kern/55932: NFS root is not read only

[martin%NetBSD.org@localhost]

>Number:         55932
>Category:       kern
>Synopsis:       NFS root is not read only
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Jan 16 12:25:00 +0000 2021
>Originator:     Martin Husemann
>Release:        NetBSD 9.99.77
>Organization:
The NetBSD Foundation, Inc.
>Environment:
System: NetBSD martins.aprisoft.de 9.99.77 NetBSD 9.99.77 (GENERIC) #42: Sat Jan 2 21:12:56 CET 2021 martin%martins.aprisoft.de@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:

When using a read-only mount for / on a diskless client the mountall
rc.d script fails:

Mounting all file systems...
mount_nfs: emmas:/fire-below on /: Operation not supported
/etc/rc.d/mountall exited with code 1

This is with a /etc/fstab entry like:

# NetBSD /etc/fstab
# See /usr/share/examples/fstab/ for more examples.
emmas:/fire-below       /               nfs     ro
tmpfs                   /tmp            tmpfs   rw,-sram%30
tmpfs                   /var            tmpfs   rw,-sram%30
kernfs                  /kern           kernfs  rw
ptyfs                   /dev/pts        ptyfs   rw
procfs                  /proc           procfs  rw


It seems that NFS can not downgrade from a rw to a ro mount (this is bug 1)
and the kernel NFS mountroot code seems to alread mount / as read-write
(different than all other file systems mounted as root - this is bug 2).

>How-To-Repeat:
s/a

>Fix:
n/a