OS_Lab1_实验报告

前置信息

Lab1对于Lab0的跳跃对我来说有点大，因此需要利用AI掌握一些前置信息。

ELF

首先是要了解ELF，先看一段ELF代码：

/* This file defines standard ELF types, structures, and macros.
   Copyright (C) 1995, 1996, 1997, 1998, 1999 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Ian Lance Taylor <ian@cygnus.com>.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Library General Public License as
   published by the Free Software Foundation; either version 2 of the
   License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Library General Public License for more details.

   You should have received a copy of the GNU Library General Public
   License along with the GNU C Library; see the file COPYING.LIB.  If not,
   write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
   Boston, MA 02111-1307, USA.  */

#ifndef _ELF_H
#define _ELF_H

#include <stdint.h>

/* ELF defination file from GNU C Library. We simplefied this
 * file for our lab, removing definations about ELF64, structs and
 * enums which we don't care.
 */

/* Type for a 16-bit quantity.  */
typedef uint16_t Elf32_Half;

/* Types for signed and unsigned 32-bit quantities.  */
typedef uint32_t Elf32_Word;
typedef int32_t Elf32_Sword;

/* Types for signed and unsigned 64-bit quantities.  */
typedef uint64_t Elf32_Xword;
typedef int64_t Elf32_Sxword;

/* Type of addresses.  */
typedef uint32_t Elf32_Addr;

/* Type of file offsets.  */
typedef uint32_t Elf32_Off;

/* Type for section indices, which are 16-bit quantities.  */
typedef uint16_t Elf32_Section;

/* Type of symbol indices.  */
typedef uint32_t Elf32_Symndx;

/* Lab 1 Key Code "readelf-struct-def" */

/* The ELF file header.  This appears at the start of every ELF file.  */

#define EI_NIDENT (16)

typedef struct {
        unsigned char e_ident[EI_NIDENT]; /* Magic number and other info */
        Elf32_Half e_type;                /* Object file type */
        Elf32_Half e_machine;             /* Architecture */
        Elf32_Word e_version;             /* Object file version */
        Elf32_Addr e_entry;               /* Entry point virtual address */
        Elf32_Off e_phoff;                /* Program header table file offset */
        Elf32_Off e_shoff;                /* Section header table file offset */
        Elf32_Word e_flags;               /* Processor-specific flags */
        Elf32_Half e_ehsize;              /* ELF header size in bytes */
        Elf32_Half e_phentsize;           /* Program header table entry size */
        Elf32_Half e_phnum;               /* Program header table entry count */
        Elf32_Half e_shentsize;           /* Section header table entry size */
        Elf32_Half e_shnum;               /* Section header table entry count */
        Elf32_Half e_shstrndx;            /* Section header string table index */
} Elf32_Ehdr;

/* Fields in the e_ident array.  The EI_* macros are indices into the
   array.  The macros under each EI_* macro are the values the byte
   may have.  */

#define EI_MAG0 0    /* File identification byte 0 index */
#define ELFMAG0 0x7f /* Magic number byte 0 */

#define EI_MAG1 1   /* File identification byte 1 index */
#define ELFMAG1 'E' /* Magic number byte 1 */

#define EI_MAG2 2   /* File identification byte 2 index */
#define ELFMAG2 'L' /* Magic number byte 2 */

#define EI_MAG3 3   /* File identification byte 3 index */
#define ELFMAG3 'F' /* Magic number byte 3 */

/* Section segment header.  */
typedef struct {
        Elf32_Word sh_name;      /* Section name */
        Elf32_Word sh_type;      /* Section type */
        Elf32_Word sh_flags;     /* Section flags */
        Elf32_Addr sh_addr;      /* Section addr */
        Elf32_Off sh_offset;     /* Section offset */
        Elf32_Word sh_size;      /* Section size */
        Elf32_Word sh_link;      /* Section link */
        Elf32_Word sh_info;      /* Section extra info */
        Elf32_Word sh_addralign; /* Section alignment */
        Elf32_Word sh_entsize;   /* Section entry size */
} Elf32_Shdr;

