linux-kernel - Re: [PATCH 3/7] kcov: x86: introduce CONFIG_KCOV_ENABLE

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Message-ID: <CAG_fn=UBVzq3V4EHQ94zOUwdFLd_awwkQUPLb5XjnMmgBoXpgg@mail.gmail.com>
Date: Thu, 24 Apr 2025 15:58:28 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Marco Elver <elver@...gle.com>
Cc: quic_jiangenj@...cinc.com, linux-kernel@...r.kernel.org, 
	kasan-dev@...glegroups.com, x86@...nel.org, 
	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>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/7] kcov: x86: introduce CONFIG_KCOV_ENABLE_GUARDS

> > --- a/arch/x86/kernel/vmlinux.lds.S
> > +++ b/arch/x86/kernel/vmlinux.lds.S
> > @@ -390,6 +390,7 @@ SECTIONS
> >                 . = ALIGN(PAGE_SIZE);
> >                 __bss_stop = .;
> >         }
> > +       SANCOV_GUARDS_BSS
>
> Right now this will be broken on other architectures, right?

Right. I'm going to make it depend on X86_64 for now.


> > - * Entry point from instrumented code.
> > - * This is called once per basic-block/edge.
> > - */
> > -void notrace __sanitizer_cov_trace_pc(void)
> > +static void sanitizer_cov_write_subsequent(unsigned long *area, int size,
>
> notrace is missing.
Ack.
> Can we give this a more descriptive name? E.g. "kcov_append" ?
I'll rename it to kcov_append_to_buffer().


> > +
> > +/*
> > + * Entry point from instrumented code.
> > + * This is called once per basic-block/edge.
> > + */
> > +#ifndef CONFIG_KCOV_ENABLE_GUARDS
>
> Negation makes it harder to read - just #ifdef, and swap the branches below.

I thought I'd better keep the default hook above, but maybe you are right.
Will do in v2.


> >
> > +config KCOV_ENABLE_GUARDS
>
> The "ENABLE" here seems redundant.
> Just KCOV_GUARDS should be clear enough.

I am already renaming this config to KCOV_UNIQUE per Dmitry's request :)

>
> > +       depends on KCOV
> > +       depends on CC_HAS_SANCOV_TRACE_PC_GUARD
> > +       bool "Use fsanitize-coverage=trace-pc-guard for kcov"
>
> The compiler option is an implementation detail - it might be more
> helpful to have this say "Use coverage guards for kcov".

Ack.

> > --- a/scripts/Makefile.kcov
> > +++ b/scripts/Makefile.kcov
> > @@ -1,5 +1,9 @@
> >  # SPDX-License-Identifier: GPL-2.0-only
> > +ifeq ($(CONFIG_KCOV_ENABLE_GUARDS),y)
> > +kcov-flags-$(CONFIG_CC_HAS_SANCOV_TRACE_PC_GUARD) += -fsanitize-coverage=trace-pc-guard
>
> This can just be kcov-flags-y, because CONFIG_KCOV_ENABLE_GUARDS
> implies CONFIG_CC_HAS_SANCOV_TRACE_PC_GUARD.
>

Agreed.