准备知识
-
ELF文件
"Executable and Linkable Format" 的简称。当编译和链接一个 C 程序的时候，编译器将每个 C 源码文件 (.c) 转为一个对象文件 (.o) ，对象文件中存放的是机器能理解的二进制格式的汇编语言指令。然后，链接器 (linker) 将所有对象文件结合为一个二进制映像 (image) 文件，即ELF文件。 -
硬盘布局
bootloader (boot.S and main.c) 存放在启动盘的第一个 sector
kernel (必须为 elf 文件)存放在第二个 sector - 启动步骤
- 将BIOS读入内存并执行
- BIOS将初始化设备，设置好中断，将设备的第一个sector读入内存并跳转。
- 执行到bootloader时，
boot.S将开启保护模式，并设置好栈指针使得系统可以执行 C 程序。然后执行bootmain()。 -
main.c中的bootmain会读入 kernel 并且跳转。
阅读代码
用到了在 inc/elf.h 中定义的两个结构体
struct Elf { // ELF文件头
uint32_t e_magic; // must equal ELF_MAGIC
uint8_t e_elf[12];
uint16_t e_type;
uint16_t e_machine;
uint32_t e_version;
uint32_t e_entry;
uint32_t e_phoff; // program header起始位置
uint32_t e_shoff; // section header起始位置
uint32_t e_flags;
uint16_t e_ehsize; // ELF文件头本身大小
uint16_t e_phentsize;
uint16_t e_phnum; // program header个数
uint16_t e_shentsize;
uint16_t e_shnum;
uint16_t e_shstrndx;
};
struct Proghdr { // 程序头表
uint32_t p_type;
uint32_t p_offset; // 段相对于ELF文件开头的偏移
uint32_t p_va;
uint32_t p_pa; // 物理地址
uint32_t p_filesz;
uint32_t p_memsz; // 在内存中的大小
uint32_t p_flags; // 读，写，执行权限
uint32_t p_align;
};
ELF文件结构
首先，ELF文件格式提供了两种视图，分别是链接视图和执行视图。
链接视图是以节（section）为单位，执行视图是以段（segment）为单位。链接视图就是在链接时用到的视图，而执行视图则是在执行时用到的视图。上图左侧的视角是从链接来看的，右侧的视角是执行来看的。可以看出，一个segment可以包含数个section。
本文关注执行，结构体Proghdr是用于描述段 (segment) 的 program header，可有多个。
bootmain()函数
#include <inc/x86.h>
#include <inc/elf.h>
/**********************************************************************
* This a dirt simple boot loader, whose sole job is to boot
* an ELF kernel image from the first IDE hard disk.
*
* DISK LAYOUT
* * This program(boot.S and main.c) is the bootloader. It should
* be stored in the first sector of the disk.
*
* * The 2nd sector onward holds the kernel image.
*
* * The kernel image must be in ELF format.
*
* BOOT UP STEPS
* * when the CPU boots it loads the BIOS into memory and executes it
*
* * the BIOS intializes devices, sets of the interrupt routines, and
* reads the first sector of the boot device(e.g., hard-drive)
* into memory and jumps to it.
*
* * Assuming this boot loader is stored in the first sector of the
* hard-drive, this code takes over...
*
* * control starts in boot.S -- which sets up protected mode,
* and a stack so C code then run, then calls bootmain()
*
* * bootmain() in this file takes over, reads in the kernel and jumps to it.
