实现步骤：

1、添加$U/_trace到Makefile中的UPROGS变量里;

2、添加声明到user/user.h，添加一个entry到user/usys.pl和一个syscall number到kernel/syscall.h中;

3、添加mask值到proc结构体;

proc结构体：

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// Per-process state
struct proc {
  struct spinlock lock;

  // p->lock must be held when using these:
  enum procstate state;        // Process state
  struct proc *parent;         // Parent process
  void *chan;                  // If non-zero, sleeping on chan
  int killed;                  // If non-zero, have been killed
  int xstate;                  // Exit status to be returned to parent's wait
  int pid;                     // Process ID

  // these are private to the process, so p->lock need not be held.
  uint64 kstack;               // Virtual address of kernel stack
  uint64 sz;                   // Size of process memory (bytes)
  pagetable_t pagetable;       // User page table
  struct trapframe *trapframe; // data page for trampoline.S
  struct context context;      // swtch() here to run process
  struct file *ofile[NOFILE];  // Open files
  struct inode *cwd;           // Current directory
  char name[16];               // Process name (debugging)
  int mask;                    // 添加mask值
};

4、添加sys_trace()到kernel/sysproc.c

sys_trace实现：

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uint64
sys_trace(void)
{
  int n;
  if (argint(0, &n) < 0) // 判断参数是否获取成功
    return -1;
  myproc()->mask = n;    // 将argv[1]保存到当前进程的mask中
  return 0;
}

5、修改kernel/proc.c中的fork函数，添加子进程复制父进程mask的功能

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int
fork(void)
{
  /* do something .... */
  safestrcpy(np->name, p->name, sizeof(p->name));
  
  // 复制 mask
  np->mask = p->mask;

  pid = np->pid;

  np->state = RUNNABLE;

  release(&np->lock);

  return pid;
}

6、修改kernel/syscall.c，首先添加声明sys_trace函数，然后添加到syscalls数组中，然后就是修改syscall函数添加trace识别功能

kernel/syscall.c：

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/*........*/
extern uint64 sys_uptime(void);
extern uint64 sys_trace(void);   // 添加sys_trace声明

static uint64 (*syscalls[])(void) = {
/* ..... */
[SYS_trace]   sys_trace,         // 添加到syscall数组中
};

// 添加识别名
char* syscalls_name[23] = {"", "fork", "exit", "wait", "pipe", "read", "kill", "exec",
                      "fstat", "chdir", "dup", "getpid", "sbrk", "sleep", "uptime",
                      "open", "write", "mknod", "unlink", "link", "mkdir", "close", "trace"};

void
syscall(void)
{
  int num;
  struct proc *p = myproc();

  num = p->trapframe->a7;
  if(num > 0 && num < NELEM(syscalls) && syscalls[num]) {
    p->trapframe->a0 = syscalls[num]();
    // 添加追踪功能
    if (p->mask & (1 << num))
    {
      printf("%d: syscall %s -> %d\n",p->pid, syscalls_name[num], p->trapframe->a0);
    }
  } else {
    printf("%d %s: unknown sys call %d\n",
            p->pid, p->name, num);
    p->trapframe->a0 = -1;
  }
}