trace.txt

1. Modify user/user.h:
   - Add the prototype for the new system call `int trace(int mask);`

2. Modify user/usys.pl:
   - Add the stub for the new system call `SYS_trace`:
     ```
     entry("trace");
     ```

3. Modify kernel/syscall.h:
   - Add the system call number for `SYS_trace`:
     ```
     #define SYS_trace 22
     ```

4. Modify kernel/syscall.c:
   - Add an array of syscall names to index into:
     ```
     static char *syscall_names[] = {
         [SYS_fork] = "fork",
         [SYS_exit] = "exit",
         [SYS_wait] = "wait",
         ...
         [SYS_trace] = "trace",
     };
     ```

   - Modify the `syscall()` function to print the trace output:
     ```

//added by rqx 2023/7/24
static const char *syscall_names[] = {
  [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",
  
};
     void
     syscall(void)
     {
         ...//added by rqx 2023/7/24
      if (p->trace_mask & (1 << num)) {
           printf("%d: syscall %s -> %d\n", p->pid, syscall_names[num], p->trapframe->a0);
         }
         }
         ...
     }
     ```

5. Modify kernel/sysproc.c:
   - Add a new variable `trace_mask` to the `proc` structure in kernel/proc.h:
     ```
     struct proc {
         ...
         int trace_mask; // Trace mask for system call tracing
         ...
     };
     ```

   - Add a new system call `sys_trace()` to implement the `trace()` system call:
     ```
     int sys_trace(void)
{
   //added by rqx 2023/7/24
   int mask;
   if (argint(0, &mask) < 0)
       return -1;
   myproc()->trace_mask = mask;
   return 0;
}
     ```

   - Modify the `fork()` function to copy the trace mask from the parent to the child process:
     ```
     int
     fork(void)
     {
         ...
         np->trace_mask = proc->trace_mask;
         ...
     }
     `

``