Date:	Thu, 4 Oct 2007 23:23:02 -0700
From:	Greg KH <greg@...ah.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Tejun Heo <htejun@...il.com>, cornelia.huck@...ibm.com,
	stern@...land.harvard.edu, kay.sievers@...y.org,
	linux-kernel@...r.kernel.org,
	Linux Containers <containers@...ts.osdl.org>
Subject: Re: [PATCHSET 3/4] sysfs: divorce sysfs from kobject and driver
	model

On Thu, Sep 27, 2007 at 01:25:48PM -0600, Eric W. Biederman wrote:
> 
> I still need to look at the code in detail but I have some concerns
> I want to inject into this conversation of future sysfs architecture.
> 
> - If we want to carefully limit sysfs from going to wild code review
>   is clearly not enough.  We need some technological measures to
>   assist us.  As the experience with sysctl has shown.

I totally agree.  You should see the ways that people have tried to
circumvent the current kobject/sysfs code over the past years.  It's so
scary it's not even funny...

> - The network namespace work scheduled to be merged in 2.6.24 is 
>   currently has a dependency in Kconfig that is "&& !SYSFS"
>   because sysfs is currently very much a moving target.
> 
>   Does it look like we can resolve Tejun's work for 2.6.24?
>   If not does it make sense to push my patches that allow
>   multiple mounts of sysfs for 2.6.24?  So I can allow
>   network namespaces in the presence of sysfs.
> 
>   Outside of sysfs and the device model I'm only talk maybe 30 lines
>   of code...    So I could easily merge that patch later in the
>   merge window after the other pieces have gone in.

I would be interested in seeing what your patches look like.  I don't
think that we should take any more sysfs changes for 2.6.24 as we do
have a lot of them right now, and I don't think that Tejun and I agree
on the future direction of the outstanding ones just yet.

But I don't think that your multiple-mount patches could make it into
.24, unless .23 is still weeks away.

> - Farther down the road we have the device namespace.
>   The bounding requirements are:
>   - We want to restrict which set of devices a subset of process
>     can access.

That's reasonable.

>   - When we migrate an application we want to preserve the device
>     numbers of all devices that show up in the new location.
>     So filesystems whose block devices reside on a SAN, ramdisks,
>     ttys, etc.
>     Other devices that really are different we can handle with
>     hotplug remove and add events, during the migration.
> 
>   So while there is lower hanging fruit the requirements for a
>   device namespace are becoming clear, and don't look like something
>   we will ultimately be able to dodge.
> 
>   For sysfs the implication is that we will need to filter the
>   hotplug events based upon the device namespace of the recipient, and
>   we will need to restrict the set of devices that show up in sysfs
>   based on who mounts it (as the prototype patches with the network
>   namespace are doing).

That is going to be interesting to see how you come up with a way to do
hat.

>   Also fun is that the dev file implementation needs to be able to
>   report different major:minor numbers based on which mount of
>   sysfs we are dealing with.

Um, no, that's not going to happen.  /dev/sda will _always_ have the
same major:minor number, as defined by the LSB spec.  You can not break
that at all.  So while you might not want to show all mounts
/sys/devices/block/sda/ the ones that you do, will all have the LSB
defined major:minor number assigned to it.

thanks,

greg k-h
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