Reproduction steps:
-------------------
1. Create an AWS EC2 instance from FreeBSD 14.0-CURRENT-amd64-20230323 UEFI , ami-02dbe14b26d93d722 in us-east-1 (or any newer ami that starts with "FreeBSD 14.0-CURRENT-amd64-")

2. run kldunload if_ena.ko

Result:
-------
Crashes every time. 100% reproducible.

Core dump stack:

__curthread () at /root/freebsd-src/sys/amd64/include/pcpu_aux.h:59
59 __asm("movq %%gs:%P1,%0" : "=r" (td) : "n" (offsetof(struct pcpu,
(kgdb) #0 __curthread () at /root/freebsd-src/sys/amd64/include/pcpu_aux.h:59
#1 doadump (textdump=textdump@entry=1)
at /root/freebsd-src/sys/kern/kern_shutdown.c:407
#2 0xffffffff80bedc6c in kern_reboot (howto=260)
at /root/freebsd-src/sys/kern/kern_shutdown.c:528
#3 0xffffffff80bee18f in vpanic (fmt=<optimized out>,
ap=ap@entry=0xfffffe01fef62ae0)
at /root/freebsd-src/sys/kern/kern_shutdown.c:972
#4 0xffffffff80bedf13 in panic (fmt=<unavailable>)
at /root/freebsd-src/sys/kern/kern_shutdown.c:896
#5 0xffffffff810e2b39 in trap_fatal (frame=0xfffffe01fef62b70, eva=0)
at /root/freebsd-src/sys/amd64/amd64/trap.c:954
#6 <signal handler called>
#7 dump_sa (nw=nw@entry=0xfffffe01fef62d08, attr=attr@entry=1,
sa=0xdeadc0dedeadc0de) at /root/freebsd-src/sys/netlink/route/iface.c:210
#8 0xffffffff80e5659a in dump_iface (nw=nw@entry=0xfffffe01fef62d08,
ifp=ifp@entry=0xfffff80109bbe800, hdr=hdr@entry=0xfffffe01fef62d48,
if_flags_mask=if_flags_mask@entry=0)
at /root/freebsd-src/sys/netlink/route/iface.c:279
#9 0xffffffff80e55e7b in rtnl_handle_ifevent (ifp=0xfffff80109bbe800,
nlmsg_type=<optimized out>, if_flags_mask=0)
at /root/freebsd-src/sys/netlink/route/iface.c:943
#10 0xffffffff80d1fc1d in do_link_state_change (arg=0xfffff80109bbe800,
pending=1) at /root/freebsd-src/sys/net/if.c:2205
#11 0xffffffff80c5233a in taskqueue_run_locked (
queue=queue@entry=0xfffff80106ce7100)
at /root/freebsd-src/sys/kern/subr_taskqueue.c:514
#12 0xffffffff80c5224d in taskqueue_run (queue=0xfffff80106ce7100)
at /root/freebsd-src/sys/kern/subr_taskqueue.c:529
#13 0xffffffff80ba8126 in intr_event_execute_handlers (ie=0xfffff80106a9d300,
p=<optimized out>) at /root/freebsd-src/sys/kern/kern_intr.c:1207
#14 ithread_execute_handlers (ie=0xfffff80106a9d300, p=<optimized out>)
at /root/freebsd-src/sys/kern/kern_intr.c:1220
#15 ithread_loop (arg=arg@entry=0xfffff80106c951c0)
at /root/freebsd-src/sys/kern/kern_intr.c:1308
#16 0xffffffff80ba45c0 in fork_exit (
callout=0xffffffff80ba7eb0 <ithread_loop>, arg=0xfffff80106c951c0,
frame=0xfffffe01fef62f40) at /root/freebsd-src/sys/kern/kern_fork.c:1102
#17 <signal handler called>
(kgdb)

Initial investigation results:
------------------------------

1. printed ifp->if_addr->ifa_addr inside do_link_state_change and it is 0xdeadc0dedeadc0de.

2. Initially I suspected that it is some kernel issue. I therefore tried to find a kernel commit that caused this:

The last non crashing instance is with ami (us-east-1):
FreeBSD 14.0-CURRENT-amd64-20230316 UEFI , ami-0d80d8baae9fea731
uname -a shows kernel commit hash cee09bda03c8


The first crashing instance is with ami (us-east-1:
FreeBSD 14.0-CURRENT-amd64-20230323 UEFI , ami-02dbe14b26d93d722
uname -a shows kernel commit hash b5d43972e394

However I saw that if the ami was a crashing ami - then no matter which kernel I built and installed from sources, the issue reproduced. And the other way, if I used a non crashing ami, no matter which kernel I build and installed form sources, the issue didn't reproduce.

So I figured it is a Userland issue. So I went on to build and install Userland without kernel until I found the commit that caused the issue. (command used make buildworld -j`sysctl -n hw.ncpu` && make installworld -j`sysctl -n hw.ncpu` && reboot)

This commit proved to be: https://reviews.freebsd.org/D39048 (commit before doesnt crash, commits >= crash).

Relevant discussions:
---------------------
Initially I commented in https://reviews.freebsd.org/D39048, which created an email thread where the following was written:

Zhenlei Huang <zlei@FreeBSD.org>:
=================================
iface.c:210 That might be line 214.
Also be aware that `sa == 0xdeadc0dedeadc0de`. 

```
static bool
dump_iface(struct nl_writer *nw, struct ifnet *ifp, const struct nlmsghdr *hdr,
    int if_flags_mask)
{
...
    if ((ifp->if_addr != NULL)) {
        dump_sa(nw, IFLA_ADDRESS, ifp->if_addr->ifa_addr);
    }
...
}
```
There probably have concurrency between ifp destroying and interface status event handling.
`ifp` might be freed before this event handler rtnl_handle_ifevent() .

So only checking `ifp->if_addr != NULL` is not enough.
=================================

Fix thoughts:
-------------
My first thought was to alter the dump_iface code that @zlei pointed out and to check if "if->addr !=0xdeadc0dedeadc0de"
But I didn't find any code that does that or a #define for 0xdeadc0dedeadc0de that I could use.
So I guess this is not the right way to do this.

Would appreciate any suggestions you may have on how to tackle this.