 20 00 47 65 0x0190 6F 6D 00 48 61 72 64 20 44 69 73 6B 00 52 65 61 0x01A0 64 00 20 45 72 72 6F 72 0D 0A 00 BB 01 00 B4 0E 0x01B0 CD 10 AC 3C 00 75 F4 C3 32 F2 49 15 00 00 80 01 0x01C0 01 00 07 EF FF FF 3F 00 00 00 D0 88 81 1D 00 FE 0x01D0 FF FF 05 FE FF FF FE 8F 81 1D 02 60 07 1C 00 EF 0x01E0 FF FF 07 EF FF FF 10 F1 88 39 80 57 AF 00 00 00 0x01F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 AA flying-bird@flyingbird:~/examples/cpp/read_mbr$

初步地分析这个码流，我们知道已成功读取MBR的数据，比如最后的55 AA；再倒数的每16个字节一组的MBR Partition Record。进一步地，可以用上一节的命令做对比：

flying-bird@flyingbird:~/examples/cpp/read_mbr$ sudo dd if=/dev/sda ibs=512 count=1 | hexdump -C00000000  eb 63 90 00 02 8e d7 bc  00 7a bb a0 07 8b ce 8e  |.c.......z......|00000010  db 8e c3 f3 a4 ea 5d 00  a0 07 10 00 01 00 00 7a  |......]........z|00000020  00 00 00 00 00 00 00 00  00 00 07 66 8b 55 08 b4  |...........f.U..|00000030  42 c6 06 1f 00 7c 32 c0  66 89 16 22 00 be 1a 00  |B....|2.f.."....|00000040  b2 80 cd 13 0f 82 cd 00  81 3e fe 03 55 aa c3 ac  |.........>..U...|00000050  0a c0 74 fa b4 0e bb 07  00 cd 00 80 01 00 00 00  |..t.............|00000060  00 00 00 00 ff fa 90 90  f6 c2 80 74 05 f6 c2 70  |...........t...p|00000070  74 02 b2 80 ea 79 7c 00  00 31 c0 8e d8 8e d0 bc  |t....y|..1......|00000080  00 20 fb a0 64 7c 3c ff  74 02 88 c2 52 bb 17 04  |. ..d|<.t...R...|00000090  80 27 03 74 06 be 88 7d  e8 17 01 be 05 7c b4 41  |.'.t...}.....|.A|000000a0  bb aa 55 cd 13 5a 52 72  3d 81 fb 55 aa 75 37 83  |..U..ZRr=..U.u7.|000000b0  e1 01 74 32 31 c0 89 44  04 40 88 44 ff 89 44 02  |..t21..D.@.D..D.|000000c0  c7 04 10 00 66 8b 1e 5c  7c 66 89 5c 08 66 8b 1e  |....f..\|f.\.f..|000000d0  60 7c 66 89 5c 0c c7 44  06 00 70 b4 42 cd 13 72  |`|f.\..D..p.B..r|000000e0  05 bb 00 70 eb 76 b4 08  cd 13 73 0d f6 c2 80 0f  |...p.v....s.....|000000f0  84 d0 00 be 93 7d e9 82  00 66 0f b6 c6 88 64 ff  |.....}...f....d.|00000100  40 66 89 44 04 0f b6 d1  c1 e2 02 88 e8 88 f4 40  |@f.D...........@|00000110  89 44 08 0f b6 c2 c0 e8  02 66 89 04 66 a1 60 7c  |.D.......f..f.`||00000120  66 09 c0 75 4e 66 a1 5c  7c 66 31 d2 66 f7 34 88  |f..uNf.\|f1.f.4.|00000130  d1 31 d2 66 f7 74 04 3b  44 08 7d 37 fe c1 88 c5  |.1.f.t.;D.}7....|00000140  30 c0 c1 e8 02 08 c1 88  d0 5a 88 c6 bb 00 70 8e  |0........Z....p.|00000150  c3 31 db b8 01 02 cd 13  72 1e 8c c3 60 1e b9 00  |.1......r...`...|00000160  01 8e db 31 f6 bf 00 80  8e c6 fc f3 a5 1f 61 ff  |...1..........a.|00000170  26 5a 7c be 8e 7d eb 03  be 9d 7d e8 34 00 be a2  |&Z|..}....}.4...|00000180  7d e8 2e 00 cd 18 eb fe  47 52 55 42 20 00 47 65  |}.......GRUB .Ge|00000190  6f 6d 00 48 61 72 64 20  44 69 73 6b 00 52 65 61  |om.Hard Disk.Rea|000001a0  64 00 20 45 72 72 6f 72  0d 0a 00 bb 01 00 b4 0e  |d. Error........|000001b0  cd 10 ac 3c 00 75 f4 c3  32 f2 49 15 00 00 80 01  |...<.u..2.I.....|000001c0  01 00 07 ef ff ff 3f 00  00 00 d0 88 81 1d 00 fe  |......?.........|000001d0  ff ff 05 fe ff ff fe 8f  81 1d 02 60 07 1c 00 ef  |...........`....|000001e0  ff ff 07 ef ff ff 10 f1  88 39 80 57 af 00 00 00  |.........9.W....|000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|记录了1+0 的读入记录了1+0 的写出512字节(512 B)已复制，0.00250075 秒，205 kB/秒00000200flying-bird@flyingbird:~/examples/cpp/read_mbr$