**********************************************************************/
// 扇区(sector)大小512
#define SECTSIZE 512
// 将0x10000设为内核起始地址
#define ELFHDR ((struct Elf *) 0x10000) // scratch space
void readsect(void*, uint32_t);
void readseg(uint32_t, uint32_t, uint32_t);
void
bootmain(void)
{
struct Proghdr *ph, *eph;
// read 1st page off disk
// 从 0 开始读取 8*512 = 4096 byte 的内容到 ELFHDR
readseg((uint32_t) ELFHDR, SECTSIZE*8, 0);
// is this a valid ELF?
if (ELFHDR->e_magic != ELF_MAGIC)
goto bad;
// load each program segment (ignores ph flags)
// 获得程序头表的起始位置 ph
ph = (struct Proghdr *) ((uint8_t *) ELFHDR + ELFHDR->e_phoff);
// 获取程序头表结束的位置 eph
eph = ph + ELFHDR->e_phnum;
for (; ph < eph; ph++)
// p_pa is the load address of this segment (as well
// as the physical address)
// 根据每个 program header 读取 segment
// 从 p_offset 开始拷贝 p_memsz 个 byte 到 p_pa
readseg(ph->p_pa, ph->p_memsz, ph->p_offset);
// call the entry point from the ELF header
// note: does not return!
((void (*)(void)) (ELFHDR->e_entry))();
bad:
outw(0x8A00, 0x8A00);
outw(0x8A00, 0x8E00);
while (1)
/* do nothing */;
}
语法难点解析
ph = (struct Proghdr *) ((uint8_t *) ELFHDR + ELFHDR->e_phoff);
首先将ELFHDR转为 uint8_t 型指针，做加法的时候按照 byte 加，获得程序头表的起始位置，再将这个位置转为 Proghdr 型指针 ph。((void (*)(void)) (ELFHDR->e_entry))();
将ELFHDR->e_entry转为一个无参数，无返回值的函数指针，并执行该函数。
读取segment
(只从逻辑分析，忽略readsect和waitdisk函数)
// Read 'count' bytes at 'offset' from kernel into physical address 'pa'.
// Might copy more than asked
void
readseg(uint32_t pa, uint32_t count, uint32_t offset)
{
uint32_t end_pa;
end_pa = pa + count;
// round down to sector boundary
// 将pa按扇区对齐
pa &= ~(SECTSIZE - 1);
// translate from bytes to sectors, and kernel starts at sector 1
// 将以byte为单位的offset转为以sector为单位
offset = (offset / SECTSIZE) + 1;
// If this is too slow, we could read lots of sectors at a time.
// We'd write more to memory than asked, but it doesn't matter --
// we load in increasing order.
while (pa < end_pa) {
// Since we haven't enabled paging yet and we're using
// an identity segment mapping (see boot.S), we can
// use physical addresses directly. This won't be the
// case once JOS enables the MMU.
// 此时，offset已经被转为以扇区(sector)为单位
// 始终是以一个 sector 为单位读取
readsect((uint8_t*) pa, offset);
pa += SECTSIZE;
offset++;
}
}
void
waitdisk(void)
{
// wait for disk reaady
while ((inb(0x1F7) & 0xC0) != 0x40)
/* do nothing */;
}
void
readsect(void *dst, uint32_t offset)
{
// wait for disk to be ready
waitdisk();
outb(0x1F2, 1); // count = 1
outb(0x1F3, offset);
