I just hit this with kernel-stable from repo.
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
commit b724e93564045d97ec1179d3acacdac60e658569 (HEAD -> makepkg, tag: v5.1.1, origin/linux-5.1.y)

Date:   Sat May 11 07:49:56 2019 +0200

    Linux 5.1.1

```
[    0.000000] microcode: microcode updated early to revision 0xaa, date = 2018-12-12
[    0.000000] Linux version 5.1.1-gb724e9356404 (user@i87k) (gcc version 8.3.0 (GCC)) #2 SMP Sun May 12 04:05:34 CEST 2019
[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-linux-stable root=UUID=2b8b9ab8-7ac5-4586-aa42-d7ffb12de92a rw root_trim=yes rd.luks.allow-discards rd.luks.options=discard ipv6.disable=1 ipv6.disable_ipv6=1 ipv6.autoconf=0 loglevel=15 log_buf_len=16M ignore_loglevel printk.always_kmsg_dump=y printk.time=y printk.devkmsg=on mminit_loglevel=4 memory_corruption_check=1 fbcon=scrollback:4096k fbcon=font:ProFont6x11 net.ifnames=0 pax_sanitize_slab=full nolvm dobtrfs console=tty1 earlyprintk=vga audit=0 systemd.log_target=kmsg systemd.journald.forward_to_console=1 enforcing=0 udev.children-max=1256 rd.udev.children-max=1256 nohz=on oops=panic crashkernel=128M panic=0 psi=1 sysrq_always_enabled random.trust_cpu=off logo.nologo lpj=0 mce=bootlog reboot=force,cold noexec=on nohibernate scsi_mod.use_blk_mq=1 consoleblank=120 noefi cpuidle.governor=teo zram.num_devices=3 zswap.enabled=0 zswap.same_filled_pages_enabled=1 zswap.compressor=zstd zswap.max_pool_percent=40 zswap.zpool=z3fold i915.alp
[    0.000000] KERNEL supported cpus:
[    0.000000]   Intel GenuineIntel

...

[   28.395363] raw_diag: Unexpected inet family 10
[   28.395372] WARNING: CPU: 8 PID: 1356 at net/ipv4/raw_diag.c:28 raw_get_hashinfo.isra.3+0x1a/0x30 [raw_diag]
[   28.395372] Modules linked in: raw_diag inet_diag af_packet_diag msr xt_TCPMSS iptable_mangle iptable_security iptable_nat nf_nat iptable_raw nf_log_ipv4 nf_log_common xt
_owner xt_LOG xt_connlimit nf_conncount xt_conntrack nf_conntrack nf_defrag_ipv4 xt_hashlimit xt_multiport xt_addrtype snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic intel_rapl x86_pkg_temp_thermal intel_powerclamp coretemp kvm irqbypass i915 crct10dif_pclmul snd_hda_intel i2c_algo_bit crc32_pclmul snd_hda_codec crc32c_intel drm_kms_helper snd_hwdep syscopyarea snd_hda_core sysfillrect sysimgblt snd_pcm fb_sys_fops iTCO_wdt ghash_clmulni_intel snd_timer drm iTCO_vendor_support intel_cstate snd intel_uncore mei_me intel_rapl_perf e1000e mq_deadline soundcore drm_panel_orientation_quirks pcspkr i2c_i801 mei video acpi_pad xhci_pci xhci_hcd
[   28.395384] CPU: 8 PID: 1356 Comm: ss Kdump: loaded Tainted: G     U            5.1.1-gb724e9356404 #2
[   28.395385] Hardware name: System manufacturer System Product Name/PRIME Z370-A, BIOS 1002 07/02/2018
[   28.395386] RIP: 0010:raw_get_hashinfo.isra.3+0x1a/0x30 [raw_diag]
[   28.395387] Code: e8 01 89 46 3c c3 66 2e 0f 1f 84 00 00 00 00 00 48 c7 c0 20 9e 00 93 40 80 ff 02 74 16 80 3d d8 1f 00 00 00 0f 84 4c 04 00 00 <0f> 0b 48 c7 c0 ea ff ff ff c3 66 66 2e 0f 1f 84 00 00 00 00 00 90
