Re: [RFC][PATCH 0/8][v2]: Enable multiple mounts of devpts

["Serge E. Hallyn" <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

Quoting Cedric Le Goater (clg-NmTC/0ZBporQT0dZR+AlfA@public.gmane.org):
> Eric W. Biederman wrote:
> > "Serge E. Hallyn" <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org> writes:
> > 
> >>>     (3.2) mnt namespace maybe ?
> >> I think the last one is the way to go.
> >>
> >> mnt_namespace points to mq_ns.
> >>
> >> At clone(CLONE_NEWMNT), the new mnt namespace receives a copy of the
> >> parent's mq_ns.
> >>
> >> If a task does
> >> 	mount -o newinstance -t mqueue none /dev/mqueue
> >> then its current->nsproxy->mnt_namespace->mqns is switched
> >> to point to a new instance of the mq_ns.
> >>
> >> mnt_ns->mq_ns has pointers to the sb (and hence root dentry) of the
> >> devpts fs.
> >>
> >> When a task does mq_open(name, flag), then name is in the mqueuefs
> >> found in current->nsproxy->mnt_namespace->mqns.
> >>
> >> But if a task does
> >>
> >> 	clone(CLONE_NEWMNT);
> >> 	mount --move /dev/mqueue /oldmqueue
> >> 	mount -o newinstance -t mqueue none /dev/mqueue
> >>
> >> then that task can find files for the old mqueuefs under
> >> /oldmqueue, while mq_open() uses /dev/mqueue since that's
> >> what it finds through its mnt_namespace.
> > 
> > Serge if we can make the lookup a pure mount namespace operation
> > i.e. a well known path.  Than I don't have any problems with it.
> > Otherwise it looks like abuse of the mount namespace.
> > 
> > In particular.  The best approximation I have is to change the
> > kernel to simply lookup "/dev/mqueue" and if not found fallback
> > to the initial kernel instance.
> > 
> > I'm staring at the code as I really haven't looked at it enough
> > but it sure looks like we can transform it into a proper filesystem
> > with just a touch of backwards compatibility logic.
> > - put the current mq_namespace in the superblock.
> 
> ok that is done. using the s_fs_info.
> 
> > - Have open/unlink lookup "/dev/mqueue" to find the filesystem
> >   if nothing is found fallback to the internal mount otherwise error.
> 
> what do you mean ? loop on the mnt_namespace of current to find a 
> 'struct vfsmount' pointing to /dev/mqueue ?

I think he means start a lookup starting from current->fs->root
to find /dev/mqueue.

-serge