kern/47040: renaming mount point panics in namei

[Taylor R Campbell <campbell+netbsd%mumble.net@localhost>]

>Number:         47040
>Category:       kern
>Synopsis:       renaming mount point panics in namei
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Oct 05 14:00:08 +0000 2012
>Originator:     Taylor R Campbell <campbell+netbsd%mumble.net@localhost>
>Release:        NetBSD 6.99.11
>Organization:
>Environment:
Architecture: amd64
Machine: amd64
>Description:

        I had a directory /foo/bar/current.ro, and I wanted to rename
        it to /foo/current.ro, so I ran

                % mv bar/current.ro .

        from /foo, forgetting that /foo/bar/current.ro had a file
        system mounted.  This immediately triggered a panic in namei:

                panic: leaf `current.ro' should be empty

        I am not sure which instance of the panic this is -- whether
        it's the one in lookup_once or the one in relookup -- because I
        can't read a kernel core dump on this machine.

>How-To-Repeat:

        Rename a mount point.

>Fix:

        Yes, please!

        (Real fix: rewrite namei...)