Linux内核无法以区的概念来识别可执行文件。内核使用包括连续页的VMA（virtual memory area）来识别进程。
    在每个VMA中可能映射了一个或多个区。每个VMA代表一个ELF文件的段。
    那么，内核如何知道哪个区属于某个VMA（段）？映射关系保存在Program Header Table（PHT）中
    readelf -S test

    1. There are 30 section headers, starting at offset 0xa88:
    3. Section Headers:
    4.   [Nr] Name              Type             Address           Offset
    5.        Size              EntSize          Flags  Link  Info  Align
    6.   [ 0]                   NULL             0000000000000000  00000000
    7.        0000000000000000  0000000000000000           0     0     0
    8.   [ 1] .interp           PROGBITS         0000000000400200  00000200
    9.        000000000000001c  0000000000000000   A       0     0     1
    10.   [ 2] .note.ABI-tag     NOTE             000000000040021c  0000021c
    11.        0000000000000020  0000000000000000   A       0     0     4
    12.   [ 3] .note.gnu.build-i NOTE             000000000040023c  0000023c
    13.        0000000000000024  0000000000000000   A       0     0     4
    14.   [ 4] .gnu.hash         GNU_HASH         0000000000400260  00000260
    15.        000000000000001c  0000000000000000   A       5     0     8
    16.   [ 5] .dynsym           DYNSYM           0000000000400280  00000280
    17.        0000000000000078  0000000000000018   A       6     1     8
    18.   [ 6] .dynstr           STRTAB           00000000004002f8  000002f8
    19.        0000000000000044  0000000000000000   A       0     0     1
    20.   [ 7] .gnu.version      VERSYM           000000000040033c  0000033c
    21.        000000000000000a  0000000000000002   A       5     0     2
    22.   [ 8] .gnu.version_r    VERNEED          0000000000400348  00000348
    23.        0000000000000020  0000000000000000   A       6     1     8
    24.   [ 9] .rela.dyn         RELA             0000000000400368  00000368
    25.        0000000000000018  0000000000000018   A       5     0     8
    26.   [10] .rela.plt         RELA             0000000000400380  00000380
    27.        0000000000000048  0000000000000018   A       5    12     8
    28.   [11] .init             PROGBITS         00000000004003c8  000003c8
    29.        0000000000000018  0000000000000000  AX       0     0     4
    30.   [12] .plt              PROGBITS         00000000004003e0  000003e0
    31.        0000000000000040  0000000000000010  AX       0     0     4
    32.   [13] .text             PROGBITS         0000000000400420  00000420
    33.        0000000000000238  0000000000000000  AX       0     0     16
    34.   [14] .fini             PROGBITS         0000000000400658  00000658
    35.        000000000000000e  0000000000000000  AX       0     0     4
    36.   [15] .rodata           PROGBITS         0000000000400668  00000668
    37.        0000000000000053  0000000000000000   A       0     0     8
    38.   [16] .eh_frame_hdr     PROGBITS         00000000004006bc  000006bc
    39.        0000000000000024  0000000000000000   A       0     0     4
    40.   [17] .eh_frame         PROGBITS         00000000004006e0  000006e0
    41.        000000000000007c  0000000000000000   A       0     0     8
    42.   [18] .ctors            PROGBITS         0000000000600760  00000760
    43.        0000000000000010  0000000000000000  WA       0     0     8
    44.   [19] .dtors            PROGBITS         0000000000600770  00000770
    45.        0000000000000010  0000000000000000  WA       0     0     8
    46.   [20] .jcr              PROGBITS         0000000000600780  00000780
    47.        0000000000000008  0000000000000000  WA       0     0     8
    48.   [21] .dynamic          DYNAMIC          0000000000600788  00000788
    49.        0000000000000190  0000000000000010  WA       6     0     8
    50.   [22] .got              PROGBITS         0000000000600918  00000918
    51.        0000000000000008  0000000000000008  WA       0     0     8
    52.   [23] .got.plt          PROGBITS         0000000000600920  00000920
    53.        0000000000000030  0000000000000008  WA       0     0     8
    54.   [24] .data             PROGBITS         0000000000600950  00000950
    55.        0000000000000008  0000000000000000  WA       0     0     4
    56.   [25] .bss              NOBITS           0000000000600958  00000958
    57.        0000000000000018  0000000000000000  WA       0     0     8
    58.   [26] .comment          PROGBITS         0000000000000000  00000958
    59.        000000000000002c  0000000000000001  MS       0     0     1
    60.   [27] .shstrtab         STRTAB           0000000000000000  00000984
    61.        00000000000000fe  0000000000000000           0     0     1
    62.   [28] .symtab           SYMTAB           0000000000000000  00001208
    63.        0000000000000648  0000000000000018          29    46     8
    64.   [29] .strtab           STRTAB           0000000000000000  00001850
    65.        000000000000021e  0000000000000000           0     0     1
    66. Key to Flags:
    67.   W (write), A (alloc), X (execute), M (merge), S (strings)
    68.   I (info), L (link order), G (group), x (unknown)
    69.   O (extra OS processing required) o (OS specific), p (processor specific)

    .text区存储的是程序的代码（二进制指令），该区的标志为X表示可执行。
    如上所示，段有多种类型，下面介绍LOAD类型
    LOAD：该段的内容从可执行文件中获取。Offset标识内核从文件读取的位置。FileSiz标识读取多少字节。