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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     David Howells <dhowells@redhat.com>
Cc:     viro@zeniv.linux.org.uk, torvalds@linux-foundation.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        mszeredi@redhat.com
References: <153754740781.17872.7869536526927736855.stgit@warthog.procyon.org.uk>
        <153754743491.17872.12115848333103740766.stgit@warthog.procyon.org.uk>
Date:   Sun, 21 Oct 2018 11:57:43 -0500
In-Reply-To: <153754743491.17872.12115848333103740766.stgit@warthog.procyon.org.uk>
        (David Howells's message of "Fri, 21 Sep 2018 17:30:34 +0100")
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Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]
Sender: linux-kernel-owner@vger.kernel.org
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David Howells <dhowells@redhat.com> writes:

> 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.

In light of recent conversations about double umount_tree.

Do we want to simply limit ourselves to attaching file->f_path of
a FMODE_NEED_UMOUNT file descriptor and clearing FMODE_NEED_UMOUNT
when it is attached?

Either that or perhaps move the logic into mntput on when to perform the
umount_tree?

Otherwise I believe we start running into surprising situations:

This works:
	ofd = open_tree(...);
	dup_fd = openat(ofd, "", O_PATH);
	mount_move(dup_fd, ...);
	close(ofd);
	close(dup_fd);

This should fail:
	ofd = open_tree(...);
	dup_fd = openat(ofd, "", O_PATH);
	close(ofd);
	mount_move(dup_fd, ...);
	close(dup_fd);

Allowing any file descriptor that points to mnt_ns == NULL without
MNT_UMOUNT set seems like it is adding flexibility for no reason.


> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
>
>  fs/namespace.c |   26 ++++++++++++++++++++------
>  1 file changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index dd38141b1723..caf5c55ef555 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -1785,8 +1785,10 @@ void dissolve_on_fput(struct vfsmount *mnt)
>  {
>  	namespace_lock();
>  	lock_mount_hash();
> -	mntget(mnt);
> -	umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
> +	if (!real_mount(mnt)->mnt_ns) {
> +		mntget(mnt);
> +		umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
> +	}

^^^^^^ This change should be unnecessary.

>  	unlock_mount_hash();
>  	namespace_unlock();
>  }
> @@ -2393,6 +2395,7 @@ 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);
> @@ -2403,10 +2406,19 @@ 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;

^^^^^^^^^^^^^^^^^^^^^^^

!old->mnt_ns  should only be allowed when it comes from a file
 descriptor with FMODE_NEED_UMOUNT.


> -	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)
> @@ -2421,7 +2433,7 @@ 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
> @@ -2435,7 +2447,7 @@ 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;

^^^^^^^^^^^^^^^^^^^
Somewhere around here we should have code that clears FMODE_NEED_UMOUNT.


> @@ -3121,6 +3133,8 @@ 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.
>   */