[PATCH 37/73] union-mount: Prevent topmost file system from beingmounted elsewhere [ver #2]

[David Howells]

From: Valerie Aurora <vaurora@xxxxxxxxxx>

The device underlying the topmost read-write layer of a file system
cannot be mounted anywhere else on the system.  We keep a pointer to
the union stack in the dentry of the topmost directory, so that dentry
can't be part of a different mount, since dentries are shared between
different mounts of the same device.

Original-author: Valerie Aurora <vaurora@xxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 fs/namespace.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index aa6b1ef..3c950fa 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2091,6 +2091,11 @@ static int do_add_mount(struct mount *newmnt, struct path *path, int mnt_flags)
 	if (S_ISLNK(newmnt->mnt.mnt_root->d_inode->i_mode))
 		goto unlock;
 
+	/* Top layers of union mounts can't be mounted elsewhere */
+	err = -EBUSY;
+	if (newmnt->mnt.mnt_sb->s_union_lower_mnts)
+		goto unlock;
+
 	newmnt->mnt.mnt_flags = mnt_flags;
 	err = graft_tree(newmnt, path);
 

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