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Subject: [PATCH 03/32] teach move_mount(2) to work with OPEN_TREE_CLONE [ver
 #9]
From:   David Howells <dhowells@redhat.com>
To:     viro@zeniv.linux.org.uk
Cc:     dhowells@redhat.com, linux-fsdevel@vger.kernel.org,
        torvalds@linux-foundation.org, linux-kernel@vger.kernel.org
Date:   Tue, 10 Jul 2018 23:41:48 +0100
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References: <153126248868.14533.9751473662727327569.stgit@warthog.procyon.org.uk>
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From: Al Viro <viro@zeniv.linux.org.uk>

Allow a detached tree created by open_tree(..., OPEN_TREE_CLONE) to be
attached by move_mount(2).

If by the time of final fput() of OPEN_TREE_CLONE-opened file its tree is
not detached anymore, it won't be dissolved.  move_mount(2) is adjusted
to handle detached source.

That gives us equivalents of mount --bind and mount --rbind.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---

 fs/namespace.c |   26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index e95b2bc8fcfe..bd2526b24afb 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1845,8 +1845,10 @@ void dissolve_on_fput(struct vfsmount *mnt)
 {
 	namespace_lock();
 	lock_mount_hash();
-	mntget(mnt);
-	umount_tree(real_mount(mnt), UMOUNT_SYNC);
+	if (!real_mount(mnt)->mnt_ns) {
+		mntget(mnt);
+		umount_tree(real_mount(mnt), UMOUNT_SYNC);
+	}
 	unlock_mount_hash();
 	namespace_unlock();
 }
@@ -2453,6 +2455,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 	struct mount *old;
 	struct mountpoint *mp;
 	int err;
+	bool attached;
 
 	mp = lock_mount(new_path);
 	err = PTR_ERR(mp);
@@ -2463,10 +2466,19 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 	p = real_mount(new_path->mnt);
 
 	err = -EINVAL;
-	if (!check_mnt(p) || !check_mnt(old))
+	/* The mountpoint must be in our namespace. */
+	if (!check_mnt(p))
+		goto out1;
+	/* The thing moved should be either ours or completely unattached. */
+	if (old->mnt_ns && !check_mnt(old))
 		goto out1;
 
-	if (!mnt_has_parent(old))
+	attached = mnt_has_parent(old);
+	/*
+	 * We need to allow open_tree(OPEN_TREE_CLONE) followed by
+	 * move_mount(), but mustn't allow "/" to be moved.
+	 */
+	if (old->mnt_ns && !attached)
 		goto out1;
 
 	if (old->mnt.mnt_flags & MNT_LOCKED)
@@ -2481,7 +2493,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 	/*
 	 * Don't move a mount residing in a shared parent.
 	 */
-	if (IS_MNT_SHARED(old->mnt_parent))
+	if (attached && IS_MNT_SHARED(old->mnt_parent))
 		goto out1;
 	/*
 	 * Don't move a mount tree containing unbindable mounts to a destination
@@ -2495,7 +2507,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 			goto out1;
 
 	err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp,
-				   &parent_path);
+				   attached ? &parent_path : NULL);
 	if (err)
 		goto out1;
 
@@ -3179,6 +3191,8 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
 
 /*
  * Move a mount from one place to another.
+ * In combination with open_tree(OPEN_TREE_CLONE [| AT_RECURSIVE]) it can be
+ * used to copy a mount subtree.
  *
  * Note the flags value is a combination of MOVE_MOUNT_* flags.
  */