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

Cedric Le Goater clg at fr.ibm.com
Wed Sep 3 05:01:28 PDT 2008

Serge E. Hallyn wrote:
> Quoting Eric W. Biederman (ebiederm at xmission.com):
>> "Serge E. Hallyn" <serue at us.ibm.com> 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.
> Why?
> Actually it may work to just put mq_ns straight in the nsproxy.

ok. that's the path I was taking.

> So let's see:
> 	mq_open(name, flag): opens name under the dentry pointed
> 		to by current->nsproxy->mq_ns->mq_dentry
> 	mount -t mqueue none /dev/mqueue: either returns -EBUSY
> 		or just mounts current->nsproxy->mq_ns->mq_sb
> 		under /dev/mqueue
> 	mount -o newinstance -t mqueue none /dev/mqueue: mounts
> 		 a new mq_ns instance under /dev/mqueue
> While doing
> 	mount --make-rshared /vs1
> 	mount --bind /dev/mqueue /vs1/dev/mqueue
> 	create_a_new_container_chrooted_at(/vs1)
> 		mount -o newinstance -t mqueue none /dev/mqueue
> would allow the host to see the child's /dev/mqueue under
> /vs1/dev/mqueue while having its own mqueuefs continue to be
> mounted under /dev/mqueue.

ok. complete isolation would require 2 steps. I guess this is
acceptable because mq uses a fs

allowing the host to see the child's /dev/mqueue is also 'a nice 
to have' feature. unfortunately, we can't do that for all namespaces,
for sysvipc for example. So I'm wondering if we should allow it
at all ?



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