linux-kernel - Re: [PATCH v4 01/11] timekeeping: add raw clock fallback for random_get

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Message-ID: <CAL_JsqJYq5Oe_zBbcwYNMpfpqGLGCyaSfGqOrPjZ_Pj=nF73mA@mail.gmail.com>
Date:   Wed, 13 Apr 2022 09:32:34 -0500
From:   Rob Herring <robh@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>, "Theodore Ts'o" <tytso@....edu>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "David S . Miller" <davem@...emloft.net>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Dinh Nguyen <dinguyen@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-m68k@...ts.linux-m68k.org,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        linux-um@...ts.infradead.org, X86 ML <x86@...nel.org>,
        linux-xtensa@...ux-xtensa.org
Subject: Re: [PATCH v4 01/11] timekeeping: add raw clock fallback for random_get_entropy()

On Wed, Apr 13, 2022 at 6:55 AM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> The addition of random_get_entropy_fallback() provides access to
> whichever time source has the highest frequency, which is useful for
> gathering entropy on platforms without available cycle counters. It's
> not necessarily as good as being able to quickly access a cycle counter
> that the CPU has, but it's still something, even when it falls back to
> being jiffies-based.
>
> In the event that a given arch does not define get_cycles(), falling
> back to the get_cycles() default implementation that returns 0 is really
> not the best we can do. Instead, at least calling
> random_get_entropy_fallback() would be preferable, because that always
> needs to return _something_, even falling back to jiffies eventually.
> It's not as though random_get_entropy_fallback() is super high precision
> or guaranteed to be entropic, but basically anything that's not zero all
> the time is better than returning zero all the time.
>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Theodore Ts'o <tytso@....edu>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  include/linux/timex.h     |  8 ++++++++
>  kernel/time/timekeeping.c | 10 ++++++++++
>  2 files changed, 18 insertions(+)
>
> diff --git a/include/linux/timex.h b/include/linux/timex.h
> index 5745c90c8800..fbbe34226044 100644
> --- a/include/linux/timex.h
> +++ b/include/linux/timex.h
> @@ -62,6 +62,8 @@
>  #include <linux/types.h>
>  #include <linux/param.h>
>
> +extern unsigned long random_get_entropy_fallback(void);
> +
>  #include <asm/timex.h>
>
>  #ifndef random_get_entropy
> @@ -74,8 +76,14 @@
>   *
>   * By default we use get_cycles() for this purpose, but individual
>   * architectures may override this in their asm/timex.h header file.
> + * If a given arch does not have get_cycles(), then we fallback to

'does not have a usable get_cycles(), ...' as clearly some arches have
get_cycles() and yet still need a fallback.

Why not handle the 'if get_cycles() returns 0 do the fallback' within
a weak random_get_entropy() function? Then more arches don't need any
random_get_entropy() implementation.

Rob