netfilter: xt_time: add support to ignore day transition

Currently, if you want to do something like:
"match Monday, starting 23:00, for two hours"
You need two rules, one for Mon 23:00 to 0:00 and one for Tue 0:00-1:00.

The rule: --weekdays Mo --timestart 23:00 --timestop 01:00

looks correct, but it will first match on monday from midnight to 1 a.m.
and then again for another hour from 23:00 onwards.

This permits userspace to explicitly ignore the day transition and
match for a single, continuous time period instead.

Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/linux/netfilter/xt_time.h

···
 	/* Match against local time (instead of UTC) */
 	XT_TIME_LOCAL_TZ = 1 << 0,
 
+	/* treat timestart > timestop (e.g. 23:00-01:00) as single period */
+	XT_TIME_CONTIGUOUS = 1 << 1,
+
 	/* Shortcuts */
 	XT_TIME_ALL_MONTHDAYS = 0xFFFFFFFE,
 	XT_TIME_ALL_WEEKDAYS  = 0xFE,
 	XT_TIME_MIN_DAYTIME   = 0,
 	XT_TIME_MAX_DAYTIME   = 24 * 60 * 60 - 1,
 };
+
+#define XT_TIME_ALL_FLAGS (XT_TIME_LOCAL_TZ|XT_TIME_CONTIGUOUS)
 
 #endif /* _XT_TIME_H */

net/netfilter/xt_time.c

···
  */
 enum {
 	DSE_FIRST = 2039,
+	SECONDS_PER_DAY = 86400,
 };
 static const u_int16_t days_since_epoch[] = {
 	/* 2039 - 2030 */
···
 	unsigned int v, w;
 
 	/* Each day has 86400s, so finding the hour/minute is actually easy. */
-	v         = time % 86400;
+	v         = time % SECONDS_PER_DAY;
 	r->second = v % 60;
 	w         = v / 60;
 	r->minute = w % 60;
···
 		if (packet_time < info->daytime_start &&
 		    packet_time > info->daytime_stop)
 			return false;
+
+		/** if user asked to ignore 'next day', then e.g.
+		 *  '1 PM Wed, August 1st' should be treated
+		 *  like 'Tue 1 PM July 31st'.
+		 *
+		 * This also causes
+		 * 'Monday, "23:00 to 01:00", to match for 2 hours, starting
+		 * Monday 23:00 to Tuesday 01:00.
+		 */
+		if ((info->flags & XT_TIME_CONTIGUOUS) &&
+		     packet_time <= info->daytime_stop)
+			stamp -= SECONDS_PER_DAY;
 	}
 
 	localtime_2(&current_time, stamp);
···
 			"stop time greater than 23:59:59\n");
 		return -EDOM;
 	}
+
+	if (info->flags & ~XT_TIME_ALL_FLAGS) {
+		pr_info("unknown flags 0x%x\n", info->flags & ~XT_TIME_ALL_FLAGS);
+		return -EINVAL;
+	}
+
+	if ((info->flags & XT_TIME_CONTIGUOUS) &&
+	     info->daytime_start < info->daytime_stop)
+		return -EINVAL;
 
 	return 0;
 }