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From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
        arnd@arndb.de
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Theodore Ts'o <tytso@mit.edu>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
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        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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        "H . Peter Anvin" <hpa@zytor.com>, Chris Zankel <chris@zankel.net>,
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        John Stultz <john.stultz@linaro.org>,
        Stephen Boyd <sboyd@kernel.org>,
        Dinh Nguyen <dinguyen@kernel.org>,
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Subject: Re: [PATCH v2 03/11] m68k: use ktime_read_raw_clock() for
 random_get_entropy() instead of zero
In-Reply-To: <20220410214951.55294-4-Jason@zx2c4.com>
References: <20220410214951.55294-1-Jason@zx2c4.com>
 <20220410214951.55294-4-Jason@zx2c4.com>
Date:   Mon, 11 Apr 2022 10:18:30 +0200
Message-ID: <87sfqkf2y1.ffs@tglx>
MIME-Version: 1.0
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Precedence: bulk

On Sun, Apr 10 2022 at 23:49, 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 ktime_read_raw_clock() would be preferable,
> because that always needs to return _something_, even falling back to
> jiffies eventually. It's not as though ktime_read_raw_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.
>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  arch/m68k/include/asm/timex.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/m68k/include/asm/timex.h b/arch/m68k/include/asm/timex.h
> index 6a21d9358280..5351b10e1b18 100644
> --- a/arch/m68k/include/asm/timex.h
> +++ b/arch/m68k/include/asm/timex.h
> @@ -35,7 +35,7 @@ static inline unsigned long random_get_entropy(void)
>  {
>  	if (mach_random_get_entropy)
>  		return mach_random_get_entropy();
> -	return 0;
> +	return ktime_read_raw_clock();

I'd rather do something like this in a common header:

unsigned long random_get_entropy_fallback(void);

and use random_get_entropy_fallback() in the architecture specific
files.

That way you can encapsulate the fallback implementation in the random
code and if it turns out that ktime_read_raw_clock() is a stupid idea or
someone has a better idea then you have to change exactly one place and
not patch the whole tree again.

Thanks,

        tglx