kern/49846: fdescfs behavior changed between 6.1.5 and 7.0_BETA - bug?

[riz%NetBSD.org@localhost]

>Number:         49846
>Category:       kern
>Synopsis:       fdescfs behavior changed between 6.1.5 and 7.0_BETA - bug?
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Apr 20 20:15:00 +0000 2015
>Originator:     Jeff Rizzo
>Release:        7.0_BETA
>Organization:
>Environment:
NetBSD vagrant 7.0_BETA NetBSD 7.0_BETA (GENERIC.201504181110Z) amd64
>Description:
On a freshly-installed 7.0_BETA virtual machine, having fdesc mounted over /dev causes "su" to fail:

vagrant% grep fdesc /etc/fstab
fdesc /dev fdesc ro,-o=union 0 0
vagrant% ls -l /dev/tty
crw-rw-rw-  1 root  wheel  1, 0 Apr 20 20:07 /dev/tty
vagrant% df -h |grep fdesc
fdesc                                            1.0K       1.0K         0B 100% /dev
vagrant% su -
 Inappropriate ioctl for device
su: Sorry: conversation failure
vagrant% uname -a
NetBSD vagrant 7.0_BETA NetBSD 7.0_BETA (GENERIC.201504181110Z) amd64

>How-To-Repeat:
- install 7.0_BETA
mount fdesc with:
fdesc /dev fdesc ro,-o=union 0 0

- try to su

This works on a 6.1.5 system.
>Fix: