Re: [PATCH v2 03/11] m68k: use ktime_read_raw_clock() for random_get_entropy() instead of zero

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>,
	"David S . Miller" <davem@davemloft.net>,
	Richard Weinberger <richard@nod.at>,
	Anton Ivanov <anton.ivanov@cambridgegreys.com>,
	Johannes Berg <johannes@sipsolutions.net>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Chris Zankel <chris@zankel.net>,
	Max Filippov <jcmvbkbc@gmail.com>,
	John Stultz <john.stultz@linaro.org>,
	Stephen Boyd <sboyd@kernel.org>,
	Dinh Nguyen <dinguyen@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org,
	linux-riscv@lists.infradead.org, sparclinux@vger.kernel.org,
	linux-um@lists.infradead.org, x86@kernel.org,
	linux-xtensa@linux-xtensa.org
Subject: Re: [PATCH v2 03/11] m68k: use ktime_read_raw_clock() for random_get_entropy() instead of zero
Date: Mon, 11 Apr 2022 10:18:30 +0200	[thread overview]
Message-ID: <87sfqkf2y1.ffs@tglx> (raw)
In-Reply-To: <20220410214951.55294-4-Jason@zx2c4.com>

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