[PATCH AUTOSEL for 4.9 087/219] bonding: refine bond_fold_stats() wrap detection

[Sasha Levin]

From: Eric Dumazet <edumazet@xxxxxxxxxx>

[ Upstream commit 142c6594acbcc32391af9c15f8cd65c6c177698f ]

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: 5f0c5f73e5ef ("bonding: make global bonding stats more reliable")
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
---
 drivers/net/bonding/bond_main.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 6b4b43319ffa..c3f3096b24ae 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -3331,12 +3331,17 @@ 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;
 	}
 }
 
-- 
2.14.1