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Subject[PATCH 37/73] union-mount: Prevent topmost file system from being mounted elsewhere [ver #2]
From: Valerie Aurora <>

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 <>
Signed-off-by: David Howells <>

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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