[Devel] [RFC][PATCH 3/4]: Enable multiple mounts of /dev/pts

[sukadev at us.ibm.com]

Pavel Emelianov [xemul at openvz.org] wrote:
| Serge E. Hallyn wrote:
| > Quoting Pavel Emelyanov (xemul at openvz.org):
| >> [snip]
| >>
| >>>> Mmm. I wanted to send one small objection to Cedric's patches with mqns,
| >>>> but the thread was abandoned by the time I decided to do-it-right-now.
| >>>>
| >>>> So I can put it here: forcing the CLONE_NEWNS is not very good, since
| >>>> this makes impossible to push a bind mount inside a new namespace, which
| >>>> may operate in some chroot environment. But this ability is heavily
| >>> Which direction do you want to go?  I'm wondering whether mounts
| >>> propagation can address it.
| >> Hardly. AFAIS there's no way to let the chroot-ed tasks see parts of
| >> vfs tree, that left behind them after chroot, unless they are in the 
| >> same mntns as you, and you bind mount this parts to their tree. No?
| > 
| > Well no, but I suspect I'm just not understanding what you want to do.
| > But if the chroot is under /jail1, and you've done, say,
| > 
| > 	mkdir -p /share/pts
| > 	mkdir -p /jail1/share
| > 	mount --bind /share /share
| > 	mount --make-shared /share
| > 	mount --bind /share /jail1/share
| > 	mount --make-slave /jail1/share
| > 
| > before the chroot-ed tasks were cloned with CLONE_NEWNS, then when you
| > do
| > 
| > 	mount --bind /dev/pts /share/pts
| > 
| > from the parent mntns (not that I know why you'd want to do *that* :)
| > then the chroot'ed tasks will see the original mntns's /dev/pts under
| > /jail1/share.
| 
| I haven't yet tried that, but :( this function
| 
| 	static inline int check_mnt(struct vfsmount *mnt)
| 	{
| 	        return mnt->mnt_ns == current->nsproxy->mnt_ns;
| 	}
| 
| and this code in do_loopback
| 
|         if (!check_mnt(nd->mnt) || !check_mnt(old_nd.mnt))
|                 goto out;
| 
| makes me think that trying to bind a mount from another mntns
| ot _to_ another is prohibited... Do I miss something?
| 
| >>> Though really, I think you're right - we shouldn't break the kernel
| >>> doing CLONE_NEWMQ or CLONE_NEWPTS without CLONE_NEWNS, so we shouldn't
| >>> force the combination.
| >>>
| >>>> exploited in OpenVZ, so if we can somehow avoid forcing the NEWNS flag
| >>>> that would be very very good :) See my next comment about this issue.
| >>>>
| >>>>> Pavel, not long ago you said you were starting to look at tty and pty
| >>>>> stuff - did you have any different ideas on devpts virtualization, or
| >>>>> are you ok with this minus your comments thus far?
| >>>> I have a similar idea of how to implement this, but I didn't thought
| >>>> about the details. As far as this issue is concerned, I see no reasons
| >>>> why we need a kern_mount-ed devtpsfs instance. If we don't make such,
| >>>> we may safely hold the ptsns from the superblock and be happy. The
| >>>> same seems applicable to the mqns, no?
| >>> But the current->nsproxy->devpts->mnt is used in several functions in
| >>> patch 3.
| >> Indeed. I overlooked this. Then we're in a deep ... problem here.
| >>
| >> Breaking this circle was not that easy with pid namespaces, so
| >> I put the strut in proc_flush_task - when the last task from the
| >> namespace exits the kern-mount-ed vfsmnt is dropped, but we can't
| >> do the same stuff with devpts.
| > 
| > But I still don't see what the problem is with my proposal?  So long as
| > you agree that if there are no tasks remaining in the devptsns,
| > then any task which has its devpts mounted should see an empty directory
| > (due to sb->s_info being NULL), I think it works.
| 
| Well, if we _do_ can handle the races with ns->devpts_mnt switch
| from not NULL to NULL, then I'm fine with this approach.
| 
| I just remember, that with pid namespaces this caused a complicated
| locking and performance degradation. This is the problem I couldn't
| remember yesterday.

Well, iirc, one problem with pid namespaces was that we need to keep
the task and pid_namespace association until the task was waited on
(for instance the wait() call from parent needs the pid_t of the
child which is tied to the pid ns in struct upid).

For this reason, we don't drop the mnt reference in free_pid_ns() but
hold the reference till proc_flush_task().

With devpts, can't we simply drop the reference in free_pts_ns() so
that when the last task using the pts_ns exits, we can unmount and
release the mnt ?

IOW, do you suspect that the circular reference leads to leaking vfsmnts ?