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

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Eric W. Biederman (ebiederm@xmission.com):
> Cedric Le Goater <clg@fr.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