Re: [PATCH RFC v1 00/10] archs/random: fallback to using sched_clock() if no cycle counter

[Jason A. Donenfeld]

Hi Eric,

On 4/11/22, Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> On Fri, Apr 08, 2022 at 08:21:35PM +0200, Jason A. Donenfeld wrote:
>> By my first guess, we have ktime_get_boottime_ns(), jiffies, and
>> sched_clock(). It seems like sched_clock() has already done a lot of
>> work in being always available with some incrementing value, falling
>> back to jiffies as necessary. So this series goes with that as a
>> fallback, for when the architecture doesn't define random_get_entropy in
>> its own way and when there's no working cycle counter.
>
> Won't this interact badly with how try_to_generate_entropy() (a.k.a. the
> "Linus
> Jitter Dance") detects the presence of an appropriate timer currently?
>
>         stack.cycles = random_get_entropy();
>
>         /* Slow counter - or none. Don't even bother */
>         if (stack.cycles == random_get_entropy())
>                 return;
>
> So if random_get_entropy() always returns 0, then try_to_generate_entropy()
> won't run.  However, if random_get_entropy() is even just a low-precision
> timer,
> then try_to_generate_entropy() will have a chance of running, since the
> timer
> might change between the two calls to random_get_entropy().  And if
> try_to_generate_entropy() does run, then it credits 1 bit of entropy for
> every
> iteration, regardless of the timer's precision.
>
> This is an existing problem, but this patchset will make it worse, as it
> changes
> a lot of cases from "no timer" to "low precision timer".
>
> Perhaps try_to_generate_entropy() should check the timer at least 3 times
> and
> verify that it changed each time?

What you've identified is actually already the case for platforms
where the cycle counter is already just slow (and there are a few such
platforms; my odroid C2 even exhibits this). As you identified, the
cycle counter might already be too slow, yet we get [un]lucky and
reach this code right on the cusp or a change.

So the problem isn't new here, per say, for this patchset. But indeed
perhaps we should consider adjusting the heuristics for that a bit in
a separate patch. Your check three times idea seems like a good
starting point, if you want to send a patch and we can poke at it.

Jason