[PATCH v16 06/12] namei: LOOKUP_NO_XDEV: block mountpoint crossing

[Al Viro]

On Sat, Nov 16, 2019 at 11:27:56AM +1100, Aleksa Sarai wrote:

> @@ -1383,6 +1398,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
>  				return -ECHILD;
>  			if (&mparent->mnt == nd->path.mnt)
>  				break;
> +			if (unlikely(nd->flags & LOOKUP_NO_XDEV))
> +				return -EXDEV;
>  			/* we know that mountpoint was pinned */
>  			nd->path.dentry = mountpoint;
>  			nd->path.mnt = &mparent->mnt;
> @@ -1397,6 +1414,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
>  			return -ECHILD;
>  		if (!mounted)
>  			break;
> +		if (unlikely(nd->flags & LOOKUP_NO_XDEV))
> +			return -EXDEV;
>  		nd->path.mnt = &mounted->mnt;
>  		nd->path.dentry = mounted->mnt.mnt_root;
>  		inode = nd->path.dentry->d_inode;

I really don't think we should return hard errors from that function.
Let the caller redo it in refwalk mode.

It's not the fast path, especially for this kind of errors.  Matter of
fact, I'm not sure about -ENOENT returned in another failure case
there - it's probably OK, but again, -ECHILD would be just as good.