[mit6.1810]Lab2: system calls

【写在前面】

这是2023-2024春季学期操作系统课程有关xv6 lab部分的实验报告，参考了很多网络资源，解释也不一定正确，仅作为留档，参考需谨慎。

Lab 地址：6.1810 / Fall 2023

参考：

MIT 6.S081_Zheyuan Zou的博客-CSDN博客

【露说xv6】Lab2-Basic and Optional（985男大的初学案例分享）_哔哩哔哩_bilibili

Using gdb

Q1: Looking at the backtrace output, which function called syscall?

A1: usertrap()调用了syscall

Q2: What is the value of p->trapframe->a7 and what does that value represent?

A2: p /x p->trapframe->a7 = 0x7 由此可知，在syscall.h中找到#define SYS_exec 7，在 user/initcode.S查看：xv6 启动的第一个用户程序为SYS_exec

Q3: What was the previous mode that the CPU was in?

A3: sstatus = 0x200000022，将sstatus转化为二进制表示，其中第八位为0（0表示用户态，1表示核心态），所以CPU处于用户态。

Q4: Write down the assembly instruction the kernel is panicing at. Which register corresponds to the variable num?

A4:instruction: lw a3,0(zero)#0<entry-0x80000000>；register: a3

Q5: Why does the kernel crash? Hint: look at figure 3-3 in the text; is address 0 mapped in the kernel address space? Is that confirmed by the value in scause above?

A5: 访问0地址时超出列表出错；0地址不会被映射到空间；由输出scause 0x000000000000000d可知，对应RISC-V表中0/13，显示 “Load page fault" 表示加载列表出现错误，相符。

Q6: What is the name of the binary that was running when the kernel paniced? What is its process id (pid)?

A6: name of the binary: initcode 000\000\000\000\000\000\000'；pid = 1

System call tracing

添加一个系统调用：

在user.h中添加：int trace(int);

在usys.pl中添加：entry("trace");

在syscall.h中添加：#define SYS_trace 23

在syscall.c中添加函数声明extern uint64 sys_trace(void);和函数列表[SYS_trace] sys_trace,
加入一个表示用于进程追踪的掩码

在proc.c中新增变量声明int traceMask;用于接受参数mask。在生成proc时会调用函数allocproc，严谨地在该函数中初始化变量p->traceMask=0;

在sysproc.c的文件中真正地实现sys_trace的逻辑

在sysproc.c中添加函数定义：

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uint64
sys_trace(void)
{
  int flag;
  argint(0, &flag); // 将trapframe->a0读入（详细查看argraw())
  struct proc* p=myproc();
  p->traceMask=flag;
  return 0; 
}

在函数fork()中添加np->traceMask = p->traceMask; 将参数复制到子进程中

在syscall.c中还需要加一个系统调用号到名称的映射表：

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static char* SyscallName[] = {
[SYS_fork]    "fork",
[SYS_exit]    "exit",
[SYS_wait]    "wait",
[SYS_pipe]    "pipe",
[SYS_read]    "read",
[SYS_kill]    "kill",
[SYS_exec]    "exec",
[SYS_fstat]   "fstat",
[SYS_chdir]   "chdir",
[SYS_dup]     "dup",
[SYS_getpid]  "getpid",
[SYS_sbrk]    "sbrk",
[SYS_sleep]   "sleep",
[SYS_uptime]  "uptime",
[SYS_open]    "open",
[SYS_write]   "write",
[SYS_mknod]   "mknod",
[SYS_unlink]  "unlink",
[SYS_link]    "link",
[SYS_mkdir]   "mkdir",
[SYS_close]   "close",
[SYS_trace]   "trace",
};

测试结果

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