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[2a02:3100:b105:e101:a437:493a:4de9:ee3e]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-499542851d6sm192408105e9.12.2026.08.07.13.50.40 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Fri, 07 Aug 2026 13:50:41 -0700 (PDT) From: Karl Mehltretter To: Andrew Morton Cc: Andrey Konovalov , Dmitry Vyukov , Alexander Potapenko , Marco Elver , Bradley Morgan , Thomas Gleixner , Anna-Maria Behnsen , Frederic Weisbecker , Peter Zijlstra , Ingo Molnar , Juri Lelli , Vincent Guittot , Steven Rostedt , Sebastian Andrzej Siewior , Clark Williams , linux-rt-devel@lists.linux.dev, kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org Subject: [PATCH 0/5] kcov: suppress timer and scheduler coverage leaks Date: Fri, 7 Aug 2026 22:50:22 +0200 Message-Id: <20260807205027.31972-1-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit KCOV excludes interrupt and scheduler coverage so syscall coverage stays input-dependent. Instrumented callees can still record when uninstrumented timer and scheduler paths run with in_task() true. CONFIG_KCOV_SELFTEST [1] exposes three cases on x86-64: deferred hrtimer rearm, __schedule() callees and PREEMPT_RT wakeups. Task-context wakeups and new-task enqueue also add scheduler coverage to ordinary syscalls. Add a nestable KCOV_PAUSED bit. Use it around deferred hrtimer rearm, __schedule(), try_to_wake_up() and wake_up_new_task(). This keeps coverage from their instrumented callees without excluding those callees from real task-context coverage. Testing: - GCC builds across x86-64, arm32, arm64, MIPS32, PowerPC 32/64, s390, RISC-V 32/64, LoongArch, Xtensa and UML - x86-64 Clang and KCOV-disabled builds - KCOV selftest, 10/10 x86-64 boots with and without PREEMPT_RT - KCOV selftest, 3/3 RISC-V 32/64, LoongArch and s390 boots after isolating unrelated architecture entry leaks - 40 dummy_hcd/g_zero remote-KCOV cycles on x86-64 and arm64 - 400 repeated fork calls on x86-64 PREEMPT_RT - syzkaller: five one-hour A/B pairs on four 2-vCPU PREEMPT_RT VMs. At ~110k executions, the five-run median was 4,142 vs. 3,216 corpus entries (+28.8%) and 54,872 vs. 50,940 coverage (+7.7%). No unsuppressed reports The USB runs no longer contained the baseline PCs from deferred rearm, hrtick and scheduler wakeup callees. The fork run no longer contained the baseline __smp_call_single_queue(), generic_exec_single() or smp_call_function_single_async() PCs. With KCOV disabled, the pause calls compile away. With KCOV enabled on x86-64, __schedule() grows by 117 bytes across patches 2 and 3, try_to_wake_up() by 106 bytes and wake_up_new_task() by 88 bytes. [1] https://lore.kernel.org/r/20260724192122.73080-1-kmehltretter@gmail.com/ Karl Mehltretter (5): kcov: add kcov_pause()/kcov_resume() helpers hrtimer: pause KCOV during deferred rearm sched: pause KCOV in __schedule() sched: pause KCOV in try_to_wake_up() sched: pause KCOV in wake_up_new_task() include/linux/hrtimer_rearm.h | 18 ++++++++++++++++-- include/linux/kcov.h | 25 ++++++++++++++++++++++++- kernel/kcov.c | 2 +- kernel/sched/core.c | 15 ++++++++++++++- 4 files changed, 55 insertions(+), 5 deletions(-) base-commit: 8ba098e6b6ff0db8edf28528d1552be261af30d4