kike: add a trivial flood detector

kike.c

@@ -92,6 +92,49 @@ setup_signal_handlers (void)
 		exit_fatal ("%s: %s", "sigaction", strerror (errno));
 }
 
+// --- Rate limiter ------------------------------------------------------------
+
+struct flood_detector
+{
+	unsigned interval;                  ///< Interval for the limit
+	unsigned limit;                     ///< Maximum number of events allowed
+
+	time_t *timestamps;                 ///< Timestamps of last events
+	unsigned pos;                       ///< Index of the oldest event
+};
+
+static void
+flood_detector_init (struct flood_detector *self,
+	unsigned interval, unsigned limit)
+{
+	self->interval = interval;
+	self->limit = limit;
+	self->timestamps = xcalloc (limit + 1, sizeof *self->timestamps);
+	self->pos = 0;
+}
+
+static void
+flood_detector_free (struct flood_detector *self)
+{
+	free (self->timestamps);
+}
+
+static bool
+flood_detector_check (struct flood_detector *self)
+{
+	time_t now = time (NULL);
+	self->timestamps[self->pos++] = now;
+	if (self->pos > self->limit)
+		self->pos = 0;
+
+	time_t begin = now - self->interval;
+	size_t count = 0;
+	for (size_t i = 0; i <= self->limit; i++)
+		if (self->timestamps[i] >= begin)
+			count++;
+	return count <= self->limit;
+}
+
 // --- IRC token validation ----------------------------------------------------
 
 // Use the enum only if applicable and a simple boolean isn't sufficient.
@@ -277,6 +320,7 @@ struct client
 	unsigned mode;                      ///< User's mode
 	char *away_message;                 ///< Away message
 	time_t last_active;                 ///< Last PRIVMSG, to get idle time
+	struct flood_detector antiflood;    ///< Flood detector
 };
 
 static void
@@ -287,6 +331,8 @@ client_init (struct client *self)
 	self->socket_fd = -1;
 	str_init (&self->read_buffer);
 	str_init (&self->write_buffer);
+	// TODO: make this configurable and more fine-grained
+	flood_detector_init (&self->antiflood, 10, 20);
 }
 
 static void
@@ -306,6 +352,7 @@ client_free (struct client *self)
 
 	free (self->hostname);
 	free (self->away_message);
+	flood_detector_free (&self->antiflood);
 }
 
 static void client_close_link (struct client *, const char *);
@@ -2299,6 +2346,12 @@ irc_process_message (const struct irc_message *msg,
 	if (c->closing_link)
 		return;
 
+	if (!flood_detector_check (&c->antiflood))
+	{
+		client_close_link (c, "Excess flood");
+		return;
+	}
+
 	struct irc_command *cmd = str_map_find (&c->ctx->handlers, msg->command);
 	if (!cmd)
 		irc_send_reply (c, IRC_ERR_UNKNOWNCOMMAND, msg->command);