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    SubjectRe: mmotm 2008-12-01-19-41: early exception (page fault -- deref of 0x20)
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    On Tue, 2008-12-02 at 15:50 -0800, Arjan van de Ven wrote: 
    > Matt Mackall wrote:
    > >
    > > If we oops or warn while picking a timesource, we'll have lots of fun?
    > >
    >
    > we really only need to mix in the tsc; ktime_get() is just an arch friendly way to get that
    > I supposed (wrongly).
    >
    > but yes we need to do a few things
    > 1) seed on demand with a platform time source

    Currently we use jiffies + get_cycles(). That's going to have somewhere
    between, oh, 3 bits of entropy (very stable boot with only jiffies) and
    25 bits of entropy (TSC with lots of waiting for hardware) at boot.
    Ideally, we'd have access to a wall clock of some sort as well. But
    that's also a fairly limited source - wall clocks are both low
    resolution and predictable/collision-prone.

    > 2) have a way where arch init can just hand semi random data during the boot process to
    > increase the randomness (even if it doesn't count as entropy)

    A simple wrapper around mix_pool_bytes probably fits the ticket.

    But I don't think this will solve the general problem of 'large numbers
    of practically identical machines booting up with the same pre-init
    random number pools'. Beyond things like MAC addresses and serial
    numbers (predictable/observable but at least not collision-prone), we
    have no way to differentiate some boxes. We may need to forcibly
    generate some timing entropy. Perhaps something like this:

    http://markmail.org/message/xwsbywr6ziil2qu2

    (which is way too slow in its current form)

    There's a related problem of systems with no way to store a seed across
    boots.

    --
    Mathematics is the supreme nostalgia of our time.



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