/* Program segment header.  */

typedef struct {
        Elf32_Word p_type;   /* Segment type */
        Elf32_Off p_offset;  /* Segment file offset */
        Elf32_Addr p_vaddr;  /* Segment virtual address */
        Elf32_Addr p_paddr;  /* Segment physical address */
        Elf32_Word p_filesz; /* Segment size in file */
        Elf32_Word p_memsz;  /* Segment size in memory */
        Elf32_Word p_flags;  /* Segment flags */
        Elf32_Word p_align;  /* Segment alignment */
} Elf32_Phdr;
/* End of Key Code "readelf-struct-def" */

/* Legal values for p_type (segment type).  */

#define PT_NULL 0            /* Program header table entry unused */
#define PT_LOAD 1            /* Loadable program segment */
#define PT_DYNAMIC 2         /* Dynamic linking information */
#define PT_INTERP 3          /* Program interpreter */
#define PT_NOTE 4            /* Auxiliary information */
#define PT_SHLIB 5           /* Reserved */
#define PT_PHDR 6            /* Entry for header table itself */
#define PT_NUM 7             /* Number of defined types.  */
#define PT_LOOS 0x60000000   /* Start of OS-specific */
#define PT_HIOS 0x6fffffff   /* End of OS-specific */
#define PT_LOPROC 0x70000000 /* Start of processor-specific */
#define PT_HIPROC 0x7fffffff /* End of processor-specific */

/* Legal values for p_flags (segment flags).  */

#define PF_X (1 << 0)          /* Segment is executable */
#define PF_W (1 << 1)          /* Segment is writable */
#define PF_R (1 << 2)          /* Segment is readable */
#define PF_MASKPROC 0xf0000000 /* Processor-specific */

#endif /* elf.h */

这是一段C语言代码，指导书的注释非常详实，这段代码主要定义了几个结构体，有Elf32_Ehdr,Elf32_Phdr,Elf32_Shdr。

Ehdr(ELF header):

e_ident:主要用来识别是不是ELF文件

e_machine:目标架构，是给x86跑的还是ARM跑的

e_version:ELF的版本号，通常为1

e_entry:程序入口虚拟地址，操作系统把程序加载到内存后，CPU 第一条执行的指令就在这个地址（通常对应代码里的_start函数，而不是main）。

e_phoff:程序头表偏移量 e_shoff:节头表偏移量 e_flags:特定标志位，一般为0

e_ehsize:Elf32_Ehdr结构体本身的字节数 e_phentsize:程序头表中，每一个表项的大小

e_phnum:程序头表里有几个表项(程序被分成了几个Segment) e_shentsize:节头表中每一个表项的大小

e_shnum:节头表有几个表项(文件被分成了几个Section) e_shstrndx:节名字符串表的索引,如.text,.main存放的位置

Shdr(Section header):

sh_name:节的名称索引 sh_type:节的类型 sh_flags:节的权限标志，是否可写，是否需要在内存中分配空间

sh_addr:虚拟地址，如果这个节在程序运行时需要放到内存里，这里就是它期望被加载到的内存地址。如果不加载到内存（比如调试信息），这里就是 0。

sh_offset:文件内偏移量，这个节的实际数据（比如一堆机器指令）在当前文件里的起始位置。

sh_size:这个节占了多少个字节 sh_link:关联节的索引 sh_info:附加信息

sh_addralign:对齐方式，要求这个节在内存里的起始地址必须是某个数的整数倍

sh_entsize:表项大小，如果这个节里面存的是一张表（比如符号表，里面全是固定大小的条目），这里记录每个条目有多大；如果是一般代码/数据，这里为 0。

Phdr(Program header):

p_type:段的类型，最重要的是 PT_LOAD (1)，意思是这个段必须被搬进内存里。其他的还有 PT_DYNAMIC（动态链接信息）等。

p_offset:文件内偏移量 p_vaddr_虚拟内存地址，操作系统应该把这个段的数据复制到内存中的哪个虚拟地址。

p_paddr:物理内存地址 p_filesz:文件大小，这个段在文件里占据了多少字节 p_memsz:内存大小，这个段在内存里需要占据多少字节

p_flags:段的访问权限 p_align:内存页对齐要求，规定这个段在内存中的起始地址 p_vaddr 和文件里的 p_offset 必须对齐到一个页面大小的整数倍（通常是 0x1000，即 4096 字节）。这方便操作系统利用内存页机制进行映射。