[   28.395388] RSP: 0018:ffffb160099f3a58 EFLAGS: 00010246
[   28.395389] RAX: 0000000000000023 RBX: ffff91a223e78b00 RCX: 0000000000000000
[   28.395390] RDX: 0000000000000000 RSI: ffff91a22da15508 RDI: ffff91a22da15508
[   28.395390] RBP: ffff91a1c8320c10 R08: 00000000000004e7 R09: 0000000000000032
[   28.395391] R10: 0000000000000001 R11: ffffb160099f3900 R12: ffff91a1c8320c10
[   28.395392] R13: ffff91a223d22708 R14: ffff91a223d22708 R15: 0000000000000000
[   28.395392] FS:  00007a609c7d3740(0000) GS:ffff91a22da00000(0000) knlGS:0000000000000000
[   28.395393] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   28.395394] CR2: 00007ffe1cd4c0a0 CR3: 00000007c88d6004 CR4: 00000000003606e0
[   28.395394] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   28.395395] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   28.395396] Call Trace:
[   28.395398]  raw_diag_dump+0x3b/0x1d0 [raw_diag]
[   28.395400]  __inet_diag_dump+0x3c/0x80 [inet_diag]
[   28.395402]  netlink_dump+0x11e/0x370
[   28.395403]  __netlink_dump_start+0x197/0x210
[   28.395404]  inet_diag_handler_cmd+0xc8/0xe0 [inet_diag]
[   28.395406]  ? inet_diag_dump_compat+0xe0/0xe0 [inet_diag]
[   28.395407]  sock_diag_rcv_msg+0x10f/0x130
[   28.395408]  ? sock_diag_bind+0x50/0x50
[   28.395409]  netlink_rcv_skb+0x44/0x110
[   28.395410]  sock_diag_rcv+0x1f/0x30
[   28.395411]  netlink_unicast+0x1b2/0x250
[   28.395412]  netlink_sendmsg+0x1ff/0x3d0
[   28.395414]  sock_sendmsg+0x31/0x40
[   28.395415]  ___sys_sendmsg+0x290/0x2f0
[   28.395417]  ? ___sys_recvmsg+0x167/0x200
[   28.395418]  ? __check_object_size+0x3c/0x181
[   28.395420]  ? _copy_to_user+0x26/0x30
[   28.395421]  ? move_addr_to_user+0xb1/0xe0
[   28.395423]  __sys_sendmsg+0x52/0xa0
[   28.395424]  do_syscall_64+0x50/0x150
[   28.395426]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   28.395427] RIP: 0033:0x7a609c8db4c8
[   28.395428] Code: ff eb b4 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 f3 0f 1e fa 48 8d 05 55 fd 0b 00 8b 00 85 c0 75 17 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 60 c3 0f 1f 80 00 00 00 00 55 48 89 e5 41 55
[   28.395429] RSP: 002b:00007ffe1ccafe38 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
[   28.395430] RAX: ffffffffffffffda RBX: 00007ffe1ccaffb0 RCX: 00007a609c8db4c8
[   28.395430] RDX: 0000000000000000 RSI: 00007ffe1ccaff40 RDI: 0000000000000003
[   28.395431] RBP: 000000000000000a R08: 0000000000007fff R09: 0000000000000000
[   28.395431] R10: 00007a609c996250 R11: 0000000000000246 R12: 00007ffe1ccafec0
[   28.395432] R13: 0000000000000003 R14: 00007ffe1ccb0000 R15: 00007ffe1ccaff40
[   28.395433] ---[ end trace 33a830973ab9aad3 ]---
```

I'm on ArchLinux

So this is because `CONFIG_IPV6` is disabled in kernel `.config`
```
include/linux/socket.h:172:#define AF_INET6	10	/* IP version 6			*/
```

and, as I understand it, some process tried to send IPv6 packet?

