kern/58082: mount_null(8) is missing important semantics

[campbell+netbsd%mumble.net@localhost]

>Number:         58082
>Category:       kern
>Synopsis:       mount_null(8) is missing important semantics
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          doc-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Mar 26 21:20:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10, 9, 8, ...
>Organization:
The NullBSD Foundationsystem
>Environment:
>Description:
mount_null(8) exposes the directory tree of a mount point, but doesn't actually hide anything in the file system from file handles, so it cannot be used to make a subtree-restricted NFS export of a file system.

The bulk of the mount_null(8) file system is about the implementation details of layerfs, not about the semantics.  This should be moved to a design document in /usr/share/doc or something and the man page should focus on semantics.
>How-To-Repeat:
man mount_null
>Fix:
Yes, please!