Re: [PATCH] fs/pnode.c: treat zero mnt_group_id-s as unequal

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Feb 16, 2016 at 11:45:33AM -0800, Maxim Patlasov wrote:
> propagate_one(m) calculates "type" argument for copy_tree() like this:
> 
> >    if (m->mnt_group_id == last_dest->mnt_group_id) {
> >        type = CL_MAKE_SHARED;
> >    } else {
> >        type = CL_SLAVE;
> >        if (IS_MNT_SHARED(m))
> >           type |= CL_MAKE_SHARED;
> >   }
> 
> The "type" argument then governs clone_mnt() behavior with respect to flags
> and mnt_master of new mount. When we iterate through a slave group, it is
> possible that both current "m" and "last_dest" are not shared (although,
> both are slaves, i.e. have non-NULL mnt_master-s). Then the comparison
> above erroneously makes new mount shared and sets its mnt_master to
> last_source->mnt_master. The patch fixes the problem by handling zero
> mnt_group_id-s as though they are unequal.
> 
> The similar problem exists in the implementation of "else" clause above
> when we have to ascend upward in the master/slave tree by calling:
> 
> >    last_source = last_source->mnt_master;
> >    last_dest = last_source->mnt_parent;
> 
> proper number of times. The last step is governed by
> "n->mnt_group_id != last_dest->mnt_group_id" condition that may lie if
> both are zero. The patch fixes this case in the same way as the former one.

Mind putting together a reproducer?