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Thu, 20 Aug 2026 02:42:44 -0700 (PDT) Received: from kernel.tail6741c6.ts.net ([116.128.244.169]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-395adaf52b7sm494590a91.0.2026.08.20.02.42.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 20 Aug 2026 02:42:43 -0700 (PDT) From: Kunwu Chan X-Google-Original-From: Kunwu Chan 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 Subject: [PATCH] rcutorture: Fix divide-by-zero with fwd_progress_div=1 Date: Thu, 20 Aug 2026 17:42:28 +0800 Message-ID: <20260820094228.540304-1-kunwu.chan@linux.dev> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Kunwu Chan 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 --- 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