系统调用流程分析

syscall 到 exec 调用

1. syscall 处理系统调用
   • 获取当前执行进程
   • 从系统调用表 syscalls 中调用系统调用函数 sys_exec
2. sys_exec
   • 检查
   • 获取 argv
   • 调用 exec
3. exec

exec() 系统调用

void exec(init, argv)

1. 创建用户地址空间
2. 从磁盘加载 path 文件, 到内存中
   • 打开文件, namei
   • 读取 ELF 头, readi

     readelf -h _init
             

     ELF Header:
       Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
       Class:                             ELF32
       Data:                              2's complement, little endian
       Version:                           1 (current)
       OS/ABI:                            UNIX - System V
       ABI Version:                       0
       Type:                              EXEC (Executable file)
       Machine:                           Intel 80386
       Version:                           0x1
       Entry point address:               0x0
       Start of program headers:          52 (bytes into file)
       Start of section headers:          14252 (bytes into file)
       Flags:                             0x0
       Size of this header:               52 (bytes)
       Size of program headers:           32 (bytes)
       Number of program headers:         2
       Size of section headers:           40 (bytes)
       Number of section headers:         18
       Section header string table index: 17
             

   • 解析代码段, readi/allocuvm/loaduvm

     readelf -l _init
             

     
     Elf file type is EXEC (Executable file)
     Entry point 0x0
     There are 2 program headers, starting at offset 52
     
     Program Headers:
       Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
       LOAD           0x000080 0x00000000 0x00000000 0x00b2c 0x00b38 RWE 0x10
       GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RWE 0x10
     
      Section to Segment mapping:
       Segment Sections...
        00     .text .rodata .eh_frame .data .bss
        01
             

3. 申请用户堆栈页面, allocuvm
4. 设置参数 args, 准备堆栈, copyout
5. 提交用户态，跳转用户态, switchuvm
   • elf.entry
   • pgdir
   • eip/esp