From: Kunwu Chan <kunwu.chan@gmail.com>
To: dave@stgolabs.net, paulmck@kernel.org, josh@joshtriplett.org,
frederic@kernel.org, neeraj.upadhyay@kernel.org,
joelagnelf@nvidia.com, boqun@kernel.org, urezki@gmail.com,
rostedt@goodmis.org, mathieu.desnoyers@efficios.com,
jiangshanlai@gmail.com, qiang.zhang@linux.dev
Cc: linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
Kunwu Chan <kunwu.chan@gmail.com>
Subject: [PATCH] rcutorture: Fix divide-by-zero with fwd_progress_div=1
Date: Thu, 20 Aug 2026 17:42:28 +0800 [thread overview]
Message-ID: <20260820094228.540304-1-kunwu.chan@linux.dev> (raw)
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>
---
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 54bec8e21e71..ed2407e888b4 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -3990,7 +3990,7 @@ 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);
base-commit: c68271c3f83a46de3fca48d3aa9e27c4dec8e58a
--
2.43.0
reply other threads:[~2026-08-20 9:42 UTC|newest]
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