Re: /etc/mtab and per-process namespaces

[Ram Pai <linuxram@us.ibm.com>]

On Sat, 2005-10-22 at 06:23, Dr. Greg Wettstein wrote:
> On Oct 13,  7:10pm, Mike Waychison wrote:
> } Subject: Re: /etc/mtab and per-process namespaces
> 
> Good morning to everyone, really behind on e-mail, my apologies for
> joining the thread late.
> 
> > Ram wrote:
> > > On Tue, Oct 04, 2005 at 12:14:47PM -0700, David Leimbach wrote:
> > > 
> > >>Hmm no responses on this thread a couple days now.  I guess:
> > >>
> > >>1) No one cares about private namespaces or the fact that they make
> > >>/etc/mtab totally inconsistent.
> > >>2) Private Namespaces aren't important to anyone and will never be
> > >>robust unless someone who cares, like me, takes it over somehow.
> > >>3) Everyone is busy with their own shit and doesn't want to deal with
> > >>me or mine right now.
> > >>
> > >>I'm seriously hoping it's 3 :).  2 Is acceptable too of course.  I
> > >>think this is important and I want to know more about the innards
> > >>anyway.  1 would make me sad as I think Linux can really show other
> > >>Unix's what-for here when it comes to showing off how good the VFS can
> > >>be.
> 
> > Or,  you bite the bullet and fix /proc/mounts and let distributions bind 
> > mount /proc/mounts over /etc/mtab.
> > 
> > Sun recognized this as a problem a long time ago and /etc/mnttab has 
> > been magic for quite some time now.
> > 
> > Add to this the fact that a textfile /etc/mtab is busted because it's 
> > whitespace seperated and pieces blows up and you do things like:
> > 
> > mount filer:/export/mikew "/home/Mike Waychison"
> 
> As to the three options above, I believe number 3 would be operative.
> Private namespaces are extremely useful concepts, we are growing
> increasingly dependent on them for systems management and
> administration.  I believe the issue is a chicken/egg problem, without
> an update in tools the concept of namespaces are less approachable
> than they should be.
> 
> Mike's comments are very apt.  The current situation with mount
> support is untenable.  Even working on private development machines it
> gets confusing as to what is or is not mounted in various
> shells/processes.  The basic infra-structure is there with process
> specific mount information (/proc/self/mounts) but mount and friends
> are a bit problematic with respect to supporting this.
> 
> I'm working on a namespace toolkit to address these issues.  I've got
> a pretty basic tool, similar to sudo, which allows spawning processes
> with a protected namespace.  I'm adding a configuration system which
> allow systems administrators to define a setup of bind mounts which
> are automatically executed before the user is given their shell.  I'm
> also working up a PAM account module to go along with this.  I would
> certainly be open to suggestions as to what else people would consider
> useful in such a toolkit.
> 
> I've been pondering the best way to take on the mount problem.
> Current mount binaries seem to fall back to /proc/mounts if /etc/mtab
> is not present.  All bets are off of course if the mount binary is
> used for the bind mount since a new /etc/mtab is created.
> 
> I'm willing to whack on the mount binary a bit as part of this.  The
> obvious solution is to teach mount to act differently if it is running
> in a private namespace.  If anybody knows of a good way to detect this
> I would be interested in knowing that.  In newns (the namespace sudo
> tool) I'm setting an environment variable for mount to detect on but a
> system level approach would be more generic.

actually there is a hackish way for a process to figure out if it is  in
a different namespace than the system namespace.

ls /proc/1/root 

in a system namespace it will allow you to see the content.
And in a per-process-namespace it will fail with permission denied.

But I think we should figure out a cleaner way to decipher this, 
and that would start with clearly defining the requirements, I think. 

RP