4.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Eric W. Biederman" <ebied...@xmission.com>

commit 0c56fe31420ca599c90240315f7959bf1b4eb6ce upstream.

If the first mount in shared subtree is locked don't unmount the
shared subtree.

This is ensured by walking through the mounts parents before children
and marking a mount as unmountable if it is not locked or it is locked
but it's parent is marked.

This allows recursive mount detach to propagate through a set of
mounts when unmounting them would not reveal what is under any locked
mount.

Signed-off-by: "Eric W. Biederman" <ebied...@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 fs/pnode.c |   32 +++++++++++++++++++++++++++++---
 fs/pnode.h |    1 +
 2 files changed, 30 insertions(+), 3 deletions(-)

--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -382,6 +382,26 @@ void propagate_mount_unlock(struct mount
 }
 
 /*
+ * Mark all mounts that the MNT_LOCKED logic will allow to be unmounted.
+ */
+static void mark_umount_candidates(struct mount *mnt)
+{
+       struct mount *parent = mnt->mnt_parent;
+       struct mount *m;
+
+       BUG_ON(parent == mnt);
+
+       for (m = propagation_next(parent, parent); m;
+                       m = propagation_next(m, parent)) {
+               struct mount *child = __lookup_mnt_last(&m->mnt,
+                                               mnt->mnt_mountpoint);
+               if (child && (!IS_MNT_LOCKED(child) || IS_MNT_MARKED(m))) {
+                       SET_MNT_MARK(child);
+               }
+       }
+}
+
+/*
  * NOTE: unmounting 'mnt' naturally propagates to all other mounts its
  * parent propagates to.
  */
@@ -398,10 +418,13 @@ static void __propagate_umount(struct mo
                struct mount *child = __lookup_mnt_last(&m->mnt,
                                                mnt->mnt_mountpoint);
                /*
-                * umount the child only if the child has no
-                * other children
+                * umount the child only if the child has no children
+                * and the child is marked safe to unmount.
                 */
-               if (child && list_empty(&child->mnt_mounts)) {
+               if (!child || !IS_MNT_MARKED(child))
+                       continue;
+               CLEAR_MNT_MARK(child);
+               if (list_empty(&child->mnt_mounts)) {
                        list_del_init(&child->mnt_child);
                        child->mnt.mnt_flags |= MNT_UMOUNT;
                        list_move_tail(&child->mnt_list, &mnt->mnt_list);
@@ -420,6 +443,9 @@ int propagate_umount(struct list_head *l
 {
        struct mount *mnt;
 
+       list_for_each_entry_reverse(mnt, list, mnt_list)
+               mark_umount_candidates(mnt);
+
        list_for_each_entry(mnt, list, mnt_list)
                __propagate_umount(mnt);
        return 0;
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -19,6 +19,7 @@
 #define IS_MNT_MARKED(m) ((m)->mnt.mnt_flags & MNT_MARKED)
 #define SET_MNT_MARK(m) ((m)->mnt.mnt_flags |= MNT_MARKED)
 #define CLEAR_MNT_MARK(m) ((m)->mnt.mnt_flags &= ~MNT_MARKED)
+#define IS_MNT_LOCKED(m) ((m)->mnt.mnt_flags & MNT_LOCKED)
 
 #define CL_EXPIRE              0x01
 #define CL_SLAVE               0x02


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