outb(0x1F4, offset >> 8);
outb(0x1F5, offset >> 16);
outb(0x1F6, (offset >> 24) | 0xE0);
outb(0x1F7, 0x20); // cmd 0x20 - read sectors
// wait for disk to be ready
waitdisk();
// read a sector
insl(0x1F0, dst, SECTSIZE/4);
}
语法难点解析
-
pa &= ~(SECTSIZE - 1);
对应汇编码：
(gdb)
=> 0x7cf1: and $0xfffffe00,%ebx
0x00007cf1 in ?? ()
uint32_t 512的十六进制表示为0x00000200，减1后为0x000001ff，按位取反得0xfffffe00，可以看出作用是将pa的后9 bit全部置0。
附录1. main.c生成的汇编代码
为了分析exercise 3，有必要对各个函数的汇编码进行一个review。
// 调用 bootmain()
=> 0x7c45: call 0x7d15
=> 0x7d15: push %ebp
=> 0x7d16: mov %esp,%ebp
=> 0x7d18: push %esi
=> 0x7d19: push %ebx
// 从右向左压入参数
=> 0x7d1a: push $0x0
=> 0x7d1c: push $0x1000
=> 0x7d21: push $0x10000
// 调用 readseg((uint32_t) ELFHDR, SECTSIZE*8, 0)
=> 0x7d26: call 0x7cdc
=> 0x7cdc: push %ebp
=> 0x7cdd: mov %esp,%ebp
=> 0x7cdf: push %edi
=> 0x7ce0: push %esi
// 利用偏移获取各参数
// ebp+8 位置是arg1
// ebp+12 位置是arg2
// ebp+16 位置是arg3
=> 0x7ce1: mov 0x10(%ebp),%edi
=> 0x7ce4: push %ebx
=> 0x7ce5: mov 0xc(%ebp),%esi
=> 0x7ce8: mov 0x8(%ebp),%ebx
=> 0x7ceb: shr $0x9,%edi // (offset / SECTSIZE)
=> 0x7cee: add %ebx,%esi
=> 0x7cf0: inc %edi
=> 0x7cf1: and $0xfffffe00,%ebx
=> 0x7cf7: cmp %esi,%ebx // while 语句比较 pa 和 end_pa
=> 0x7cf9: jae 0x7d0d // 大于等于则跳转
=> 0x7cfb: push %edi
=> 0x7cfc: push %ebx
=> 0x7cfd: inc %edi // offset++
=> 0x7cfe: add $0x200,%ebx // pa += SECTSIZE
// 调用 readsect((uint8_t*) pa, offset)
=> 0x7d04: call 0x7c7c
=> 0x7c7c: push %ebp
=> 0x7c7d: mov %esp,%ebp
=> 0x7c7f: push %edi
=> 0x7c80: mov 0xc(%ebp),%ecx
// 调用 waitdisk(void)
=> 0x7c83: call 0x7c6a
=> 0x7c6a: push %ebp
=> 0x7c6b: mov $0x1f7,%edx
=> 0x7c70: mov %esp,%ebp
=> 0x7c72: in (%dx),%al
=> 0x7c73: and $0xffffffc0,%eax
=> 0x7c76: cmp $0x40,%al
=> 0x7c78: jne 0x7c72
=> 0x7c7a: pop %ebp
=> 0x7c7b: ret
// waitdisk 结束，返回 readsect 函数继续执行
=> 0x7c88: mov $0x1f2,%edx
=> 0x7c8d: mov $0x1,%al
=> 0x7c8f: out %al,(%dx)
=> 0x7c90: mov $0x1f3,%edx
=> 0x7c95: mov %cl,%al
=> 0x7c97: out %al,(%dx)
=> 0x7c98: mov %ecx,%eax
=> 0x7c9a: mov $0x1f4,%edx
=> 0x7c9f: shr $0x8,%eax
=> 0x7ca2: out %al,(%dx)
=> 0x7ca3: mov %ecx,%eax
=> 0x7ca5: mov $0x1f5,%edx
=> 0x7caa: shr $0x10,%eax
=> 0x7cad: out %al,(%dx)
=> 0x7cae: mov %ecx,%eax
=> 0x7cb0: mov $0x1f6,%edx
=> 0x7cb5: shr $0x18,%eax
=> 0x7cb8: or $0xffffffe0,%eax
=> 0x7cbb: out %al,(%dx)
=> 0x7cbc: mov $0x1f7,%edx
=> 0x7cc1: mov $0x20,%al
=> 0x7cc3: out %al,(%dx)
// 调用 waitdisk(void)
=> 0x7cc4: call 0x7c6a
=> 0x7c6a: push %ebp
=> 0x7c6b: mov $0x1f7,%edx
=> 0x7c70: mov %esp,%ebp
