linux-kernel - Re: [patch -mm 0/4] mqueue namespace

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Date:	Fri, 20 Jun 2008 09:50:31 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Dave Hansen <haveblue@...ibm.com>
Cc:	Cedric Le Goater <clg@...ibm.com>,
	Linux Containers <containers@...ts.osdl.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Pavel Emelianov <xemul@...nvz.org>
Subject: Re: [patch -mm 0/4] mqueue namespace

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> Cedric Le Goater <clg@...ibm.com> writes:
> 
> > Hello ! 
> >
> > Here's a small patchset introducing a new namespace for POSIX
> > message queues. 
> >
> > Nothing really complex a part from the mqueue filesystem which 
> > needed some special care
> 
> This looks stalled.

It actually isn't really - Cedric had resent it a few weeks ago but had
troubles with the mail server so it never hit the lists.  I think Dave
made a few more changes from there and was getting ready to resend
again.  Dave?

> I have a brainstorm that might takes a totally
> different perspective on things.
> 
> The only reason we don't just allow multiple mounts of mqueuefs to
> solve this problem is because there is a kernel syscall on the path.
> 
> If we just hard coded a mount point into the kernel and required user
> space to always mount mqueuefs there the problem would be solved.
> 
> hard coding a mount point is unfortunately violates the unix rule
> of separating mechanism and policy.
> 
> One way to fix that is to add a hidden directory to the mnt namespace.
> Where magic in kernel filesystems can be mounted.  Only visible
> with a magic openat flag.  Then:
> 
> fd = openat(AT_FDKERN, ".", O_DIRECTORY)
> fchdir(fd);
> umount("./mqueue", MNT_DETACH);
> mount(("none", "./mqueue", "mqueue", 0, NULL);
> 
> Would unshare the mqueue namespace.
> 
> Implemented for plan9 this would solve a problem of how do you get
> access to all of it's special filesystems.  As only bind mounts
> and remote filesystem mounts are available.  For linux thinking about
> it might shake the conversation up a bit.

It is unfortunate that two actions are needed to properly complete the
unshare, and we had definately talked about just using the mount before.
I forget why we decided it wasn't practical, so maybe what you describe
solves it...

But at least the current patch reuses CLONE_NEWIPC for posix ipc, which
also seems to make sense.

-serge
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