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

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Message-ID: <12040808.ISNuLM4SqW@tauon.atsec.com>
Date:	Tue, 21 Jun 2016 18:51:32 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Arnd Bergmann <arnd@...db.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

Am Dienstag, 21. Juni 2016, 09:47:23 schrieb John Stultz:

Hi John,

> On Tue, Jun 21, 2016 at 9:34 AM, Stephan Mueller <smueller@...onox.de> 
wrote:
> > Am Dienstag, 21. Juni 2016, 09:22:31 schrieb John Stultz:
> > 
> > Hi John,
> > 
> >> 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.
> > 
> > Funnily it does not seem like that. All tests that I have conducted show
> > that monotonic clocks behave equally as realtime clocks, because the
> > uncertainty lies in the execution time of a set of instructions. All we
> > need to do is to measure it with a timer that has a resolution that
> > allows detecting these variations.
> 
> Ok. If you're only using it for interval measurements, then either way
> shouldn't matter. I just wanted to make sure the entropy wasn't coming
> from the actual time.
> 
> >> > - "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.
> > 
> > Perfect, that is what I would be interested in.
> 
> But documenting *why* clearly is the thing I'd very strongly suggest.
> If we need to make some slight semantic change for whatever reason, I
> don't want folks worried "we can't do that because the crypto code is
> using it for voodoo".

I hope my explanation is sufficient to not count as voodoo: I only need an 
interval measurement capability which has a sufficient high resolution similar 
or better than RDTSC on x86.
> 
> >> > - "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
> > 
> > My code would not see that as an issue.
> > 
> >> 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 a lot. With that, I would think that the proposed
> > ktime_get_raw_fast_ns is good for use, which is supported with testing on
> > my system.
> 
> So.. again, I'd avoid using the "fast" accessor unless there is a
> clear need or obvious benefit. Which should be documented.

So, you suggest ktime_get_raw_ns? If yes, let me test that for one use case. 

Thanks
Stephan