2.3 几个问题分析

1. 在上面的代码中，有一大段是输出码流的代码，而且这个功能也相对独立。因此需要把这一部分代码重构到一个独立的函数中。

2. 再者，对设备文件是硬编码的，因此如果能够参数化，则更加灵活。再进一步，如果能够自动识别MBR所在的设备名称，代码就更具有可移植性。

3. MBR中每一个组成部分还没有自动分析出来，因此需要增加结构化分析。

接下来就是通过每一个迭代来实现上述功能。

3. 迭代2：领域无关代码的重构

这一步，我们把和读取&解析MBR无关的代码分离出去，也就是把输出二进制码流部分的代码利用Extract Method手法提取出去。

另外从现在开始，我们从C切换到C++。此时代码包括如下几个文件：

main.cpp：领域代码；
file-utils.h：提取出来的打印二进制码流的实用小函数，头文件；
file-tuils.cpp：使用小函数的实现文件。

3.1 main.cpp

#include <cstdio>#include <sys/types.h>#include <sys/stat.h>#include <fcntl.h>#include <unistd.h>#include <file-utils.h>int main() {    int fd = -1;    const unsigned int SECTOR_SIZE = 512;    unsigned char buf[SECTOR_SIZE];    ssize_t read_count;    const char* DEV_NAME = "/dev/sda";    fd = open(DEV_NAME, O_RDONLY);    if (fd == -1) {        printf("Open file failed.\n");        return -1;    }    read_count = read(fd, buf, SECTOR_SIZE);    if (read_count != SECTOR_SIZE) {        printf("Read error: %d\n", read_count);        close(fd);        return -1;    }    dump_binary(buf, SECTOR_SIZE);    close(fd);    return 0;}

3.2 file-utils.h

/* * File utilities. *  */#ifndef __FILE_UTILS_H#define __FILE_UTILS_H/* * Dump the binary stream. * 16 bytes each line. */void dump_binary(unsigned char *buf, unsigned int len);#endif //__FILE_UTILS_H

3.3 file-utils.cpp

#include <file-utils.h>#include <cstdio>void dump_binary(unsigned char *buf, unsigned int len) {    printf("\n\n Addr  00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F\n");    for (unsigned int i = 0; i < len; i++) {        if (i % 16 == 0) {            printf("\n0x%04X ", i);        }        printf("%02X ", buf[i]);    }    printf("\n\n");}

