linux-kernel - Re: [PATCH v4 03/11] riscv: use fallback for random_get

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Date:   Wed, 13 Apr 2022 09:40:13 -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 03/11] riscv: use fallback for random_get_entropy()
 instead of zero

On Wed, Apr 13, 2022 at 6:56 AM Jason A. Donenfeld <Jason@...c4.com> 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 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: Paul Walmsley <paul.walmsley@...ive.com>
> Cc: Palmer Dabbelt <palmer@...belt.com>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  arch/riscv/include/asm/timex.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/timex.h b/arch/riscv/include/asm/timex.h
> index 507cae273bc6..d6a7428f6248 100644
> --- a/arch/riscv/include/asm/timex.h
> +++ b/arch/riscv/include/asm/timex.h
> @@ -41,7 +41,7 @@ static inline u32 get_cycles_hi(void)
>  static inline unsigned long random_get_entropy(void)
>  {
>         if (unlikely(clint_time_val == NULL))

Moving this check to get_cycles() implementation would eliminate the
RiscV implementation of random_get_entropy() if you follow my other
suggestion.

I guess there's some advantage to skipping a NULL check every time for
get_cycles(), but really the register read time will be much slower
than an added check.

> -               return 0;
> +               return random_get_entropy_fallback();
>         return get_cycles();
>  }
>  #define random_get_entropy()   random_get_entropy()