作为一个高视角的科普吧，动态链接在装载过程中做了非常复杂的操作，还需要深入学习。
关于分段和分页
核心思想就是虚拟化，如何共用同一块内存，如何实现物理内存的复用，MMU和交换空间的发展使之一步一步成为可能，至于如何进行换页，如何进行映射这是OS该管的事，OS使用MMU提供的接口实现一些列换页算法，如LRU等等，装载器的实现的核心代码也是定义在内核中的，主要分为以下几步：
按我的理解就是，根据elf的各个段的大小以及类型等各个字段，来建立虚拟内存和文件系统以及物理内存之间的映射关系（俗称页表）。当然进程管理系统，也应该做事，这里只讨论文件装载。决定装载的映射关系的一个重要的地方就是内部碎片和外部碎片，感觉这两个非常矛盾，需要trade-off。段页式管理，多级页表虽然缓解了外部碎片，但是同样加剧了内部碎片，使本应该不占一个页的段，占据一个页空间。感觉这里面极度需要平衡，至于该如何装载那就是OS的事了。但是装载之前elf也是做了一些工作，可执行文件有一个叫程序头的东西，俗称段表，貌似也可以对装载的过程一窥一二。
其中描述了各个段的信息，大概就是描述了什么文件偏移该映射到什么地方，但是程序是有pie这个选项的，不知道内核是如何处理的。，可以观察到段表比节表少了好多，段表会将部分节表进行合并导致的。
typedef struct
{
Elf64_Word p_type;
Elf64_Word p_flags;
Elf64_Off p_offset;
Elf64_Addr p_vaddr;
Elf64_Addr p_paddr;
Elf64_Xword p_filesz;
Elf64_Xword p_memsz;
Elf64_Xword p_align;
} Elf64_Phdr;
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .interp PROGBITS 0000000000000318 00000318
000000000000001c 0000000000000000 A 0 0 1
[ 2] .note.gnu.pr[...] NOTE 0000000000000338 00000338
0000000000000030 0000000000000000 A 0 0 8
[ 3] .note.gnu.bu[...] NOTE 0000000000000368 00000368
0000000000000024 0000000000000000 A 0 0 4
[ 4] .note.ABI-tag NOTE 000000000000038c 0000038c
0000000000000020 0000000000000000 A 0 0 4
[ 5] .gnu.hash GNU_HASH 00000000000003b0 000003b0
0000000000000024 0000000000000000 A 6 0 8
[ 6] .dynsym DYNSYM 00000000000003d8 000003d8
00000000000000a8 0000000000000018 A 7 1 8
[ 7] .dynstr STRTAB 0000000000000480 00000480
000000000000008f 0000000000000000 A 0 0 1
[ 8] .gnu.version VERSYM 0000000000000510 00000510
000000000000000e 0000000000000002 A 6 0 2
[ 9] .gnu.version_r VERNEED 0000000000000520 00000520
0000000000000030 0000000000000000 A 7 1 8
[10] .rela.dyn RELA 0000000000000550 00000550
00000000000000c0 0000000000000018 A 6 0 8
[11] .rela.plt RELA 0000000000000610 00000610
0000000000000018 0000000000000018 AI 6 24 8
[12] .init PROGBITS 0000000000001000 00001000
000000000000001b 0000000000000000 AX 0 0 4
[13] .plt PROGBITS 0000000000001020 00001020
0000000000000020 0000000000000010 AX 0 0 16
[14] .plt.got PROGBITS 0000000000001040 00001040
0000000000000010 0000000000000010 AX 0 0 16
[15] .plt.sec PROGBITS 0000000000001050 00001050
0000000000000010 0000000000000010 AX 0 0 16
[16] .text PROGBITS 0000000000001060 00001060
0000000000000117 0000000000000000 AX 0 0 16
[17] .fini PROGBITS 0000000000001178 00001178
000000000000000d 0000000000000000 AX 0 0 4
[18] .rodata PROGBITS 0000000000002000 00002000
0000000000000007 0000000000000000 A 0 0 4
[19] .eh_frame_hdr PROGBITS 0000000000002008 00002008
0000000000000034 0000000000000000 A 0 0 4
[20] .eh_frame PROGBITS 0000000000002040 00002040
00000000000000ac 0000000000000000 A 0 0 8
[21] .init_array INIT_ARRAY 0000000000003db8 00002db8
0000000000000008 0000000000000008 WA 0 0 8
[22] .fini_array FINI_ARRAY 0000000000003dc0 00002dc0
0000000000000008 0000000000000008 WA 0 0 8
[23] .dynamic DYNAMIC 0000000000003dc8 00002dc8
00000000000001f0 0000000000000010 WA 7 0 8
[24] .got PROGBITS 0000000000003fb8 00002fb8
0000000000000048 0000000000000008 WA 0 0 8
[25] .data PROGBITS 0000000000004000 00003000
0000000000000010 0000000000000000 WA 0 0 8
[26] .bss NOBITS 0000000000004010 00003010
0000000000000008 0000000000000000 WA 0 0 1
[27] .comment PROGBITS 0000000000000000 00003010
000000000000002b 0000000000000001 MS 0 0 1
[28] .symtab SYMTAB 0000000000000000 00003040
0000000000000360 0000000000000018 29 18 8
[29] .strtab STRTAB 0000000000000000 000033a0
00000000000001e0 0000000000000000 0 0 1
[30] .shstrtab STRTAB 0000000000000000 00003580
000000000000011a 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
D (mbind), l (large), p (processor specific)
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
PHDR 0x0000000000000040 0x0000000000000040 0x0000000000000040
0x00000000000002d8 0x00000000000002d8 R 0x8
INTERP 0x0000000000000318 0x0000000000000318 0x0000000000000318
0x000000000000001c 0x000000000000001c R 0x1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
LOAD 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000628 0x0000000000000628 R 0x1000
LOAD 0x0000000000001000 0x0000000000001000 0x0000000000001000
0x0000000000000185 0x0000000000000185 R E 0x1000
LOAD 0x0000000000002000 0x0000000000002000 0x0000000000002000
0x00000000000000ec 0x00000000000000ec R 0x1000
LOAD 0x0000000000002db8 0x0000000000003db8 0x0000000000003db8
0x0000000000000258 0x0000000000000260 RW 0x1000
DYNAMIC 0x0000000000002dc8 0x0000000000003dc8 0x0000000000003dc8
0x00000000000001f0 0x00000000000001f0 RW 0x8
NOTE 0x0000000000000338 0x0000000000000338 0x0000000000000338
0x0000000000000030 0x0000000000000030 R 0x8
NOTE 0x0000000000000368 0x0000000000000368 0x0000000000000368
0x0000000000000044 0x0000000000000044 R 0x4
GNU_PROPERTY 0x0000000000000338 0x0000000000000338 0x0000000000000338
0x0000000000000030 0x0000000000000030 R 0x8
GNU_EH_FRAME 0x0000000000002008 0x0000000000002008 0x0000000000002008
0x0000000000000034 0x0000000000000034 R 0x4
GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 RW 0x10
GNU_RELRO 0x0000000000002db8 0x0000000000003db8 0x0000000000003db8
0x0000000000000248 0x0000000000000248 R 0x1
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