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...

[Warner Losh]

> On Jul 22, 2015, at 4:53 PM, Jeff Roberson <jroberson at jroberson.net> wrote:
> 
> 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.

I’m not sure I’m happy about this answer. Do you have some research backing up such cavalier claims?

>>> 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.

A certain video streaming company will be pushing the envelope to get to 100Gbps very soon. Even a few extra instructions on every packet / allocation will be a killer. Especially if one is an almost guaranteed cache miss. This most certainly will be burdensome. There absolutely must be a way to turn this off at compile time. We don’t care that much about entropy to leave performance on the table.

Warner
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