[PATCH] unbindable mounts

An unbindable mount does not forward or receive propagation.  Also
unbindable mount disallows bind mounts.  The semantics is as follows.

Bind semantics:
  It is invalid to bind mount an unbindable mount.

Move semantics:
  It is invalid to move an unbindable mount under shared mount.

Clone-namespace semantics:
  If a mount is unbindable in the parent namespace, the corresponding
  cloned mount in the child namespace becomes unbindable too.  Note:
  there is subtle difference, unbindable mounts cannot be bind mounted
  but can be cloned during clone-namespace.

Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 62 insertions, 26 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 089670363704..caa9187f67e5 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -213,6 +213,16 @@ static struct vfsmount *next_mnt(struct vfsmount *p, struct vfsmount *root)
         return list_entry(next, struct vfsmount, mnt_child);
 }
 
+static struct vfsmount *skip_mnt_tree(struct vfsmount *p)
+{
+        struct list_head *prev = p->mnt_mounts.prev;
+        while (prev != &p->mnt_mounts) {
+                p = list_entry(prev, struct vfsmount, mnt_child);
+                prev = p->mnt_mounts.prev;
+        }
+        return p;
+}
+
 static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root,
                                         int flag)
 {
@@ -650,6 +660,9 @@ struct vfsmount *copy_tree(struct vfsmount *mnt, struct dentry *dentry,
         struct vfsmount *res, *p, *q, *r, *s;
         struct nameidata nd;
 
+        if (!(flag & CL_COPY_ALL) && IS_MNT_UNBINDABLE(mnt))
+                return NULL;
+
         res = q = clone_mnt(mnt, dentry, flag);
         if (!q)
                 goto Enomem;
@@ -661,6 +674,10 @@ struct vfsmount *copy_tree(struct vfsmount *mnt, struct dentry *dentry,
                         continue;
 
                 for (s = r; s; s = next_mnt(s, r)) {
+                        if (!(flag & CL_COPY_ALL) && IS_MNT_UNBINDABLE(s)) {
+                                s = skip_mnt_tree(s);
+                                continue;
+                        }
                         while (p != s->mnt_parent) {
                                 p = p->mnt_parent;
                                 q = q->mnt_parent;
@@ -698,18 +715,18 @@ Enomem:
  *
  *  NOTE: in the table below explains the semantics when a source mount
  *  of a given type is attached to a destination mount of a given type.
- *      -------------------------------------------------------------
- *      |         BIND MOUNT OPERATION                               |
- *      |*************************************************************
- *      | source-->| shared        |       private  |       slave    |
- *      | dest     |               |                |                |
- *      |   |      |               |                |                |
- *      |   v      |               |                |                |
- *      |*************************************************************
- *      |  shared  | shared (++)   |     shared (+) |     shared(+++)|
- *      |          |               |                |                |
- *      |non-shared| shared (+)    |      private   |      slave (*) |
- *      **************************************************************
+ * ---------------------------------------------------------------------------
+ * |         BIND MOUNT OPERATION                                            |
+ * |**************************************************************************
+ * | source-->| shared        |       private  |       slave    | unbindable |
+ * | dest     |               |                |                |            |
+ * |   |      |               |                |                |            |
+ * |   v      |               |                |                |            |
+ * |**************************************************************************
+ * |  shared  | shared (++)   |     shared (+) |     shared(+++)|  invalid   |
+ * |          |               |                |                |            |
+ * |non-shared| shared (+)    |      private   |      slave (*) |  invalid   |
+ * ***************************************************************************
  * A bind operation clones the source mount and mounts the clone on the
  * destination mount.
  *
@@ -726,18 +743,18 @@ Enomem:
  * (*)   the cloned mount is made a slave of the same master as that of the
  *       source mount.
  *
- *      --------------------------------------------------------------
- *      |                       MOVE MOUNT OPERATION                 |
- *      |*************************************************************
- *      | source-->| shared        |       private  |       slave    |
- *      | dest     |               |                |                |
- *      |   |      |               |                |                |
- *      |   v      |               |                |                |
- *      |*************************************************************
- *      |  shared  | shared (+)    |     shared (+) |    shared(+++) |
- *      |          |               |                |                |
- *      |non-shared| shared (+*)   |      private   |    slave (*)   |
- *      **************************************************************
+ * ---------------------------------------------------------------------------
+ * |                    MOVE MOUNT OPERATION                                 |
+ * |**************************************************************************
+ * | source-->| shared        |       private  |       slave    | unbindable |
+ * | dest     |               |                |                |            |
+ * |   |      |               |                |                |            |
+ * |   v      |               |                |                |            |
+ * |**************************************************************************
+ * |  shared  | shared (+)    |     shared (+) |    shared(+++) |  invalid   |
+ * |          |               |                |                |            |
+ * |non-shared| shared (+*)   |      private   |    slave (*)   | unbindable |
+ * ***************************************************************************
  *
  * (+)  the mount is moved to the destination. And is then propagated to
  *      all the mounts in the propagation tree of the destination mount.
@@ -854,6 +871,9 @@ static int do_loopback(struct nameidata *nd, char *old_name, int recurse)
 
         down_write(&namespace_sem);
         err = -EINVAL;
+        if (IS_MNT_UNBINDABLE(old_nd.mnt))
+                goto out;
+
         if (!check_mnt(nd->mnt) || !check_mnt(old_nd.mnt))
                 goto out;
 
@@ -911,6 +931,16 @@ static int do_remount(struct nameidata *nd, int flags, int mnt_flags,
         return err;
 }
 
+static inline int tree_contains_unbindable(struct vfsmount *mnt)
+{
+        struct vfsmount *p;
+        for (p = mnt; p; p = next_mnt(p, mnt)) {
+                if (IS_MNT_UNBINDABLE(p))
+                        return 1;
+        }
+        return 0;
+}
+
 static int do_move_mount(struct nameidata *nd, char *old_name)
 {
         struct nameidata old_nd, parent_nd;
@@ -954,6 +984,12 @@ static int do_move_mount(struct nameidata *nd, char *old_name)
          */
         if (old_nd.mnt->mnt_parent && IS_MNT_SHARED(old_nd.mnt->mnt_parent))
                 goto out1;
+        /*
+         * Don't move a mount tree containing unbindable mounts to a destination
+         * mount which is shared.
+         */
+        if (IS_MNT_SHARED(nd->mnt) && tree_contains_unbindable(old_nd.mnt))
+                goto out1;
         err = -ELOOP;
         for (p = nd->mnt; p->mnt_parent != p; p = p->mnt_parent)
                 if (p == old_nd.mnt)
@@ -1266,7 +1302,7 @@ long do_mount(char *dev_name, char *dir_name, char *type_page,
                                     data_page);
         else if (flags & MS_BIND)
                 retval = do_loopback(&nd, dev_name, flags & MS_REC);
-        else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE))
+        else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
                 retval = do_change_type(&nd, flags);
         else if (flags & MS_MOVE)
                 retval = do_move_mount(&nd, dev_name);
@@ -1311,7 +1347,7 @@ int copy_namespace(int flags, struct task_struct *tsk)
         down_write(&namespace_sem);
         /* First pass: copy the tree topology */
         new_ns->root = copy_tree(namespace->root, namespace->root->mnt_root,
-                                        CL_EXPIRE);
+                                        CL_COPY_ALL | CL_EXPIRE);
         if (!new_ns->root) {
                 up_write(&namespace_sem);
                 kfree(new_ns);