Re: svn commit: r239569 - head/etc/rc.d

On Fri, Sep 07, 2012 at 11:51:57AM +1000, Peter Jeremy wrote:
> I've done some experiments on a couple of systems to look at gzip and
> sha256 speed.  On one box, "sysctl -an" returns 109989 bytes (though
> it has been up for a while) which gzip's to 12511 bytes (still too
> large for a single write to /dev/random).  The following is the
> wallclock time to run sha256 or gzip on that input (based on multiple
> runs of 100 loops).
> sha256   gzip -6   CPU
>  3.3ms    5.9ms    2.5GHz amd64 (Athlon 4850e)
>  6.8ms   13.3ms    1.6GHz i386 (Atom N270)
> 85  ms   34  ms    700MHz ARMv6 (Raspberry PI, running Linux)
> These times are all in the noise compared to overall startup time.

I got my slowest times on a CAVIUM OCTEON 52XX CPU Rev. 0.8 with no FPU.
This is the source of my performance concerns.  I agree your times are
"in the noise" and thus feel this diff deals with most of the concerns.

* Updates the comment about blocking -- it hasn't been true for 8 years.

* Document the natural limitations of the harvesting subsystem due to
  it having finite resources (space & time).

* Apply above documentation and don't write over 100k to /dev/random
  thinking it is all processed.  [or even the reduced 50k of output
  from using more selective commands]

* Apply Bruce Schneier's advice WRT not reusing seed material to
  the 'better_than_nothing' seed material and only use it on first
  post-installation boot.


Index: initrandom
===================================================================

--- initrandom	(revision 239610)
+++ initrandom	(working copy)
@@ -18,18 +18,40 @@ feed_dev_random()
 {
 	if [ -f "${1}" -a -r "${1}" -a -s "${1}" ]; then
 		cat "${1}" | dd of=/dev/random bs=8k 2>/dev/null
+	else
+		return 1
 	fi
 }
 
 better_than_nothing()
 {
-	# XXX temporary until we can improve the entropy
-	# harvesting rate.
 	# Entropy below is not great, but better than nothing.
-	# This unblocks the generator at startup
-	( ps -fauxww; sysctl -a; date; df -ib; dmesg; ps -fauxww ) \
+
+	# Entropy below is not great, but better than nothing.
+	# Overwhelming the internal entropy seeding buffers is a NOP.
+	# Once the internal buffers are filled, additional input is
+	# dropped on the floor until the buffers are processed.
+	# For FreeBSD's current yarrow implementation that means
+	# there is little need to seed with more than 4k of input.
+	# In order to reduce the size of the seed input we hash it.
+
+	# The output of a cryptographic hash function whose input
+	# contained 'n' bits of entropy will have 'm' bits of entropy,
+	# where 'm' is either 'n' or slightly less due to collisions.
+	# So we operate under the premise that there is essentially
+	# no loss of entropy in hashing these inputs.
+
+	/sbin/sha256 -q `sysctl -n kern.bootfile` \
 	    | dd of=/dev/random bs=8k 2>/dev/null
-	cat /bin/ls | dd of=/dev/random bs=8k 2>/dev/null
+
+	# Note: commands are ordered based on least changing across reboots
+	# to most:
+	( dmesg; kenv; df -ib; \
+	    ps -fauxrH -o nwchan,nivcsw,nvcsw,time,re,sl; \
+	    sysctl -n kern.cp_times kern.geom kern.lastpid kern.timecounter \
+	    kern.tty_nout kern.tty_nin vm vfs debug dev.cpu; \
+	    date ) \
+	    | /sbin/sha256 -q | dd of=/dev/random bs=8k 2>/dev/null
 }
 
 initrandom_start()
@@ -67,16 +89,16 @@ initrandom_start()
 		#
 		case ${entropy_file} in
 		[Nn][Oo] | '')
+			better_than_nothing
 			;;
 		*)
 			if [ -w /dev/random ]; then
-				feed_dev_random "${entropy_file}"
+				feed_dev_random "${entropy_file}" \
+				    || better_than_nothing
 			fi
 			;;
 		esac
 
-		better_than_nothing
-
 		echo -n ' kickstart'
 	fi
 

-- 
-- David  (obrien@FreeBSD.org)

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