linux-kernel - Re: sysfs: tagged directories not merged completely yet

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Message-ID: <20081008001203.GA21918@us.ibm.com>
Date:	Tue, 7 Oct 2008 19:12:03 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Greg KH <greg@...ah.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	Benjamin Thery <benjamin.thery@...l.net>,
	linux-kernel@...r.kernel.org, Al Viro <viro@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: sysfs: tagged directories not merged completely yet

Quoting Greg KH (greg@...ah.com):
> On Tue, Oct 07, 2008 at 05:54:24PM -0500, Serge E. Hallyn wrote:
> > Quoting Greg KH (greg@...ah.com):
> > > On Tue, Oct 07, 2008 at 01:27:17AM -0700, Eric W. Biederman wrote:
> > > > Unless someone will give an example of how having multiple superblocks
> > > > sharing inodes is a problem in practice for sysfs and call it good
> > > > for 2.6.28.  Certainly it shouldn't be an issue if the network namespace
> > > > code is compiled out.  And it should greatly improve testing of the
> > > > network namespace to at least have access to sysfs.
> > > 
> > > But if the network namespace code is in?  THen we have problems, right?
> > > And that's the whole point here.
> > > 
> > > The fact that you are trying to limit userspace view of in-kernel data
> > > structures, based on that specific user, is, in my opinion, crazy.
> > > 
> > > Why not just keep all users from seeing sysfs, and then have a user
> > > daemon doing something on top of FUSE if you really want to see this
> > > kind of stuff.
> > 
> > Well the blocker is really that when you create a new network namespace,
> > it wants to create a new loopback interface, but
> > /sys/devices/virtual/net/lo already exists.  That's the same issue with
> > user namespace when the fair scheduler is enabled, which tries to
> > re-create /sys/kernel/uids/0.
> > 
> > Otherwise yeah at least for my own uses, containers wouldn't need to
> > look at /sys at all.
> > 
> > Heck you wouldn't even need FUSE, just mount -t tmpfs /sys/class/net
> > and manually link the right devices from /sys/devices/virtual/net.
> 
> Great, that sounds like a solution.
> 
> So tell me again why we need these huge sysfs reworks? :)

Because :

> > Well the blocker is really that when you create a new network namespace,
> > it wants to create a new loopback interface, but
> > /sys/devices/virtual/net/lo already exists.  That's the same issue with

So at least we'd have to do something to allow creation of 'duplicate'
devices in different namespaces.  It might be fine if we just ended up
with /sys/devices/virtual/net/lo, if created in a child net namespace,
be named /sys/devices/virtual/net/lo.childXYZ.  Then userspace can
mount -t tmpfs none /sys/class/net and ln -s
/sys/devices/virtual/net/lo.childXYZ /sys/class/net/lo.

-serge
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