kern/54541: kernel panic using "zfs diff"

[joernc%posteo.de@localhost]

>Number:         54541
>Category:       kern
>Synopsis:       kernel panic using "zfs diff"
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Sep 10 13:35:00 +0000 2019
>Originator:     Joern Clausen
>Release:        NetBSD 9.0/amd64 BETA
>Organization:
>Environment:
NetBSD localhost 9.0_BETA NetBSD 9.0_BETA (GENERIC) #0: Sat Sep  7 18:06:30 UTC 2019  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64
>Description:
Environment: VirtualBox VM with 2 vCPUs and 4 GB RAM

$ zpool status
  pool: data-cc
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        data-cc     ONLINE       0     0     0
          sd0       ONLINE       0     0     0
          sd1       ONLINE       0     0     0

errors: No known data errors

  pool: data-mr
 state: ONLINE
  scan: resilvered 77K in 0h0m with 0 errors on Tue Sep 10 14:20:18 2019
config:

        NAME        STATE     READ WRITE CKSUM
        data-mr     ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            sd2     ONLINE       0     0     0
            sd3     ONLINE       0     0     0

errors: No known data errors

$ zfs list
NAME             USED  AVAIL  REFER  MOUNTPOINT
data-cc          560M  30.2G    23K  /data-cc
data-cc/pkgsrc   559M  30.2G   559M  /usr/pkgsrc
data-mr          192M  15.2G    23K  /data-mr
data-mr/pkg      191M  15.2G   191M  /usr/pkg

$ zfs list -t snapshot
zfs list -t snapshot
NAME                        USED  AVAIL  REFER  MOUNTPOINT
data-mr/pkg@pre_wireshark   331K      -  77.8M  -

(BTW: IMHO it would be nice if "zfs list" would already show snapshots.)


The command

$ zfs diff data-mr/pkg@pre_wireshark

results in a kernel panic. After the next boot, this message is in /var/log/messages:

Sep 10 15:14:19 localhost savecore: reboot after panic: [ 3294.7831635] panic: kernel diagnostic assertion "fdm != NULL" failed: file "/usr/src/sys/kern/vfs_trans.c", line 166 mount 0x0 invalid
Sep 10 15:14:19 localhost savecore: system went down at Tue Sep 10 15:12:21 2019

and unfurtunately on my machine

Sep 10 15:14:19 localhost savecore: no dump, not enough free space in /var/crash

This error is repeatable, i.e. after a reboot, "zfs diff" will crash the machine again.

>How-To-Repeat:

>Fix: