From: Felix Fietkau <nbd@nbd.name>
Date: Sat, 28 May 2022 16:44:53 +0200
Subject: [PATCH] mac80211: fix overflow issues in airtime fairness code

The airtime weight calculation overflows with a default weight value of 256
whenever more than 8ms worth of airtime is reported.
Bigger weight values impose even smaller limits on maximum airtime values.
This can mess up airtime based calculations for drivers that don't report
per-PPDU airtime values, but batch up values instead.

Fix this by reordering multiplications/shifts and by reducing unnecessary
intermediate precision (which was lost in a later stage anyway).

The new shift value limits the maximum weight to 4096, which should be more
than enough. Any values bigger than that will be clamped to the upper limit.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
---

--- a/net/mac80211/ieee80211_i.h
+++ b/net/mac80211/ieee80211_i.h
@@ -1666,50 +1666,34 @@ static inline struct airtime_info *to_ai
 /* To avoid divisions in the fast path, we keep pre-computed reciprocals for
  * airtime weight calculations. There are two different weights to keep track
  * of: The per-station weight and the sum of weights per phy.
- *
- * For the per-station weights (kept in airtime_info below), we use 32-bit
- * reciprocals with a devisor of 2^19. This lets us keep the multiplications and
- * divisions for the station weights as 32-bit operations at the cost of a bit
- * of rounding error for high weights; but the choice of divisor keeps rounding
- * errors <10% for weights <2^15, assuming no more than 8ms of airtime is
- * reported at a time.
- *
- * For the per-phy sum of weights the values can get higher, so we use 64-bit
- * operations for those with a 32-bit divisor, which should avoid any
- * significant rounding errors.
+ * The per-sta shift value supports weight values of 1-4096
  */
-#define IEEE80211_RECIPROCAL_DIVISOR_64 0x100000000ULL
-#define IEEE80211_RECIPROCAL_SHIFT_64 32
-#define IEEE80211_RECIPROCAL_DIVISOR_32 0x80000U
-#define IEEE80211_RECIPROCAL_SHIFT_32 19
+#define IEEE80211_RECIPROCAL_SHIFT_SUM	24
+#define IEEE80211_RECIPROCAL_SHIFT_STA	12
+#define IEEE80211_WEIGHT_SHIFT		8
 
-static inline void airtime_weight_set(struct airtime_info *air_info, u16 weight)
+static inline void airtime_weight_set(struct airtime_info *air_info, u32 weight)
 {
+	weight = min_t(u32, weight, BIT(IEEE80211_RECIPROCAL_SHIFT_STA));
 	if (air_info->weight == weight)
 		return;
 
 	air_info->weight = weight;
-	if (weight) {
-		air_info->weight_reciprocal =
-			IEEE80211_RECIPROCAL_DIVISOR_32 / weight;
-	} else {
-		air_info->weight_reciprocal = 0;
-	}
+	if (weight)
+		weight = BIT(IEEE80211_RECIPROCAL_SHIFT_STA) / weight;
+	air_info->weight_reciprocal = weight;
 }
 
 static inline void airtime_weight_sum_set(struct airtime_sched_info *air_sched,
-					  int weight_sum)
+					  u32 weight_sum)
 {
 	if (air_sched->weight_sum == weight_sum)
 		return;
 
 	air_sched->weight_sum = weight_sum;
-	if (air_sched->weight_sum) {
-		air_sched->weight_sum_reciprocal = IEEE80211_RECIPROCAL_DIVISOR_64;
-		do_div(air_sched->weight_sum_reciprocal, air_sched->weight_sum);
-	} else {
-		air_sched->weight_sum_reciprocal = 0;
-	}
+	if (weight_sum)
+		weight_sum = BIT(IEEE80211_RECIPROCAL_SHIFT_SUM) / weight_sum;
+	air_sched->weight_sum_reciprocal = weight_sum;
 }
 
 /* A problem when trying to enforce airtime fairness is that we want to divide
--- a/net/mac80211/sta_info.c
+++ b/net/mac80211/sta_info.c
@@ -1894,9 +1894,9 @@ void ieee80211_register_airtime(struct i
 {
 	struct ieee80211_sub_if_data *sdata = vif_to_sdata(txq->vif);
 	struct ieee80211_local *local = sdata->local;
-	u64 weight_sum, weight_sum_reciprocal;
 	struct airtime_sched_info *air_sched;
 	struct airtime_info *air_info;
+	u64 weight_sum_reciprocal;
 	u32 airtime = 0;
 
 	air_sched = &local->airtime[txq->ac];
@@ -1907,27 +1907,21 @@ void ieee80211_register_airtime(struct i
 	if (local->airtime_flags & AIRTIME_USE_RX)
 		airtime += rx_airtime;
 
-	/* Weights scale so the unit weight is 256 */
-	airtime <<= 8;
-
 	spin_lock_bh(&air_sched->lock);
 
 	air_info->tx_airtime += tx_airtime;
 	air_info->rx_airtime += rx_airtime;
 
-	if (air_sched->weight_sum) {
-		weight_sum = air_sched->weight_sum;
+	if (air_sched->weight_sum)
 		weight_sum_reciprocal = air_sched->weight_sum_reciprocal;
-	} else {
-		weight_sum = air_info->weight;
+	else
 		weight_sum_reciprocal = air_info->weight_reciprocal;
-	}
 
 	/* Round the calculation of global vt */
-	air_sched->v_t += (u64)((airtime + (weight_sum >> 1)) *
-				weight_sum_reciprocal) >> IEEE80211_RECIPROCAL_SHIFT_64;
-	air_info->v_t += (u32)((airtime + (air_info->weight >> 1)) *
-			       air_info->weight_reciprocal) >> IEEE80211_RECIPROCAL_SHIFT_32;
+	air_sched->v_t += ((u64)airtime * weight_sum_reciprocal) >>
+			  (IEEE80211_RECIPROCAL_SHIFT_SUM - IEEE80211_WEIGHT_SHIFT);
+	air_info->v_t += (airtime * air_info->weight_reciprocal) >>
+			 (IEEE80211_RECIPROCAL_SHIFT_STA - IEEE80211_WEIGHT_SHIFT);
 	ieee80211_resort_txq(&local->hw, txq);
 
 	spin_unlock_bh(&air_sched->lock);