linux-kernel - [PATCH 23/34] union-mount: Prevent topmost file system from being mounted elsewhere

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Message-Id: <1284675145-4391-24-git-send-email-vaurora@redhat.com>
Date:	Thu, 16 Sep 2010 15:12:14 -0700
From:	Valerie Aurora <vaurora@...hat.com>
To:	Alexander Viro <viro@...iv.linux.org.uk>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	Christoph Hellwig <hch@...radead.org>,
	Andreas Gruenbacher <agruen@...e.de>,
	Nick Piggin <npiggin@...nel.dk>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Valerie Aurora <vaurora@...hat.com>
Subject: [PATCH 23/34] union-mount: Prevent topmost file system from being mounted elsewhere

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.

Signed-off-by: Valerie Aurora <vaurora@...hat.com>
---
 fs/namespace.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 61256e6..26efaf3 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1998,6 +1998,11 @@ int do_add_mount(struct vfsmount *newmnt, struct path *path,
 	if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
 		goto unlock;
 
+	/* Top layers of union mounts can't be mounted elsewhere */
+	err = -EBUSY;
+	if (newmnt->mnt_sb->s_union_lower_mnts)
+		goto unlock;
+
 	newmnt->mnt_flags = mnt_flags;
 	if ((err = graft_tree(newmnt, path)))
 		goto unlock;
-- 
1.6.3.3

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