3.4 编译&运行

flying-bird@flyingbird:~/examples/cpp/read_mbr$ g++ main.cpp file-utils.cpp -I.flying-bird@flyingbird:~/examples/cpp/read_mbr$ sudo ./a.out  Addr  00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F0x0000 EB 63 90 00 02 8E D7 BC 00 7A BB A0 07 8B CE 8E 0x0010 DB 8E C3 F3 A4 EA 5D 00 A0 07 10 00 01 00 00 7A 0x0020 00 00 00 00 00 00 00 00 00 00 07 66 8B 55 08 B4 0x0030 42 C6 06 1F 00 7C 32 C0 66 89 16 22 00 BE 1A 00 0x0040 B2 80 CD 13 0F 82 CD 00 81 3E FE 03 55 AA C3 AC 0x0050 0A C0 74 FA B4 0E BB 07 00 CD 00 80 01 00 00 00 0x0060 00 00 00 00 FF FA 90 90 F6 C2 80 74 05 F6 C2 70 0x0070 74 02 B2 80 EA 79 7C 00 00 31 C0 8E D8 8E D0 BC 0x0080 00 20 FB A0 64 7C 3C FF 74 02 88 C2 52 BB 17 04 0x0090 80 27 03 74 06 BE 88 7D E8 17 01 BE 05 7C B4 41 0x00A0 BB AA 55 CD 13 5A 52 72 3D 81 FB 55 AA 75 37 83 0x00B0 E1 01 74 32 31 C0 89 44 04 40 88 44 FF 89 44 02 0x00C0 C7 04 10 00 66 8B 1E 5C 7C 66 89 5C 08 66 8B 1E 0x00D0 60 7C 66 89 5C 0C C7 44 06 00 70 B4 42 CD 13 72 0x00E0 05 BB 00 70 EB 76 B4 08 CD 13 73 0D F6 C2 80 0F 0x00F0 84 D0 00 BE 93 7D E9 82 00 66 0F B6 C6 88 64 FF 0x0100 40 66 89 44 04 0F B6 D1 C1 E2 02 88 E8 88 F4 40 0x0110 89 44 08 0F B6 C2 C0 E8 02 66 89 04 66 A1 60 7C 0x0120 66 09 C0 75 4E 66 A1 5C 7C 66 31 D2 66 F7 34 88 0x0130 D1 31 D2 66 F7 74 04 3B 44 08 7D 37 FE C1 88 C5 0x0140 30 C0 C1 E8 02 08 C1 88 D0 5A 88 C6 BB 00 70 8E 0x0150 C3 31 DB B8 01 02 CD 13 72 1E 8C C3 60 1E B9 00 0x0160 01 8E DB 31 F6 BF 00 80 8E C6 FC F3 A5 1F 61 FF 0x0170 26 5A 7C BE 8E 7D EB 03 BE 9D 7D E8 34 00 BE A2 0x0180 7D E8 2E 00 CD 18 EB FE 47 52 55 42 20 00 47 65 0x0190 6F 6D 00 48 61 72 64 20 44 69 73 6B 00 52 65 61 0x01A0 64 00 20 45 72 72 6F 72 0D 0A 00 BB 01 00 B4 0E 0x01B0 CD 10 AC 3C 00 75 F4 C3 32 F2 49 15 00 00 80 01 0x01C0 01 00 07 EF FF FF 3F 00 00 00 D0 88 81 1D 00 FE 0x01D0 FF FF 05 FE FF FF FE 8F 81 1D 02 60 07 1C 00 EF 0x01E0 FF FF 07 EF FF FF 10 F1 88 39 80 57 AF 00 00 00 0x01F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 AA flying-bird@flyingbird:~/examples/cpp/read_mbr$

4. 迭代3：MBR领域建模

前面的演示，仅打印了第一个扇区的码流，具体内容是我们根据MBR格式，去查找对应位置的二进制数据。接下来我们对MBR建模，即对MBR所在第一个扇区的内容重构到对应的数据结构中。

此时新增了mbr.h,.cpp，并对main.cpp进行了重构。同时，我们重用了 Windows上面读磁盘分区表一文中的代码。

4.1 mbr.h

#ifndef __MBR_H#define __MBR_Hstruct CHS {      unsigned int header;      unsigned int sector;       unsigned int cylinder;  };  struct partition_record {      unsigned char boot_indicator;      unsigned char os_type;      struct CHS starting_chs;      struct CHS ending_chs;      unsigned int starting_LBA;      unsigned int size_in_LBA;  };  const unsigned int BOOT_LOADER_CODE_SIZE = 446;struct mbr {    unsigned char boot_loader_code[BOOT_LOADER_CODE_SIZE];    struct partition_record partition_records[4];};/*buffer: the 512-bytes of first sector.        The caller ensure that it is the right data.return: 0 - SUCCESS.        -1: invalid mbr, i.e. the signature is not 0xaa55.*/int get_mbr(unsigned char buffer[], struct mbr &mbr);#endif //__MBR_H

