Collecting entropy from device_attach() times.

[Ben Laurie]

On Wed, Sep 19, 2012 at 9:20 PM, Pawel Jakub Dawidek <pjd at freebsd.org> wrote:
> On Wed, Sep 19, 2012 at 08:59:15PM +0100, Ben Laurie wrote:
>> On Wed, Sep 19, 2012 at 8:29 PM, Pawel Jakub Dawidek <pjd at freebsd.org> wrote:
>> > On Wed, Sep 19, 2012 at 07:30:52PM +0100, Jonathan Anderson wrote:
>> >> > If all the times are more or less equally probable in this range […]
>> >>
>> >> They're very unlikely to be equally probable. It would make sense to do some characterization of these times and their statistics: a highly non-uniform distribution would mean that we don't actually get many bits per attach.
>> >
>> > I have times for ~2000 device_attach() calls when loading sound card
>> > driver on totally idle system. If someone could take those and analyse
>> > the distribution that would be great.
>> >
>> >> > […] we have more
>> >> > than 19 bits of entropy from this one call, but I reduced if to four
>> >> > bits only, because there are devices that are much faster to attach.
>> >> >
>> >>
>> >> Another reason for doing the above characterization is that, if a particular device_attach() really does provide 12 bits of uncertainty, it's a shame to drop eight of them on the floor.
>> >
>> > Rights. That's why I've prepared another patch:
>> >
>> >         http://people.freebsd.org/~pjd/patches/harvest_device_attach.2.patch
>> >
>> > which effectively discards top ten bits, which means we expect 0.1% of
>> > the attach time to be unpredictable (the attach time in most cases vary
>> > by few percent, not sure yet how much of this variation is really
>> > unpredictable).
>>
>> This is the wrong thing to do! There's no reason to discard bits on
>> input (modulo the device throwing away inputs, that is) - just reduce
>> your entropy estimate. "Extra" bits do no harm.
>
> I 'discard' ten bits from the estimation. I don't discard them by
> zeroing them out. If the number is a 26 bit value then I feed entire
> number, but pass estimation of 16 bits.

Sorry, should've read the code first! This is great. I also like your
friend's analysis.