linux-kernel - Re: freeze vs freezer

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Message-ID: <47800C5F.3080907@nigel.suspend2.net>
Date:	Sun, 06 Jan 2008 10:01:51 +1100
From:	Nigel Cunningham <nigel@...el.suspend2.net>
To:	Pavel Machek <pavel@....cz>
CC:	Oliver Neukum <oliver@...kum.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Kyle Moffett <mrmacman_g4@....com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	David Chinner <dgc@....com>,
	Jeremy Fitzhardinge <jeremy@...p.org>, xfs-masters@....sgi.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: freeze vs freezer

Hi.

Pavel Machek wrote:
> On Fri 2008-01-04 21:54:06, Oliver Neukum wrote:
>> Am Donnerstag, 3. Januar 2008 23:06:07 schrieb Nigel Cunningham:
>>> Oliver Neukum wrote:
>>>> Am Donnerstag, 3. Januar 2008 10:52:53 schrieb Nigel Cunningham:
>>>>> Oliver Neukum wrote:
>>>>>> Am Donnerstag 03 Januar 2008 schrieb Nigel Cunningham:
>>>>>>> On top of this, I made a (too simple at the moment) freeze_filesystems
>>>>>>> function which iterates through &super_blocks in reverse order, freezing
>>>>>>> fuse filesystems or ordinary ones. I say 'too simple' because it doesn't
>>>>>>> currently allow for the possibility of someone mounting (say) ext3 on
>>>>>>> fuse, but that would just be an extension of what's already done.
>>>>>> How do you deal with fuse server tasks using other fuse filesystems?
>>>>> Since they're frozen in reverse order, the dependant one would be frozen
>>>>> first.
>>>> Say I do:
>>>>
>>>> a) mount fuse on /tmp/first
>>>> b) mount fuse on /tmp/second
>>>>
>>>> Then the server task for (a) does "ls /tmp/second". So it will be frozen,
>>>> right? How do you then freeze (a)? And keep in mind that the server task
>>>> may have forked.
>>> I guess I should first ask, is this a real life problem or a
>>> hypothetical twisted web? I don't see why you would want to make two
>>> filesystems interdependent - it sounds like the way to create livelock
>>> and deadlocks in normal use, before we even begin to think about
>>> hibernating.
>> Good questions. I personally don't use fuse, but I do care about power
>> management. The problem I see is that an unprivileged user could make
>> that dependency, even inadvertedly.
> 
> Other problem is that unprivileged user can do it with evil intent. So
> called "denial-of-service" attack.

Only in this case it would be a denial-of-denial-of-service attack,
since it would stop you hibernating or suspending :).

This is still all hypothetical. If I could have a real life case where
this could actually happen, it would help a lot.

Nigel
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