Re: [Devel] Re: [patch 05/10] add "permit user mounts in new namespace" clone flag

[Ram Pai <linuxram@us.ibm.com>]

On Wed, 2007-04-18 at 11:19 +0200, Miklos Szeredi wrote:
> > > Allowing this and other flags to NOT be propagated just makes it
> > > possible to have a set of shared mounts with asymmetric properties,
> > > which may actually be desirable.
> > 
> > The shared mount feature was designed to ensure that the mount remained
> > identical at all the locations.
> 
> OK, so remount not propagating mount flags is a bug then?

As I said earlier, are there any flags currently that if not propagated 
can lead to conflicts with the shared subtree semantics? I am not aware
of any.  If you did notice a case, than according to me its a bug.

But the new proposed 'allow unpriviledged mounts' flag; if not
propagated among peers (and slaves) of a shared mount can lead to
conflicts with shared subtree semantics. Since mount in one
shared-mount; when propagated to its peer fails to mount and hence lead
to un-identical peers.



> 
> > Now designing features to make it un-identical but still naming it
> > shared, will break its original purpose.  Slave mounts were designed
> > to make it asymmetric.
> 
> What if I want to modify flags in a master mount, but not the slave
> mount?  Would I be screwed?  For example: mount is read-only in both
> master and slave.  I want to mark it read-write in master but not in
> slave.  What do I do?

Making mounts read-only or read-write -- will that effect mount
propagation in such a way that future mounts in any one of the
peers will not be able to propagate that mount to its peers or slaves?

I don't think it will. Hence its ok to selectively mark some mounts
read-only and some mounts read-write.

However with the introduction of unpriviledged mount semantics, there 
can be cases where a user has priviledges to mount at one location but
not at a different location. if these two location happen to share
a peer-relationship than I see a case of interference of read-write
flag semantics with shared subtree semantics. And hence we will end up
propagating the read-write flag too or have to craft a different
semantics that stays consistent. 



> 
> > Whatever feature that is desired to be exploited; can that be exploited
> > with the current set of semantics that we have? Is there a real need to
> > make the mounts asymmetric but at the same time name them as shared?
> > Maybe I dont understand what the desired application is? 
> 
> I do think this question of propagating mount flags is totally
> independent of user mounts.
> 
> As it stands, currently remount doesn't propagate mount flags, and I
> don't see any compelling reasons why it should.
> 
> The patchset introduces a new mount flag "allowusermnt", but I don't
> see any compelling reason to propagate this flag _either_.
> 
> Please say so if you do have such a reason.  As I've explained, having
> this flag set differently in parts of a propagation tree does not
> interfere with or break propagation in any way.

As I said earlier, I see a case where two mounts that are peers of each
other can become un-identical if we dont propagate the "allowusermnt".

As a practical example.

/tmp and /mnt are peers of each other.
/tmp has its "allowusermnt" flag set, which has not been propagated
to /mnt.

now a normal-user mounts an ext2 file system under /tmp at /tmp/1

unfortunately the mount wont appear under /mnt/1 

and this breaks the shared-subtree semantics which promises: whatever is
mounted under /tmp will also be visible under /mnt


and in case if you allow the mount to appear under /mnt/1, you will
break unpriviledge mounts semantics which promises: a normal user will
not be able to mount at a location that does not allow user-mounts.



RP


> 
> Miklos
>