4.2 mbr.cpp

#include "mbr.h"#include <cstdio>#include <cstring> //memcpy#include "file-utils.h"static void dump_partition_record(const partition_record& record) {      const double DISK_SECTOR_SIZE = 512.0;      const size_t GB = 1024 * 1024 * 1024;        printf(          "\tboot indicator: 0x%02x\n"          "\tOS Type: 0x%02x\n"          "\tStarting CHS: (%d, %d, %d)\n"          "\tEnding_CHS: (%d, %d, %d)\n"          "\tSize in LBA: %d\n"          "\tSize:%.2f\n\n",          record.boot_indicator,           record.os_type,           record.starting_chs.cylinder,          record.starting_chs.header,          record.starting_chs.sector,          record.ending_chs.cylinder,          record.ending_chs.header,          record.ending_chs.sector,          record.size_in_LBA,          record.size_in_LBA * DISK_SECTOR_SIZE / GB);  }    // transform the 16-bytes buffer to partition record  static void get_partition_record(unsigned char buffer[], struct partition_record& record) {      record.boot_indicator = buffer[0];        record.starting_chs.header = buffer[1];      record.starting_chs.sector = buffer[2] & 0x3f;      record.starting_chs.cylinder = (((buffer[2] & 0xff) >> 6) << 8) + buffer[3];        record.os_type = buffer[4];        record.ending_chs.header = buffer[5];      record.ending_chs.sector = buffer[6] & 0x3f;      record.ending_chs.cylinder = (((buffer[6] & 0xff) >> 6) << 8) + buffer[7];        record.size_in_LBA = *(unsigned int*)(buffer + 12);  }int get_mbr(unsigned char buffer[], struct mbr &mbr) {    // check the signature, it must be 0xaa55    const unsigned int SIGNATURE_OFFSET = 510;    if (buffer[SIGNATURE_OFFSET] != 0x55 && buffer[SIGNATURE_OFFSET + 1] != 0xaa) {        printf("ERROR: invalid mbr!\n");        return -1;    }    memcpy(mbr.boot_loader_code, buffer, BOOT_LOADER_CODE_SIZE);    printf("boot loader:");    dump_binary(mbr.boot_loader_code, BOOT_LOADER_CODE_SIZE);    unsigned char *partition_record = buffer + BOOT_LOADER_CODE_SIZE;    const unsigned int PARTITION_RECORD_SIZE = 16;    for (unsigned int i = 0; i < 4; i++) {      get_partition_record(            buffer + BOOT_LOADER_CODE_SIZE + i * PARTITION_RECORD_SIZE,             mbr.partition_records[0]);        dump_partition_record(mbr.partition_records[0]);      }    return 0;}

4.3 main.cpp

#include <cstdio>#include <sys/types.h>#include <sys/stat.h>#include <fcntl.h>#include <unistd.h>#include "mbr.h"int main() {    int fd = -1;    const unsigned int SECTOR_SIZE = 512;    unsigned char buf[SECTOR_SIZE];    ssize_t read_count;    const char* DEV_NAME = "/dev/sda";    fd = open(DEV_NAME, O_RDONLY);    if (fd == -1) {        printf("Open file failed.\n");        return -1;    }    read_count = read(fd, buf, SECTOR_SIZE);    if (read_count != SECTOR_SIZE) {        printf("Read error: %d\n", read_count);        close(fd);        return -1;    }    struct mbr mbr = {0};    get_mbr(buf, mbr);    close(fd);    return 0;}

