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

[Andrew Vagin <avagin@virtuozzo.com>]

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.
>

Acked-by: Andrei Vagin <avagin@virtuozzo.com>

> Signed-off-by: Maxim Patlasov <mpatlasov@virtuozzo.com>
> ---
>  fs/pnode.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/pnode.c b/fs/pnode.c
> index 6367e1e..abf156a 100644
> --- a/fs/pnode.c
> +++ b/fs/pnode.c
> @@ -212,7 +212,7 @@ static int propagate_one(struct mount *m)
>  	/* skip if mountpoint isn't covered by it */
>  	if (!is_subdir(mp->m_dentry, m->mnt.mnt_root))
>  		return 0;
> -	if (m->mnt_group_id == last_dest->mnt_group_id) {
> +	if (m->mnt_group_id && m->mnt_group_id == last_dest->mnt_group_id) {
>  		type = CL_MAKE_SHARED;
>  	} else {
>  		struct mount *n, *p;
> @@ -223,7 +223,9 @@ static int propagate_one(struct mount *m)
>  					last_source = last_source->mnt_master;
>  					last_dest = last_source->mnt_parent;
>  				}
> -				if (n->mnt_group_id != last_dest->mnt_group_id) {
> +				if (n->mnt_group_id != last_dest->mnt_group_id ||
> +				    (!n->mnt_group_id &&
> +				     !last_dest->mnt_group_id)) {
>  					last_source = last_source->mnt_master;
>  					last_dest = last_source->mnt_parent;
>  				}
> 
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