[PATCH net] net_sched: sch_fq: fix pacing delay underflow with pacing offload

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

From: Eric Dumazet <edumazet@google.com>

When pacing offload is enabled (q->offload_horizon > 0),
FQ can dequeue packets early (now < f->time_next_packet).

In this case, the drift calculation (now - f->time_next_packet)
underflows to a large unsigned value.

min(len/2, now - f->time_next_packet) then evaluates to len/2,
incorrectly halving the pacing delay for the next packet.

Fix this by only applying drift compensation if now > f->time_next_packet.

This bug was triggered when flow_max_rate was set on the qdisc
or for non EDT packets (packets with a zero skb->tstamp).

Fixes: f26080d47007 ("net_sched: sch_fq: add the ability to offload pacing")
Reported-by: Willem de Bruijn <willemb@google.com>
Closes: https://lore.kernel.org/netdev/CANn89iK6O7ujR9zCJzd04MNLQoDi3mA+HWsR-hgQWYzLS3gZfw@mail.gmail.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 net/sched/sch_fq.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index 7cae082a9847..4b5f6d896c6d 100644
--- a/net/sched/sch_fq.c
+++ b/net/sched/sch_fq.c
@@ -828,8 +828,12 @@ static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 		 * f->time_next_packet was set when prior packet was sent,
 		 * and current time (@now) can be too late by tens of us.
 		 */
-		if (f->time_next_packet)
-			len -= min(len/2, now - f->time_next_packet);
+		if (f->time_next_packet) {
+			s64 drift = now - f->time_next_packet;
+
+			if (drift > 0)
+				len -= min_t(u64, len / 2, drift);
+		}
 		f->time_next_packet = now + len;
 	}
 out:
-- 
2.55.0.766.g2966f0265a-goog