No way to forcibly unmount an NFS mount

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No way to forcibly unmount an NFS mount

Demi M. Obenour
>Synopsis: No way to forcibly unmount an NFS mount after the server dies
>Category: kernel
        System      : OpenBSD 6.6
        Details     : OpenBSD 6.6 (GENERIC.MP) #7: Thu Mar 12 11:55:22 MDT 2020
                         [hidden email]:/usr/src/sys/arch/amd64/compile/GENERIC.MP

        Architecture: OpenBSD.amd64
        Machine     : amd64
        I am working on an OpenBSD QubesOS TemplateVM, which requires bind
        mounts so that some directories can persist across AppVM reboots.
        OpenBSD doesn't have bind mounts, so I am using loopback NFS mounts
        instead.  However, when I enter single-user mode, I have no way to
        forcefully unmount the NFS mounts, since the server is dead.
        Setup a loopback NFS mount, then enter single-user mode with
        `kill -TERM 1`.
        Add an option to unmount(2) and umount(8) to forcibly unmount an NFS
        filesystem, even if the server is completely dead.  Adding bind mounts,
        such as FreeBSD's nullfs, would avoid the need for NFS mounts.

OpenBSD 6.6 (GENERIC.MP) #7: Thu Mar 12 11:55:22 MDT 2020
    [hidden email]:/usr/src/sys/arch/amd64/compile/GENERIC.MP
real mem = 385871872 (367MB)
avail mem = 361562112 (344MB)
mpath0 at root
scsibus0 at mpath0: 256 targets
mainbus0 at root
bios0 at mainbus0: SMBIOS rev. 2.4 @ 0xfc001000 (12 entries)
bios0: vendor Xen version "4.8.5-14.fc25" date 12/11/2019
bios0: Xen HVM domU
acpi0 at bios0: ACPI 4.0
acpi0: sleep states S3 S4 S5
acpi0: wakeup devices
acpitimer0 at acpi0: 3579545 Hz, 32 bits
acpimadt0 at acpi0 addr 0xfee00000: PC-AT compat
ioapic0 at mainbus0: apid 1 pa 0xfec00000, version 11, 48 pins, remapped
cpu0 at mainbus0: apid 0 (boot processor)
cpu0: Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz, 2808.58 MHz, 06-9e-09
cpu0: 256KB 64b/line 8-way L2 cache
cpu0: smt 0, core 0, package 0
mtrr: Pentium Pro MTRR support, 8 var ranges, 88 fixed ranges
cpu0: apic clock running at 99MHz
cpu1 at mainbus0: apid 2 (application processor)
cpu1: Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz, 2808.23 MHz, 06-9e-09
cpu1: 256KB 64b/line 8-way L2 cache
cpu1: smt 0, core 1, package 0
acpihpet0 at acpi0: 62500000 Hz
acpiprt0 at acpi0: bus 0 (PCI0)
acpicpu0 at acpi0: C1(@1 halt!)
acpicpu1 at acpi0: C1(@1 halt!)
acpipci0 at acpi0 PCI0: _OSC failed
acpicmos0 at acpi0
"ACPI0007" at acpi0 not configured
"ACPI0007" at acpi0 not configured
cpu0: using VERW MDS workaround (except on vmm entry)
pvbus0 at mainbus0: Xen 4.8
xen0 at pvbus0: features 0x2705, 32 grant table frames, event channel 1
xbf0 at xen0 backend 0 channel 6: disk
scsibus1 at xbf0: 2 targets
sd0 at scsibus1 targ 0 lun 0: <Xen, phy xvda 51712, 0000>
sd0: 10240MB, 512 bytes/sector, 20971520 sectors
xbf1 at xen0 backend 0 channel 7: disk
scsibus2 at xbf1: 2 targets
sd1 at scsibus2 targ 0 lun 0: <Xen, phy xvdb 51728, 0000>
sd1: 2048MB, 512 bytes/sector, 4194304 sectors
xbf2 at xen0 backend 0 channel 8: disk
scsibus3 at xbf2: 2 targets
sd2 at scsibus3 targ 0 lun 0: <Xen, phy xvdc 51744, 0000>
sd2: 10240MB, 512 bytes/sector, 20971520 sectors
xnf0 at xen0 backend 8 channel 9: address 00:16:3e:5e:6c:00
pci0 at mainbus0 bus 0
pchb0 at pci0 dev 0 function 0 "Intel 82441FX" rev 0x02
pcib0 at pci0 dev 1 function 0 "Intel 82371SB ISA" rev 0x00
pciide0 at pci0 dev 1 function 1 "Intel 82371SB IDE" rev 0x00: DMA, channel 0 wired to compatibility, channel 1 wired to compatibility
pciide0: channel 0 disabled (no drives)
pciide0: channel 1 disabled (no drives)
piixpm0 at pci0 dev 1 function 3 "Intel 82371AB Power" rev 0x03: SMBus disabled
xspd0 at pci0 dev 2 function 0 "XenSource Platform Device" rev 0x01
vga1 at pci0 dev 3 function 0 "Bochs VGA" rev 0x02
wsdisplay0 at vga1 mux 1: console (80x25, vt100 emulation)
wsdisplay0: screen 1-5 added (80x25, vt100 emulation)
ehci0 at pci0 dev 4 function 0 "Intel 82801DB USB" rev 0x10: apic 1 int 35
usb0 at ehci0: USB revision 2.0
uhub0 at usb0 configuration 1 interface 0 "Intel EHCI root hub" rev 2.00/1.00 addr 1
isa0 at pcib0
isadma0 at isa0
fdc0 at isa0 port 0x3f0/6 irq 6 drq 2
pckbc0 at isa0 port 0x60/5 irq 1 irq 12
pckbd0 at pckbc0 (kbd slot)
wskbd0 at pckbd0: console keyboard, using wsdisplay0
pms0 at pckbc0 (aux slot)
wsmouse0 at pms0 mux 0
pcppi0 at isa0 port 0x61
spkr0 at pcppi0
uhidev0 at uhub0 port 1 configuration 1 interface 0 "QEMU QEMU USB Tablet" rev 2.00/0.00 addr 2
uhidev0: iclass 3/0
ums0 at uhidev0: 3 buttons, Z dir
wsmouse1 at ums0 mux 0
vscsi0 at root
scsibus4 at vscsi0: 256 targets
softraid0 at root
scsibus5 at softraid0: 256 targets
root on sd0a (1869297ee7933887.a) swap on sd0b dump on sd0b
WARNING: / was not properly unmounted
fd0 at fdc0 drive 1: density unknown

Controller /dev/usb0:
addr 01: 8086:0000 Intel, EHCI root hub
         high speed, self powered, config 1, rev 1.00
         driver: uhub0
addr 02: 0627:0001 QEMU, QEMU USB Tablet
         high speed, power 100 mA, config 1, rev 0.00, iSerial 42
         driver: uhidev0