kern/60807: inconsistent reporting of absent birthtime

["campbell+netbsd%mumble.net@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60807
>Category:       kern
>Synopsis:       inconsistent reporting of absent birthtime
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Sep 26 21:10:00 +0000 2026
>Originator:     Taylor R Campbell
>Release:        current, 11, 10, ...
>Organization:
The FileBSD Birthdation, Inc.
>Environment:
>Description:

	File systems that don't support birthtime report it
	inconsistently.  They should report it consistently, but how?

	- Those whose VOP_GETATTR operations leave it uninitialized
	  (such as v7fs, sysvbfs) will, owing to the memset in vn_stat,
	  return {0,0}, which is to say, 1970-01-01T00:00:00Z
	  (proleptic UTC).

	- Those whose VOP_GETATTR operations use vattr_null and then
	  don't initialize va_birthtimespec will return {-1,-1}, which
	  is appealing because it is an invalid struct timespec.  nfs
	  also does this (explicitly, with VNOVAL).

	- FreeBSD seems to use {-1,0}, which is unappealing because
	  that just means 1969-01-01T23:59:59Z (proleptic UTC).

>How-To-Repeat:

	try to write birthtime tests for other bugs

>Fix:

	Yes, please!