rnd entropy estimate running low?

[Havard Eidnes <he%NetBSD.org@localhost>]

Hi,

on a couple of arm boxes I have I've been observing the
development of the entropy estimate, what "rndctl -s" calls "bits
currently stored in pool" over time.

I've also tried to read some of the code to understand the
behaviour.

If I understand correctly, randomness sources come in two basic
flavours: those which offer up randomness samples based on
(possibly external) events, and those which only provide samples
when "asked" to do so.  The hardware randomness generator on my
amlogic arm boards appears to fall into the last category.

On a system with little other active randomness sources (e.g. FS
activity or keyboard / mouse activity), it appears that the "bits
currently stored in pool" will be allowed to decrease quite close
to zero (or even *to* zero) before the polled sources are
queried, via e.g. rnd_extract() only triggering a rnd_getmore()
if it could not initially fulfill the request.  The same also
appears to hold for rnd_tryextract().

Meanwhile the hardware random generator sits there unused.

I would have thought it would make more sense to keep the "bits
currently stored in pool" more "topped up", and that a re-fill
could with benefit be done before the estimate crept down towards
zero?  Especially if you have a half-way decent hardware random
generator at hand?

(This has been observed with both 7.99.47 and 7.99.58, fwiw.)

Regards,

- Håvard