系统调用之sys_settimeofday

这个函数用于设置系统当前时间。
其源码分析如下：
SYSCALL_DEFINE1(stime, time_t __user *, tptr)
{
	struct timespec64 tv;
	int err;
	#从user space的buffer中copy要设置的时间到tv.tv_sec中
	if (get_user(tv.tv_sec, tptr))
		return -EFAULT;

	tv.tv_nsec = 0;
	#security_settime64->cap_settime检查是否有权限设置时间
	err = security_settime64(&tv, NULL);
	if (err)
		return err;
	#开始设置时间
	do_settimeofday64(&tv);
	return 0;
}


int do_settimeofday64(const struct timespec64 *ts)
{
	#得到timekeeper的时间
	struct timekeeper *tk = &tk_core.timekeeper;
	struct timespec64 ts_delta, xt;
	unsigned long flags;
	int ret = 0;
	#检查时间是否合法
	if (!timespec64_valid_strict(ts))
		return -EINVAL;

	raw_spin_lock_irqsave(&timekeeper_lock, flags);
	write_seqcount_begin(&tk_core.seq);

	timekeeping_forward_now(tk);
	#将timekeeper转成timespec64
	xt = tk_xtime(tk);
	#计算当前时间和要设置时间的偏差
	ts_delta.tv_sec = ts->tv_sec - xt.tv_sec;
	ts_delta.tv_nsec = ts->tv_nsec - xt.tv_nsec;
	#检查这个偏差是合法
	if (timespec64_compare(&tk->wall_to_monotonic, &ts_delta) > 0) {
		ret = -EINVAL;
		goto out;
	}
	#设置墙上时间
	tk_set_wall_to_mono(tk, timespec64_sub(tk->wall_to_monotonic, ts_delta));
	#设置timekeeper
	tk_set_xtime(tk, ts);
out:
	timekeeping_update(tk, TK_CLEAR_NTP | TK_MIRROR | TK_CLOCK_WAS_SET);

	write_seqcount_end(&tk_core.seq);
	raw_spin_unlock_irqrestore(&timekeeper_lock, flags);

	/* signal hrtimers about time change */
	clock_was_set();

	return ret;
}