[Bug 253593] Process hangs if nullfs mounted cwd moved out of chroot

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=253593

            Bug ID: 253593
           Summary: Process hangs if nullfs mounted cwd moved out of
                    chroot
           Product: Base System
           Version: 12.2-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: koscak.gregor at gmail.com

Overview:
Moving a current working directory of a chrooted/jailed process, which is
accessed via nullfs mount, to the outside of chroot/jail will hang the process
in R state with CPU maxed out. The process cannot be killed. Whether mount is
rw or ro makes no difference.

Steps to reproduce:
Setup directories
/test/mounted/subdir
/test/outside
/test/jail  <--- minimum chroot environment, or static shell

With nullfs mount:
host# mount
/test/inside on /test/jail/mnt (nullfs, local, soft-updates)

Procedure:
[terminal1]
host# chroot /test/jail /bin/sh
jail# cd /mnt/subdir
[terminal2]
host# mv /test/mounted/subdir /test/outside
! at this point we have:
! /test/mounted
! /test/outside/subdir
[terminal1]
jail# pwd
/mnt/subdir
jail# cd .

Actual result:
Shell process hangs w/ CPU utilization maxed and cannot be killed (R state).
Nullfs mount cannot be unmounted, results in D state.
Return to normal operation requires reboot.

Expected result:
Graceful failure.

Tested on:
12.2-RELEASE-p1, 12.2-RELEASE-p3, physical and virtual (virtualbox).
14.0-CURRENT will panic the kernel.

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