linux-kernel - Re: [PATCH v2 06/11] kcov: x86: introduce CONFIG_KCOV

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Message-ID: <CAG_fn=UrOBF=hQ5y6VN9VuA67GeVOyaaWtrnaSLz4TnC7u1fiw@mail.gmail.com>
Date: Fri, 27 Jun 2025 16:24:36 +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, 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>, Marco Elver <elver@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 06/11] kcov: x86: introduce CONFIG_KCOV_UNIQUE

On Fri, Jun 27, 2025 at 10:11 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Jun 26, 2025 at 03:41:53PM +0200, Alexander Potapenko wrote:
> > The new config switches coverage instrumentation to using
> >   __sanitizer_cov_trace_pc_guard(u32 *guard)
> > instead of
> >   __sanitizer_cov_trace_pc(void)
> >
> > This relies on Clang's -fsanitize-coverage=trace-pc-guard flag [1].
> >
> > Each callback receives a unique 32-bit guard variable residing in the
> > __sancov_guards section. Those guards can be used by kcov to deduplicate
> > the coverage on the fly.
>
> This sounds like a *LOT* of data; how big is this for a typical kernel
> build?

I have a 1.6Gb sized vmlinux, which has a .text section of 176Mb.
There are 1809419 calls to __sanitizer_cov_trace_pc_guard, and the
__sancov_guards section has a size of 6Mb, which are only allocated at
runtime.

If we take a vmlinux image from syzbot (e.g.
https://storage.googleapis.com/syzbot-assets/dadedf20b2e3/vmlinux-67a99386.xz),
its .text section is 166Mb, and there are 1893023 calls to
__sanitizer_cov_trace_pc, which will translate to exactly the same
number of __sanitizer_cov_trace_pc_guard, if we apply the unique
coverage instrumentation.