linux-kernel - [PATCH 4.0 113/220] mnt: Dont propagate unmounts to locked mounts

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Message-Id: <20150502185859.202299997@linuxfoundation.org>
Date:	Sat,  2 May 2015 21:00:28 +0200
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 4.0 113/220] mnt: Dont propagate unmounts to locked mounts

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

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

From: "Eric W. Biederman" <ebiederm@...ssion.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" <ebiederm@...ssion.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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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