linux-kernel - Re: [PATCH v2 01/11] x86: kcov: disable instrumentation of arch/x86/kernel/tsc.c

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Message-ID: <CAG_fn=XvYNkRp00A_BwL4xRn5hTFcGmvJw=M0XU1rWPMWEZNjA@mail.gmail.com>
Date: Fri, 27 Jun 2025 12:51:47 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: quic_jiangenj@...cinc.com, linux-kernel@...r.kernel.org, 
	kasan-dev@...glegroups.com, Aleksandr Nogikh <nogikh@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Ingo Molnar <mingo@...hat.com>, Josh Poimboeuf <jpoimboe@...nel.org>, Marco Elver <elver@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 01/11] x86: kcov: disable instrumentation of arch/x86/kernel/tsc.c

On Fri, Jun 27, 2025 at 9:59 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Jun 26, 2025 at 03:41:48PM +0200, Alexander Potapenko wrote:
> > sched_clock() appears to be called from interrupts, producing spurious
> > coverage, as reported by CONFIG_KCOV_SELFTEST:
>
> NMI context even. But I'm not sure how this leads to problems. What does
> spurious coverage even mean?

This leads to KCOV collecting slightly different coverage when
executing the same syscall multiple times.
For syzkaller that means higher chance to pick a less interesting
input incorrectly assuming it produced some new coverage.

There's a similar discussion at
https://lore.kernel.org/all/20240619111936.GK31592@noisy.programming.kicks-ass.net/T/#u