Re: [PATCH] crypto: Jitter RNG - use ktime_get_raw_ns as fallback

On Tue, Jun 21, 2016 at 12:37 PM, Arnd Bergmann [off-list ref] wrote:

On Tuesday, June 21, 2016 12:05:06 PM CEST John Stultz wrote:

quoted

On Tue, Jun 21, 2016 at 11:49 AM, Stephan Mueller [off-list ref] wrote:

quoted

Am Dienstag, 21. Juni 2016, 11:11:42 schrieb John Stultz:

Hi John,

quoted

I don't see in the above an explanation of *why* you're using
ktime_get_raw_ns() instead of ktime_get_ns().

Could you help me understand what the difference is or point me to some
documentation? I understood that we only talked about the _raw variant.

Using specialized interfaces with subtle semantics w/o understanding
them is sort of my concern here.

There are reasons why you might want to use the ktime_get_raw_ns()
interface over ktime_get_ns(), but they have not been made clear in
the comment. Arnd discussed some potential concerns that the freq
adjustment done by ntp might be somewhat predictable/controlled by
remote parties, which could have some effect in the calculation. That
feels a little overly vague to me, but I'm no crypto expert, so if
that is a reasonable concern, then it should be a conscious and
documented decision.

My original patch changed __getnstimeofday() to __getnstimeofday64(),
which kept the original semantics of not warning in case the clock
source is suspended (which is the only different to the normal
getnstimeofday{,64}().

I did the patch a while time ago along with a number of other patches
that I never sent out until last week, so I don't remember the
reasoning for suggesting ktime_get_raw_fast_ns() over ktime_get_raw_ns(),
but I sure wanted to keep the non-warning behavior, and ktime_get_ns()
warns on timekeeping_suspended() while the other two don't.

Right. But this code isn't called in late suspend or early resume is it?

If we don't care about the non-warning aspect, ktime_get_ns() makes
most sense here, and the original code should probably have used
getnstimeofday() as well.

This is what I suspect as well.  But again, I don't mind if folks use
the specialized interfaces, as long as they document a clear reason
for it. Especially for things like crypto where intuition isn't always
the best guide.

thanks
-john
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