this shows it happens every 1 second:

```
diff --git a/net/ipv4/raw_diag.c b/net/ipv4/raw_diag.c
index c200065ef9a5..d9d9a4e3e07a 100644
--- a/net/ipv4/raw_diag.c
+++ b/net/ipv4/raw_diag.c
@@ -23,7 +23,8 @@ raw_get_hashinfo(const struct inet_diag_req_v2 *r)
 		return &raw_v6_hashinfo;
 #endif
 	} else {
-		pr_warn_once("Unexpected inet family %d\n",
+		//pr_warn_once("Unexpected inet family %d\n",
+		pr_warn("Unexpected inet family %d\n",
 			     r->sdiag_family);
 		WARN_ON_ONCE(1);
 		return ERR_PTR(-EINVAL);
```

and it's caused by running `ss -anptSudw0eom`, for me.
/usr/bin/ss is owned by iproute2 5.0.0-1
So, `ss` tries to send some ipv6 packet(s) every second, or something... and it even shows it in itself like this:
```
Netid        State          Recv-Q         Send-Q                 Local Address:Port                   Peer Address:Port                                                                                                                                        
p_dgr        UNCONN         0              0                                  *:*                                  *             ino:39983 sk:6
	ver:1 cpy_thresh:0 flags( running auxdata )
	ring_rx(blk_size:524288,blk_nr:8,frm_size:262240,frm_nr:8,tmo:0,features:0x0) skmem:(r0,rb1048576,t0,tb1048576,f0,w0,o96,bl0,d0)
```

```
-a, --all
              Display both listening and non-listening (for TCP this means established connections) sockets.
 -n, --numeric
              Do not try to resolve service names.
-p, --processes
              Show process using socket.
       -t, --tcp
              Display TCP sockets.
       -S, --sctp
              Display SCTP sockets.
       -u, --udp
              Display UDP sockets.
       -d, --dccp
              Display DCCP sockets.
       -w, --raw
              Display RAW sockets.
       -0, --packet
              Display PACKET sockets (alias for -f link).
       -e, --extended
              Show detailed socket information. The output format is:

              uid:<uid_number> ino:<inode_number> sk:<cookie>

              <uid_number>
                     the user id the socket belongs to

              <inode_number>
                     the socket's inode number in VFS

              <cookie>
                     an uuid of the socket
      -o, --options
              Show timer information. For tcp protocol, the output format is:

              timer:(<timer_name>,<expire_time>,<retrans>)

              <timer_name>
                     the name of the timer, there are five kind of timer names:

                     on: means one of these timers: tcp retrans timer, tcp early retrans timer and tail loss probe timer

                     keepalive: tcp keep alive timer

                     timewait: timewait stage timer

                     persist: zero window probe timer

                     unknown: none of the above timers

              <expire_time>
                     how long time the timer will expire

              <retrans>
                     how many times the retran occurs

      -m, --memory
              Show socket memory usage. The output format is:

              skmem:(r<rmem_alloc>,rb<rcv_buf>,t<wmem_alloc>,tb<snd_buf>,
                            f<fwd_alloc>,w<wmem_queued>,
                            o<opt_mem>,bl<back_log>)

              <rmem_alloc>
                     the memory allocated for receiving packet

              <rcv_buf>
                     the total memory can be allocated for receiving packet

              <wmem_alloc>
                     the memory used for sending packet (which has been sent to layer 3)

              <snd_buf>
                     the total memory can be allocated for sending packet

              <fwd_alloc>
                     the  memory  allocated  by  the socket as cache, but not used for receiving/sending packet yet. If need
                     memory to send/receive packet, the memory in this cache will be used before allocate additional memory.

              <wmem_queued>
                     The memory allocated for sending packet (which has not been sent to layer 3)

              <opt_mem>
                     The memory used for storing socket option, e.g., the key for TCP MD5 signature

              <back_log>
                     The memory used for the sk backlog queue. On a process context, if the process is receiving packet, and
                     a  new  packet  is  received,  it  will  be put into the sk backlog queue, so it can be received by the
                     process immediately



