linux-kernel - Re: [RFC] mlock/stack guard interaction fixup

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Message-ID: <1282633694.2605.2254.camel@laptop>
Date:	Tue, 24 Aug 2010 09:08:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ian Jackson <ijackson@...ark.greenend.org.uk>,
	Greg KH <gregkh@...e.de>, Ian Campbell <ijc@...lion.org.uk>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	stable-review@...nel.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk
Subject: Re: [RFC] mlock/stack guard interaction fixup

On Mon, 2010-08-23 at 12:54 -0700, Jeremy Fitzhardinge wrote:
> On 08/23/2010 12:26 PM, Peter Zijlstra wrote:
> > On Mon, 2010-08-23 at 12:23 -0700, Jeremy Fitzhardinge wrote:
> >> On 08/23/2010 12:07 PM, Peter Zijlstra wrote:
> >>> mlock() simply avoids major faults, nothing more.
> >>>
> >>> I think both page migration and page-out for shared pages where some
> >>> maps are !mlocked can cause unmaps and thus minor faults.
> >>>
> >>> mlock and dirty do not interact, they will still be cleaned/written out
> >>> as normal.
> >> So mlock is useless for preventing secret stuff from being written to disk.
> > Well, if you put your sekrit in a file map, sure.
> >
> > Use a mmap(MAP_ANONYMOUS|MAP_LOCK) and madvise(MADV_DONTFORK) for your
> > sekrits.
> 
> Won't dirty anonymous pages also get written to swap?

Not if all the maps are mlocked (private like above would only have a
single map), there'd be no point would there.
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