dd if=/dev/mem can hang a machine?

[Julian Stacey]

Gary Jennejohn wrote:
> On Fri, 28 Nov 2008 15:28:35 +0200
> Andriy Gapon <avg at icyb.net.ua> wrote:
> 
> > 
> > I have a new machine with DG33TL mainboard (ICH9/G33).
> > In a course of some hacking I ran dd if=/dev/mem ... to scan all memory,
> > this caused the machine to hang.
> > I tried to reproduce and this is 100% reproducible.
> > 
> > I am not used to such behavior. In older days I could scan all the
> > memory without any issues.
> > 
> > Could this be related to some modern form of memory-mapped IO? Or to
> > Intel Management Engine (that seems t bite into DRAM)?
> > Or something else?
> > 
> > Just wondering.
> > 
> 
> That's what I would assume.  With some hardware just reading a register
> can be harmful.

I just crashed 3 normally stable machines trying that.
I only tried for casual interest. I acknowledge Gary's comment above :-)

	dd if=/dev/mem of=/dev/null

	FreeBSD lapa.js.berklix.net 4.11-RELEASE FreeBSD 4.11-RELEASE
	#0: Fri Jul  7 17:56:30 CEST 2006
	jhs at lapa.jhs.private:/usr/src/sys/compile/LAPA.small  i386

	FreeBSD laps.js.berklix.net 7.0-RELEASE FreeBSD 7.0-RELEASE
	#2: Mon Sep  8 15:39:53 CEST 2008
	jhs at laps.js.berklix.net:/usr1/ftp/pri/FreeBSD/releases/7.0-RELEASE/src/sys/i386/compile/LAPS.small
	i386

	FreeBSD john.js.berklix.net 7.1-BETA2 FreeBSD 7.1-BETA2 #0:
	Sun Oct 12 20:59:28 UTC 2008
	root at driscoll.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  amd64

Cheers,
Julian
-- 
Julian Stacey: BSDUnixLinux C Prog Admin SysEng Consult Munich www.berklix.com
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