4.4 运行结果

flying-bird@flyingbird:~/examples/cpp/read_mbr$ g++ main.cpp file-utils.cpp mbr.cpp -I.flying-bird@flyingbird:~/examples/cpp/read_mbr$ sudo ./a.out boot loader: Addr  00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F0x0000 EB 63 90 00 02 8E D7 BC 00 7A BB A0 07 8B CE 8E 0x0010 DB 8E C3 F3 A4 EA 5D 00 A0 07 10 00 01 00 00 7A 0x0020 00 00 00 00 00 00 00 00 00 00 07 66 8B 55 08 B4 0x0030 42 C6 06 1F 00 7C 32 C0 66 89 16 22 00 BE 1A 00 0x0040 B2 80 CD 13 0F 82 CD 00 81 3E FE 03 55 AA C3 AC 0x0050 0A C0 74 FA B4 0E BB 07 00 CD 00 80 01 00 00 00 0x0060 00 00 00 00 FF FA 90 90 F6 C2 80 74 05 F6 C2 70 0x0070 74 02 B2 80 EA 79 7C 00 00 31 C0 8E D8 8E D0 BC 0x0080 00 20 FB A0 64 7C 3C FF 74 02 88 C2 52 BB 17 04 0x0090 80 27 03 74 06 BE 88 7D E8 17 01 BE 05 7C B4 41 0x00A0 BB AA 55 CD 13 5A 52 72 3D 81 FB 55 AA 75 37 83 0x00B0 E1 01 74 32 31 C0 89 44 04 40 88 44 FF 89 44 02 0x00C0 C7 04 10 00 66 8B 1E 5C 7C 66 89 5C 08 66 8B 1E 0x00D0 60 7C 66 89 5C 0C C7 44 06 00 70 B4 42 CD 13 72 0x00E0 05 BB 00 70 EB 76 B4 08 CD 13 73 0D F6 C2 80 0F 0x00F0 84 D0 00 BE 93 7D E9 82 00 66 0F B6 C6 88 64 FF 0x0100 40 66 89 44 04 0F B6 D1 C1 E2 02 88 E8 88 F4 40 0x0110 89 44 08 0F B6 C2 C0 E8 02 66 89 04 66 A1 60 7C 0x0120 66 09 C0 75 4E 66 A1 5C 7C 66 31 D2 66 F7 34 88 0x0130 D1 31 D2 66 F7 74 04 3B 44 08 7D 37 FE C1 88 C5 0x0140 30 C0 C1 E8 02 08 C1 88 D0 5A 88 C6 BB 00 70 8E 0x0150 C3 31 DB B8 01 02 CD 13 72 1E 8C C3 60 1E B9 00 0x0160 01 8E DB 31 F6 BF 00 80 8E C6 FC F3 A5 1F 61 FF 0x0170 26 5A 7C BE 8E 7D EB 03 BE 9D 7D E8 34 00 BE A2 0x0180 7D E8 2E 00 CD 18 EB FE 47 52 55 42 20 00 47 65 0x0190 6F 6D 00 48 61 72 64 20 44 69 73 6B 00 52 65 61 0x01A0 64 00 20 45 72 72 6F 72 0D 0A 00 BB 01 00 B4 0E 0x01B0 CD 10 AC 3C 00 75 F4 C3 32 F2 49 15 00 00 boot indicator: 0x80OS Type: 0x07Starting CHS: (0, 1, 1)Ending_CHS: (1023, 239, 63)Size in LBA: 495028432Size:236.05boot indicator: 0x00OS Type: 0x05Starting CHS: (1023, 254, 63)Ending_CHS: (1023, 254, 63)Size in LBA: 470245378Size:224.23boot indicator: 0x00OS Type: 0x07Starting CHS: (1023, 239, 63)Ending_CHS: (1023, 239, 63)Size in LBA: 11491200Size:5.48boot indicator: 0x00OS Type: 0x00Starting CHS: (0, 0, 0)Ending_CHS: (0, 0, 0)Size in LBA: 0Size:0.00flying-bird@flyingbird:~/examples/cpp/read_mbr$

可以看到，和 Windows上面读磁盘分区表一文的partition record数据一样，但boot loader不一样。这是因为在windows系统上，后来装成了双系统，所以更新了boot loader。

5. 迭代4：自动分析设备名称

前面的代码中，设备名称是hard code成/dev/sda。但不同的环境可能并不都是这个设备名称，比如双硬盘的情况，还可能MBR在sdb上面。为此，可否自动分析出MBR所在设备名称呢？接下来就实现这个功能。

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