linux-kernel - Re: [patch 1/4] Add routine for generating an ID for kernel pointer

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Date:	Sat, 31 Dec 2011 11:51:02 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [patch 1/4] Add routine for generating an ID for kernel pointer

On Fri, Dec 30, 2011 at 06:51:37PM -0500, KOSAKI Motohiro wrote:
...
> >
> >Guys, this become more and more complex, finally I fear someone
> >propose to do ideal hashing run-time ;) Maybe we can step back and
> >live with root-only and plain pointers here? I'm not sure who else
> >might need such facility except us, and if once there will be a candidate
> >-- we could take a look on hashing again and provide safe hashes there. No?
> 
> But recently kernel security fashion are, we don't expose a kernel
> pointer at all even though the file is root only. I'm not sure how
> much effective such fashion. but you seems run opposite way.
> 
> I doubt user land can implement good comparison way. Why you gave up
> Andrew's sys_are_these_files_the_same() idea?
> 

Because of speed, as Pavel mentioned
 |
 | With this the complexity of determining sharing for N files scattered across
 | several tasks would be N^2, since we'll have to compare each file to each file.
 |
 | On the other hand having just N IDs at hands would allow us to use more
 | efficient algorithms resulting in e.g. N*log(N) complexity.
 |

	Cyrill
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