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teach move_mount(2) to work with OPEN_TREE_CLONE

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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.

Thanks also to Alan Jenkins <alan.christopher.jenkins@gmail.com> for
providing a whole bunch of ways to break things using this interface.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
David Howells 2018-11-05 17:40:31 +00:00 committed by Al Viro
parent 2db154b3ea
commit 44dfd84a6d
1 changed files with 55 additions and 7 deletions
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62
fs/namespace.c
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@ -1842,10 +1842,16 @@ void dissolve_on_fput(struct vfsmount *mnt)
namespace_lock();
lock_mount_hash();
ns = real_mount(mnt)->mnt_ns;
umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
if (ns) {
if (is_anon_ns(ns))
umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
else
ns = NULL;
}
unlock_mount_hash();
namespace_unlock();
free_mnt_ns(ns);
if (ns)
free_mnt_ns(ns);
}
void drop_collected_mounts(struct vfsmount *mnt)
@ -2081,6 +2087,10 @@ static int attach_recursive_mnt(struct mount *source_mnt,
attach_mnt(source_mnt, dest_mnt, dest_mp);
touch_mnt_namespace(source_mnt->mnt_ns);
} else {
if (source_mnt->mnt_ns) {
/* move from anon - the caller will destroy */
list_del_init(&source_mnt->mnt_ns->list);
}
mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
commit_tree(source_mnt);
}
@ -2539,13 +2549,37 @@ static inline int tree_contains_unbindable(struct mount *mnt)
return 0;
}
/*
* Check that there aren't references to earlier/same mount namespaces in the
* specified subtree. Such references can act as pins for mount namespaces
* that aren't checked by the mount-cycle checking code, thereby allowing
* cycles to be made.
*/
static bool check_for_nsfs_mounts(struct mount *subtree)
{
struct mount *p;
bool ret = false;
lock_mount_hash();
for (p = subtree; p; p = next_mnt(p, subtree))
if (mnt_ns_loop(p->mnt.mnt_root))
goto out;
ret = true;
out:
unlock_mount_hash();
return ret;
}
static int do_move_mount(struct path *old_path, struct path *new_path)
{
struct path parent_path = {.mnt = NULL, .dentry = NULL};
struct mnt_namespace *ns;
struct mount *p;
struct mount *old;
struct mountpoint *mp;
int err;
bool attached;
mp = lock_mount(new_path);
if (IS_ERR(mp))
@ -2553,12 +2587,19 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
old = real_mount(old_path->mnt);
p = real_mount(new_path->mnt);
attached = mnt_has_parent(old);
ns = old->mnt_ns;
err = -EINVAL;
if (!check_mnt(p) || !check_mnt(old))
/* The mountpoint must be in our namespace. */
if (!check_mnt(p))
goto out;
if (!mnt_has_parent(old))
/* The thing moved should be either ours or completely unattached. */
if (attached && !check_mnt(old))
goto out;
if (!attached && !is_anon_ns(ns))
goto out;
if (old->mnt.mnt_flags & MNT_LOCKED)
@ -2573,7 +2614,7 @@ 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 out;
/*
* Don't move a mount tree containing unbindable mounts to a destination
@ -2582,12 +2623,14 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
if (IS_MNT_SHARED(p) && tree_contains_unbindable(old))
goto out;
err = -ELOOP;
if (!check_for_nsfs_mounts(old))
goto out;
for (; mnt_has_parent(p); p = p->mnt_parent)
if (p == old)
goto out;
err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp,
&parent_path);
attached ? &parent_path : NULL);
if (err)
goto out;
@ -2596,8 +2639,11 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
list_del_init(&old->mnt_expire);
out:
unlock_mount(mp);
if (!err)
if (!err) {
path_put(&parent_path);
if (!attached)
free_mnt_ns(ns);
}
return err;
}
@ -3289,6 +3335,8 @@ 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.
*/