linux-kernel - Re: [PATCH] kcov: remove stale RANDOMIZE

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Message-ID: <CACT4Y+avSyKoW-i=GzDT0iAi7SnQg21BanNK1GjkQsKKBUCKNg@mail.gmail.com>
Date:   Mon, 4 Dec 2023 18:58:35 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        alex.popov@...ux.com, andreyknvl@...il.com, bp@...en8.de
Subject: Re: [PATCH] kcov: remove stale RANDOMIZE_BASE text

On Mon, 4 Dec 2023 at 18:18, Mark Rutland <mark.rutland@....com> wrote:
>
> The Kconfig help text for CONFIG_KCOV describes that recorded PC values
> will not be stable across machines or reboots when RANDOMIZE_BASE is
> selected. This was the case when KCOV was introduced in commit:
>
>   5c9a8750a6409c63 ("kernel: add kcov code coverage")
>
> However, this changed in commit:
>
>   4983f0ab7ffaad1e ("kcov: make kcov work properly with KASLR enabled")
>
> Since that commit KCOV always subtracts the KASLR offset from PC values,
> which ensures that these are stable across machines and across reboots
> even when RANDOMIZE_BASE is selected.
>
> Unfortunately, that commit failed to update the Kconfig help text, which
> still suggests disabling RANDOMIZE_BASE even though this is no longer
> necessary.
>
> Remove the stale Kconfig text.
>
> Reported-by: Borislav Petkov <bp@...en8.de>
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Alexander Popov <alex.popov@...ux.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Andrey Konovalov <andreyknvl@...il.com>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>

Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>

Thanks for fixing this.

> ---
>  lib/Kconfig.debug | 4 ----
>  1 file changed, 4 deletions(-)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index cc7d53d9dc019..e77873cf85c78 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2103,10 +2103,6 @@ config KCOV
>           KCOV exposes kernel code coverage information in a form suitable
>           for coverage-guided fuzzing (randomized testing).
>
> -         If RANDOMIZE_BASE is enabled, PC values will not be stable across
> -         different machines and across reboots. If you need stable PC values,
> -         disable RANDOMIZE_BASE.
> -
>           For more details, see Documentation/dev-tools/kcov.rst.
>
>  config KCOV_ENABLE_COMPARISONS
> --
> 2.30.2
>