MIT 6.S081 Lab syscall

Compulsory exercises

Preparation

• reading
  

• To start the lab, switch to the syscall branch:

$ git fetch
$ git checkout syscall
$ make clean

System call tracing (moderate)

• 了解trace的过程

  • trace.c调用系统调用trace()
  • 系统调用trace()调用sys_trace()设置进程的mask

• 理解mask的含义

  • mask作为作为系统调用序号的掩码，如mask = (1 << SYS_read) | (1 << SYS_fork)代表同时trace read 和 fork系统调用
  • 当mask = 2147483647时代表trace序号1 - 31的系统调用
  • question: mask的类型? uint64 or uint32? 理论上都是可行的

• 添加编译依赖
  

• 添加trace系统调用
  

code

• add sys_trace()

// add in sysproc.c
uint64
sys_trace(void)
{
    uint64 m;
    if(argaddr(0, &m) < 0)
        return -1;
    myproc()->mask = m;
    return 0;
}

// add in proc.h struct proc
  uint64 mask;                 // 64bit Mask for syscall trace

• Modify fork()

// add in proc.c
  // copy the trace mask from the parent to the child process
  np->mask = p->mask;

• Modify the syscall()

// add and modify in syscall.c
char const *syscallNames[] = {
  "", "fork", "exit", "wait", "pipe", "read",
  "kill", "exec", "fstat", "chdir", "dup", 
  "getpid", "sbrk", "sleep","uptime", "open", 
  "write", "mknod", "unlink", "link", "mkdir",
  "close","trace","sysinfo"
};

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]();

    // added in lab syscall
    if ((1 << num) & p->mask) {
        printf("%d: syscall %s -> %d\n", p->pid, syscallNames[num], p->trapframe->a0);
    }
  } else {
    printf("%d %s: unknown sys call %d\n",
            p->pid, p->name, num);
    p->trapframe->a0 = -1;
  }

}

Sysinfo (moderate)

• 了解sysinfo的过程

  • sysinfotest调用sysinfo()
  • 系统调用sysinfo()调用sys_sysinfo获取struct sysinfo

• 添加编译依赖
  

• 添加sysinfo系统调用
  

code

• add sysinfo()

// add in sysproc.c
uint64
sys_sysinfo(void)
{
    uint64 addr;
    if (argaddr(0, &addr) < 0)
        return -1;
    struct sysinfo info;
    info.freemem = freemem();
    info.nproc = procnumber();
    if(copyout(myproc()->pagetable, addr, (char *)&info, sizeof(info)) < 0)
        return -1;
    return 0;
}

• add freemem()

// add in kalloc.c
uint64
freemem(void)
{
    struct run *r;

    r = kmem.freelist;
    uint64 count_mem = 0;
    while (r) {
        count_mem += 4096;
        r = r->next;
    }

    return count_mem;
}

• add procnumber()

// add in proc.c
uint64
procnumber(void)
{
    uint64 count = 0;
    struct proc *p;

    for (p = proc; p < &proc[NPROC]; p++) {
        if (p->state != UNUSED) {
            count++;
        }
    }
    return count;
}

Optional challenge exercises

• Print the system call arguments for traced system calls (easy).

• Compute the load average and export it through sysinfo(moderate).