Re: Collecting entropy from device_attach() times.
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On Tue, Sep 25, 2012 at 12:10:13AM +0200, Mariusz Gromada wrote:
> W dniu 2012-09-24 23:56, Mariusz Gromada pisze:
>=20
> > Ok, finally I have some formal results. To be completely honest I need
> > to point out that, in fact, we have a discrete data (for example
> > integers 0, 1, ..., 63, but not continues numbers spread across 0 and
> > 63). That is way I am going to use two sample Kolmogorov-Smirnov test.
>=20
> Another clarification is needed. KS test in general (and in theory)=20
> should be used for continuous distributions. But in our case we can=20
> easily say that we observe our distribution in integers only (rounding),=
=20
> and the whole rest is easily estimated.
Thanks a lot!
To the list:
phk@ asked me privately to check if there is no correclation between
consecutive device_attach() calls during single boot.
For example each device_attach() separately can yield great entropy in
every tests, but all those calls combined might be somehow related, ie.
during one boot all calls take a bit longer and in another boot all
calls take a bit less, which could decrease total entropy we should
estimate out of it.
I created dummy driver which was registering three dummy drivers, so it
was provoking three device_attach() calls on every kldload. Mariusz
verified the observations and there was no correlation between the
times.
I believe everyone is bored at this point, so I'd like to propose a way
forward:
I'll perform one more test with CPU clock speed reduced as much as it
can be and see if rejecting 7 top bits is still fine. If it is, I'd like
to commit my patch. I was wondering if I should hide it under
#ifdef __amd64__, but the only risk in having it on all platforms is
eventually being overestimating available entropy, which is bad, but I
think better than not providing any entropy this method. On the other
hand having it on one or two platforms only would maybe motivate people
to verify it on other platforms.
--=20
Pawel Jakub Dawidek http://www.wheelsystems.com
FreeBSD committer http://www.FreeBSD.org
Am I Evil? Yes, I Am! http://tupytaj.pl
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