bonding: refine bond_fold_stats() wrap detection

Some device drivers reset their stats at down/up events, possibly
fooling bonding stats, since they operate with relative deltas.

It is nearly not possible to fix drivers, since some of them compute the
tx/rx counters based on per rx/tx queue stats, and the queues can be
reconfigured (ethtool -L) between the down/up sequence.

Lets avoid accumulating 'negative' values that render bonding stats
useless.

It is better to lose small deltas, assuming the bonding stats are
fetched at a reasonable frequency.

Fixes: 5f0c5f7 ("bonding: make global bonding stats more reliable")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -3332,12 +3332,17 @@ static void bond_fold_stats(struct rtnl_link_stats64 *_res,
 	for (i = 0; i < sizeof(*_res) / sizeof(u64); i++) {
 		u64 nv = new[i];
 		u64 ov = old[i];
+		s64 delta = nv - ov;
 
 		/* detects if this particular field is 32bit only */
 		if (((nv | ov) >> 32) == 0)
-			res[i] += (u32)nv - (u32)ov;
-		else
-			res[i] += nv - ov;
+			delta = (s64)(s32)((u32)nv - (u32)ov);
+
+		/* filter anomalies, some drivers reset their stats
+		 * at down/up events.
+		 */
+		if (delta > 0)
+			res[i] += delta;
 	}
 }