linux-kernel - Re: [Y2038] [PATCH] crypto: use timespec64 for jent_get

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Date:	Tue, 21 Jun 2016 09:22:31 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Stephan Mueller <smueller@...onox.de>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Alexander Kuleshov <kuleshovmail@...il.com>,
	y2038 Mailman List <y2038@...ts.linaro.org>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [Y2038] [PATCH] crypto: use timespec64 for jent_get_nstime

On Tue, Jun 21, 2016 at 1:32 AM, Arnd Bergmann <arnd@...db.de> wrote:
> On Tuesday, June 21, 2016 8:20:10 AM CEST Stephan Mueller wrote:
>> Am Freitag, 17. Juni 2016, 17:59:41 schrieb Arnd Bergmann:
>>
> Compared to the previous __getnstimeofday(), the difference is
>
> - using "monotonic" timebase instead of "real", so the zero time
>   is when the system booted rather than Jan 1 1970

I haven't looked at the details of the calling code, but I'd worry for
crypto uses, especially if its being used for entropy collection,
using the monotonic clock instead of the realtime clock might be
problematic.

> - "raw" means we don't honor updates for the rate based on ntp,
>   which is probably better as the ntp state might be observable
>   over the net (it probably doesn't matter, but it can't hurt)

So... this feels like a very vague explanation, and the lack of
frequency correction here probably need a really good comment. Keeping
multiple time domains is usually asking for trouble, but we added the
MONOTONIC_RAW clock to address a few cases where people really wanted
an abstract hardware counter, which was unaffected by frequency
corrections. I'd really make sure its clear why this is what you want
vs the standard system time domain so we don't run into problems
understanding it later.

> - "fast" means that in very rare cases, the time might appear
>   to go backwards (it probably can't happen here because you are not
>   called in an NMI).

"fast" really means "safe-for-nmi wrt to locking".  The tradeoff being
that when frequency adjustments occur, and if your code is delayed,
you might see time go backwards by a small amount. This allows
tracing/sched code (or other code called from NMI)  to not have to
duplicate the timekeeping infrastructure.

I think without a much better explanation, using the "fast" method
isn't really warranted here.

thanks
-john