linux-kernel - Re: [PATCH RFC v1 07/10] arm64: use sched_clock() for random_get

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Date:   Fri, 8 Apr 2022 20:40:08 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        linux-arm-kernel@...ts.infradead.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-riscv@...ts.infradead.org, sparclinux@...r.kernel.org,
        linux-um@...ts.infradead.org, x86@...nel.org,
        linux-xtensa@...ux-xtensa.org
Subject: Re: [PATCH RFC v1 07/10] arm64: use sched_clock() for
 random_get_entropy() instead of zero

Hi Mark,

On Fri, Apr 08, 2022 at 07:33:53PM +0100, Mark Rutland wrote:
> On Fri, Apr 08, 2022 at 08:21:42PM +0200, Jason A. Donenfeld wrote:
> > In the event that random_get_entropy() can't access a cycle counter or
> > similar, falling back to returning 0 is really not the best we can do.
> > Instead, at least calling sched_clock() would be preferable, because
> > that always needs to return _something_, even falling back to jiffies
> > eventually. It's not as though sched_clock() 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.
> > 
> > If CONFIG_ARM_ARCH_TIMER=n, then get_cycles() will return 0, so we only
> > need the fallback code for that case.
> 
> In arch/arm64/Kconfig we unconditionally select CONFIG_ARM_ARCH_TIMER,
> so that configuration shouldn't be possible, and I don't think this
> patch is necessary.
> 
> On arm64 we depend on the architected timer in a bunch of places, so
> anyone hacking that out has bigger problems.

Thanks for the tip. I'll drop this patch from the series.

Jason