svn commit: r284959 - in head: . share/man/man4 share/man/man9 sys/conf sys/dev/glxsb sys/dev/hifn sys/dev/random sys/dev/rndtest sys/dev/safe sys/dev/syscons sys/dev/ubsec sys/dev/virtio/random sy...
Jeff Roberson
jroberson at jroberson.net
Wed Jul 22 23:26:44 UTC 2015
On Wed, 22 Jul 2015, Mark R V Murray wrote:
>
>> On 22 Jul 2015, at 22:42, Jeff Roberson <jroberson at jroberson.net> wrote:
>>
>> On Tue, 30 Jun 2015, Mark Murray wrote:
>>
>>> - Add harvesting of slab allocator events. This needs to be checked for
>>> weighing down the allocator code.
>>
>> Neither filesystem operations nor allocations are random events. They are trivially influenced by user code. A malicious attacker could create repeated patterns of allocations or filesystem activity through the syscall path to degrade your random sample source.
>
> I?m not sure I accept that - Fortuna is very careful about using
> non-reversible hashing in it?s accumulation, and countering such
> degradation is one of the algorithm?s strong points. There is perhaps
> risk of *no* entropy, but even the per-event timing jitter will be
> providing this, if nothing else.
>
>> Perhaps more importantly to me, this is an unacceptable performance burden for the allocator. At a minimum it should compile out by default. Great care has been taken to reduce the fast path of the allocator to the minimum number of cycles and even cache misses.
>
> As currently set up in etc/rc.d/* by default, there is a simple check at
> each UMA harvesting opportunity, and no further action. I asked Robert
> Watson if this was burdensome, and he said it was not.
I find this burdensome. You can easily add a macro around the calls or
hide them in an inline with a default to off. Even a function call that
checks a global and does nothing else is a handful of new cache misses. A
microbenchmark will not realize the full cost of this. You will instead
get the dozen or so instructions of overhead which I still find
objectionable.
Kip's observations about packet cycle budgets in high-performance
applications are accurate and this is something we have put great care
into over time.
Thanks,
Jeff
>
> I?m willing to discuss optimising this, and have plans for some
> micro-benchmarks.
>
> M
> --
> Mark R V Murray
>
> PS: Please trim mail when responding - was it necessary to send me back the whole commit message and diff?
>
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