net/sched: netem: fix probability gaps in 4-state loss model

[ Upstream commit 732b463449fd0ef90acd13cda68eab1c91adb00c ]

The 4-state Markov chain in loss_4state() has gaps at the boundaries
between transition probability ranges. The comparisons use:

  if (rnd < a4)
  else if (a4 < rnd && rnd < a1 + a4)

When rnd equals a boundary value exactly, neither branch matches and
no state transition occurs. The redundant lower-bound check (a4 < rnd)
is already implied by being in the else branch.

Remove the unnecessary lower-bound comparisons so the ranges are
contiguous and every random value produces a transition, matching
the GI (General and Intuitive) loss model specification.

This bug goes back to original implementation of this model.

Fixes: 661b79725f ("netem: revised correlated loss generator")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260418032027.900913-2-stephen@networkplumber.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Ulrich Hecht <uli@kernel.org>

net/sched/sch_netem.c

@@ -219,10 +219,10 @@ static bool loss_4state(struct netem_sched_data *q)
 		if (rnd < clg->a4) {
 			clg->state = LOST_IN_GAP_PERIOD;
 			return true;
-		} else if (clg->a4 < rnd && rnd < clg->a1 + clg->a4) {
+		} else if (rnd < clg->a1 + clg->a4) {
 			clg->state = LOST_IN_BURST_PERIOD;
 			return true;
-		} else if (clg->a1 + clg->a4 < rnd) {
+		} else {
 			clg->state = TX_IN_GAP_PERIOD;
 		}
 
@@ -239,9 +239,9 @@ static bool loss_4state(struct netem_sched_data *q)
 	case LOST_IN_BURST_PERIOD:
 		if (rnd < clg->a3)
 			clg->state = TX_IN_BURST_PERIOD;
-		else if (clg->a3 < rnd && rnd < clg->a2 + clg->a3) {
+		else if (rnd < clg->a2 + clg->a3) {
 			clg->state = TX_IN_GAP_PERIOD;
-		} else if (clg->a2 + clg->a3 < rnd) {
+		} else {
 			clg->state = LOST_IN_BURST_PERIOD;
 			return true;
 		}