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[2a02:3100:b262:3601:d0d3:269d:11e6:9b86]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-45eb6d47b82sm34902217f8f.19.2026.05.26.04.47.16 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Tue, 26 May 2026 04:47:16 -0700 (PDT) From: Karl Mehltretter To: Dmitry Vyukov Cc: Andrey Konovalov , Marco Elver , Andrew Morton , Peter Zijlstra , Kees Cook , kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org, Karl Mehltretter Subject: [PATCH] kcov: Use WRITE_ONCE() for selftest mode stores Date: Tue, 26 May 2026 13:47:15 +0200 Message-Id: <20260526114715.38280-1-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: quoted-printable Content-Type: text/plain; charset="utf-8" The KCOV selftest enables coverage by setting current->kcov_mode to KCOV_MODE_TRACE_PC without installing a coverage area. If an interrupt records coverage in that window, the access should fault and expose the bug. When building for QEMU raspi0 (Raspberry Pi Zero, ARMv6, CONFIG_CPU_V6K=3Dy, CONFIG_CURRENT_POINTER_IN_TPIDRURO=3Dy) with GCC 13.3.0, the store that enables the mode is removed. The generated kcov_init() code only stores zero after the wait loop: mrc 15, 0, r3, cr13, cr0, {3} str r4, [r3, #2028] where r4 is zero. There is no store of KCOV_MODE_TRACE_PC before the loop, so the selftest reports success without exercising coverage. Use WRITE_ONCE() for the temporary mode stores. With the same compiler and config, kcov_init() contains the intended mode store: mov r3, #2 mrc 15, 0, r2, cr13, cr0, {3} str r3, [r2, #2028] Now that the KCOV selftest is actually executed, it may expose KCOV instrumentation issues depending on the kernel config. That is expected for a selftest that was intended to catch coverage from interrupt paths. Fixes: 6cd0dd934b03 ("kcov: Add interrupt handling self test") Assisted-by: Codex:gpt-5 Signed-off-by: Karl Mehltretter Reviewed-by: Alexander Potapenko --- Reviewer note: This is related to my pending trace_irqsoff.o KCOV report. Once the KCOV selftest actually enables coverage, that trace_irqsoff.o KCOV issue can become visible depending on the kernel config. That issue is separate from this patch; this patch only fixes the selftest false negative. https://lore.kernel.org/r/20260525170428.67211-1-kmehltretter@gmail.com kernel/kcov.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/kcov.c b/kernel/kcov.c index 0b369e88c7c9..f3a5ae960ce5 100644 --- a/kernel/kcov.c +++ b/kernel/kcov.c @@ -1109,10 +1109,10 @@ static void __init selftest(void) * potentially traced functions in this region. */ start =3D jiffies; - current->kcov_mode =3D KCOV_MODE_TRACE_PC; + WRITE_ONCE(current->kcov_mode, KCOV_MODE_TRACE_PC); while ((jiffies - start) * MSEC_PER_SEC / HZ < 300) ; - current->kcov_mode =3D 0; + WRITE_ONCE(current->kcov_mode, 0); pr_err("done running self test\n"); } #endif base-commit: e8c2f9fdadee7cbc75134dc463c1e0d856d6e5c7 --=20 2.39.5 (Apple Git-154)