```

To be clear, I meant, running `ss` every second like this:
```
while true; do
  ss -anptSudw0eom --vsock                                                                                                      
  #^ thanks to dreisner on freenode for suggesting 'ss' instead of the ancient 'netstat'
  echo '------------'
  sleep 1
done
```

Actually I should've ran it as `sudo ss` to see that it was `tcpdump` causing the above output inside `ss`:

```
Netid         State          Recv-Q         Send-Q                 Local Address:Port                 Peer Address:Port         
p_dgr         UNCONN         0              0                                  *:*                                *              users:(("tcpdump",pid=4895,fd=3)) ino:65078 sk:a
	ver:1 cpy_thresh:0 flags( running auxdata )
	ring_rx(blk_size:524288,blk_nr:8,frm_size:262240,frm_nr:8,tmo:0,features:0x0) skmem:(r0,rb1048576,t0,tb1048576,f0,w0,o96,bl0,d0)
------------
```

Not running `tcpdump` at all, still causes the `raw_diag: Unexpected inet family 10` every 1 second because it was indeed causes by running `ss` every second.

So, in conclusion, something (in my case `ss`) was trying to send out an ipv6 packet in a kernel compiled with unset `CONFIG_IPV6` and it is reported only once, per boot(session), by default!

PS: `ss -n` or `ss -a` is enough to trigger that. Actually it's `ss -w`(raw, displa RAW sockets) that causes it.

Looking back at OP, it was `ss` for them too:
`[  173.185512] CPU: 1 PID: 3904 Comm: ss Tainted: G           O      4.20.1 #1`

so in my case just removing `-w` gets rid of the dmesg message, ie.
`sudo ss -anptSudw0eom --vsock`
became:
`sudo ss -anptSud0eom --vsock`

but note that specifying just `ss -a` alone will imply `-w`, but the above("`sudo ss -anptSud0eom --vsock`" that is) somehow doesn't(imply `-w`).

```
┌── INET: socket monitoring interface ────────────────────────────────────────┐
│                                                                             │
│ CONFIG_INET_DIAG:                                                           │
│                                                                             │
│ Support for INET (TCP, DCCP, etc) socket monitoring interface used by       │
│ native Linux tools such as ss. ss is included in iproute2, currently        │
│ downloadable at:                                                            │
│                                                                             │
│   http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2 │
│                                                                             │
│ If unsure, say Y.                                                           │
│                                                                             │
│ Symbol: INET_DIAG [=m]                                                      │
│ Type  : tristate                                                            │
│ Prompt: INET: socket monitoring interface                                   │
│   Location:                                                                 │
│     -> Networking support (NET [=y])                                        │
│       -> Networking options                                                 │
│         -> TCP/IP networking (INET [=y])                                    │
│   Defined at net/ipv4/Kconfig:426                                           │
│   Depends on: NET [=y] && INET [=y]                                         │
│                                                                             │
│                                                                             │
│                                                                             │
│                                   <OK>                                      │
└─────────────────────────────────────────────────────────────────────────────┘
```

that file `net/ipv4/raw_diag.c` is under `CONFIG_INET_RAW_DIAG` so this would be another way to get rid of seeing the message.

```
┌── RAW: socket monitoring interface ─────────────────────────────────────────────────┐
│                                                                                     │
│ CONFIG_INET_RAW_DIAG:                                                               │
│                                                                                     │
│ Support for RAW socket monitoring interface used by the ss tool.                    │
│ If unsure, say Y.                                                                   │
│                                                                                     │
│ Symbol: INET_RAW_DIAG [=m]                                                          │
│ Type  : tristate                                                                    │
│ Prompt: RAW: socket monitoring interface                                            │
│   Location:                                                                         │
│     -> Networking support (NET [=y])                                                │
│       -> Networking options                                                         │
│         -> TCP/IP networking (INET [=y])                                            │
│           -> INET: socket monitoring interface (INET_DIAG [=m])                     │
│   Defined at net/ipv4/Kconfig:450                                                   │
│   Depends on: NET [=y] && INET [=y] && INET_DIAG [=m] && (IPV6 [=n] || IPV6 [=n]=n) │
│                                                                                     │
│                                                                                     │
│                                                                                     │
│                                       <OK>                                          │
└─────────────────────────────────────────────────────────────────────────────────────┘
```

I suspect this has already been fixed.

