[PATCH 1/3] rcutorture: Fix divide-by-zero with fwd_progress_div=1

["Paul E. McKenney" <paulmck@kernel.org>]

From: Kunwu Chan <kunwu.chan@gmail.com>

When fwd_progress_div=1, the forward-progress test computes:
  sd4 = (sd + div - 1) / div = sd
  dur = sd4 + torture_random(&trs) % (sd - sd4) = sd4 + % 0

The modulo operation with a zero divisor triggers an integer
division by zero (undefined behavior at the C level, #DE trap on x86),
causing a kernel Oops and panic. On x86_64, this manifests as:

  rcu_torture_fwd_prog_nr: Starting forward-progress test 0
  Oops: divide error: 0000 [#1] SMP PTI
  RIP: 0010:rcu_torture_fwd_prog+0x90b/0x1160
  R12: 0000000000000000

The existing guard only handles non-positive values. However,
fwd_progress_div=1 also makes the random range empty because
sd4 == sd.

Change the guard to reject values below 2. The forward-progress test
only reaches this calculation when stall_dur() is positive, so
sd = stall_dur() + 1 >= 2. For fwd_progress_div >= 2, sd4 < sd,
ensuring that sd - sd4 is at least 1.

Keep the existing fallback to the default value of 4 for invalid
values.

Verified with QEMU/KVM: a 138-second run with fwd_progress_div=1
completed 81 forward-progress test cycles without a crash.

Fixes: 1b27291b1ea4f ("rcutorture: Add forward-progress tests for RCU grace periods")
Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/rcutorture.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 182d47975efd..79807475b672 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -4024,7 +4024,7 @@ static int __init rcu_torture_fwd_prog_init(void)
 	}
 	if (fwd_progress_holdoff <= 0)
 		fwd_progress_holdoff = 1;
-	if (fwd_progress_div <= 0)
+	if (fwd_progress_div < 2)
 		fwd_progress_div = 4;
 	rfp = kzalloc_objs(*rfp, fwd_progress);
 	fwd_prog_tasks = kzalloc_objs(*fwd_prog_tasks, fwd_progress);
-- 
2.40.1