=> 0x7c72: in (%dx),%al
=> 0x7c73: and $0xffffffc0,%eax
=> 0x7c76: cmp $0x40,%al
=> 0x7c78: jne 0x7c72
=> 0x7c7a: pop %ebp
=> 0x7c7b: ret
=> 0x7cc9: mov 0x8(%ebp),%edi
=> 0x7ccc: mov $0x80,%ecx
=> 0x7cd1: mov $0x1f0,%edx
=> 0x7cd6: cld
=> 0x7cd7: repnz insl (%dx),%es:(%edi) //repeats instruction while Z flag is cleared
=> 0x7cd9: pop %edi
=> 0x7cda: pop %ebp
=> 0x7cdb: ret // 退出 readsect 函数
=> 0x7d09: pop %eax
=> 0x7d0a: pop %edx
// 返回 while 语句判断
=> 0x7d0b: jmp 0x7cf7
... // 重复直到跳出 while 循环
=> 0x7d0d: lea -0xc(%ebp),%esp
=> 0x7d10: pop %ebx
=> 0x7d11: pop %esi
=> 0x7d12: pop %edi
=> 0x7d13: pop %ebp
=> 0x7d14: ret // 退出 readseg 函数
=> 0x7d2b: add $0xc,%esp
=> 0x7d2e: cmpl $0x464c457f,0x10000 // 判断 e_magic
=> 0x7d38: jne 0x7d71
=> 0x7d3a: mov 0x1001c,%eax
=> 0x7d3f: movzwl 0x1002c,%esi
=> 0x7d46: lea 0x10000(%eax),%ebx
=> 0x7d4c: shl $0x5,%esi
=> 0x7d4f: add %ebx,%esi // ebx存放ph，esi存放eph
=> 0x7d51: cmp %esi,%ebx // for 语句中比较 ph 与 eph
=> 0x7d53: jae 0x7d6b // 大于等于则跳转
// 压入参数
=> 0x7d55: pushl 0x4(%ebx)
=> 0x7d58: pushl 0x14(%ebx)
=> 0x7d5b: add $0x20,%ebx
=> 0x7d5e: pushl -0x14(%ebx)
// 调用 readseg(ph->p_pa, ph->p_memsz, ph->p_offset)
=> 0x7d61: call 0x7cdc
... // 重复直到跳出for循环
附录2. ELF详细介绍
-
ELF executable
可看作包含加载信息的文件头 (header) 以及一些程序段 (program section)。每个程序段是相邻的代码块或数据块，需要被加载到内存的特定位置。boot loader 不更改代码或数据，只是加载到内存并且执行。 -
ELF binary
以一个定长 ELF header 开头，然后是变长的 program header，包含了所有需要加载的程序段。 -
program section
只关注三个会用到的section。 - .text
程序的可执行指令。 - .rodata
只读数据。例如 C 编译器产生的 ASCII 字符串常量。 - .data
保存程序的初始数据。例如某个有初始值的全局变量int x = 5;。
~/OS/lab/obj/kern$ objdump -h kernel
kernel: file format elf32-i386
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00001871 f0100000 00100000 00001000 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .rodata 00000714 f0101880 00101880 00002880 2**5
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 .stab 000038d1 f0101f94 00101f94 00002f94 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
3 .stabstr 000018bb f0105865 00105865 00006865 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .data 0000a300 f0108000 00108000 00009000 2**12
CONTENTS, ALLOC, LOAD, DATA
5 .bss 00000644 f0112300 00112300 00013300 2**5
ALLOC
6 .comment 00000034 00000000 00000000 00013300 2**0
CONTENTS, READONLY
重点关注的是 .text 部分的 VMA (link address) 和 LMA (load address)。link address 是开始执行该 section 的内存地址。而 load address 则顾名思义，是加载该 section 的内存地址。一般而言这两者是相同的。
boot loader 利用 ELF program header 来决定如何加载 section，而 program header 指定应该读取 ELF 对象的哪个部分进内存，以及应该放在哪里。
