linux-kernel - Re: issue with /dev/random? gets depleted very quick

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Message-ID: <20090614190417.GC7272@vanheusden.com>
Date:	Sun, 14 Jun 2009 21:04:17 +0200
From:	Folkert van Heusden <folkert@...heusden.com>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: issue with /dev/random? gets depleted very quick

> [cc:ed to lkml]

> > On an idle system (no gui, no daemons, nothing) system, /dev/random gets
> > empty in a matter of 20 seconds with a 2.6.26 kernel.
> > My test:
> > add 1000 bits to the device:
> > zolder:/tmp# cat test-RNDADDENTROPY.c 
...
> > }
> > and then check whayt is in it:
> > zolder:/tmp# ./a.out ; while true ; do echo `date` `cat /proc/sys/kernel/random/entropy_avail` ; sleep 1 ; done
> > 0
> > Sun Jun 14 14:50:44 CEST 2009 1117
...
> > Sun Jun 14 14:50:55 CEST 2009 157
> > Is there something wrong with it?
> Does it go below 128? If not, that's the behavior of something depleting
> the pool down to the anti-starvation threshold via either /dev/urandom
> or get_random_bytes.

No, it stays above 128. Sometimes around 13x, sometimes 151, so not
always close to 128.

> On my system, I'm seeing that behavior as well. fuser reports a bunch of
> processes hold /dev/urandom open, but stracing them doesn't reveal a
> culprit. Which means there's now probably something in the kernel
> calling get_random_bytes continuously.

Yes. On the systems I tried, nothing had /dev/*random open, also no
cronjobs that could use it. And still it gets lower.

> Is this a problem? It really shouldn't be. Everyone should be
> using /dev/urandom anyway. And the anti-starvation threshold guarantees

Well, if I understood correctly how /dev/*random works, urandom is fed
by /dev/random. So if there's almost nothing left in the main pool and
urandom demands bits then we have an issue.
Also, if you frequently want to generate keys (thing gpg, ssl), I think
you want bits from /dev/random and not urandom.

> that if there's entropy being collected, readers of /dev/random can
> always make forward progress.

Also if it is used so heavily, you need quit an entropy-source to keep
it filled.

Folkert van Heusden

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