Please try and reproduce this with a supported kernel 4.19.42, 5.0.15, or 5.1.1

The 4.20 kernel is not supported by upstream developers.
If you are using a distribution kernel please report it to them.

The issue can still be reproduced with 5.1.15.

> Jun 29 08:12:38 kernel: raw_diag: Unexpected inet family 10
> Jun 29 08:12:38 kernel: WARNING: CPU: 0 PID: 5182 at 0xffffffffa021a03a
> Jun 29 08:12:38 kernel: Modules linked in: raw_diag inet_diag unix_diag evdev
> hv_netvsc hv_utils ptp pps_core cn hyperv_keyboard serio 8250 hv_balloon
> ucs2_str>
> Jun 29 08:12:38 kernel: CPU: 0 PID: 5182 Comm: ss Not tainted 5.1.15-1-ck #2
> Jun 29 08:12:38 kernel: Hardware name: Microsoft Corporation Virtual
> Machine/Virtual Machine, BIOS 090007  06/02/2017
> Jun 29 08:12:38 kernel: RIP: 0010:0xffffffffa021a03a
> Jun 29 08:12:38 kernel: Code: c8 89 46 3c c3 66 66 2e 0f 1f 84 00 00 00 00 00
> 48 c7 c0 20 9d f1 81 40 80 ff 02 74 16 80 3d d4 1f 00 00 00 0f 84 bc 03 00 00
> <0f>
> Jun 29 08:12:38 kernel: RSP: 0018:ffffc9000058ba10 EFLAGS: 00010246
> Jun 29 08:12:38 kernel: RAX: 0000000000000023 RBX: 0000000000000000 RCX:
> 0000000000000000
> Jun 29 08:12:38 kernel: RDX: 0000000000000000 RSI: ffff8881bba14f78 RDI:
> ffff8881bba14f78
> Jun 29 08:12:38 kernel: RBP: ffff8881a64d0900 R08: ffff8881bba14f78 R09:
> 0000000000000004
> Jun 29 08:12:38 kernel: R10: 0000000000000000 R11: 0000000000000001 R12:
> ffff8881a5e69010
> Jun 29 08:12:38 kernel: R13: ffff8881b8d7e2f8 R14: ffff8881a64d0900 R15:
> ffff8881b8d7e2f8
> Jun 29 08:12:38 kernel: FS:  00007f07d1f5a740(0000) GS:ffff8881bba00000(0000)
> knlGS:0000000000000000
> Jun 29 08:12:38 kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> Jun 29 08:12:38 kernel: CR2: 000055be2322a188 CR3: 00000001a896e004 CR4:
> 00000000001606b0

as per my previous comments, this is just the kernel telling you that some program (`ss`) was the first to try to send an ipv6 packet while you have ipv6 support disabled in your kernel(probably not built in? or does the same happen with ipv6.disable=1 in kernel /proc/cmdline? unsure)

not technically an issue, so nothing to fix really.

IPv6 stack is not built in my kernel, because i don't use IPv6. Also, everything else works fine.

If you want to ignore this message see Comment 3 and just comment out those:

```
pr_warn_once("Unexpected inet family %d\n",
           r->sdiag_family);
```
and
```
    WARN_ON_ONCE(1);
```
in file `net/ipv4/raw_diag.c`

You can do this since you're building your own kernel.

As you can see it's not a bug, just kernel printing some info once, only the first time it happens (it = when something tries to send an ipv6 packet )

The `WARN_ON_ONCE(1)` is the one that dumps the stacktrace.