Seems that lo0, pflog0 and pfsync0 can be checked.
I think I managed to debug some but not sure if it is useful.
$ gdb ifconfig
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Reading symbols from /sbin/ifconfig...(no debugging symbols found)...done.
(gdb) run lo0
Starting program: /sbin/ifconfig lo0
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
options=600003<RXCSUM,TXCSUM,RXCSUM_IPV6,TXCSUM_IPV6>
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x6
inet 127.0.0.1 netmask 0xff000000
inet6 ::1 prefixlen 64
nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
[Inferior 1 (process 18483) exited normally]
(gdb) run nfe0
Starting program: /sbin/ifconfig nfe0
nfe0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=82008<VLAN_MTU,WOL_MAGIC,LINKSTATE>
Program received signal SIGSEGV, Segmentation fault.
0x00000008010c88ec in _IO_vfprintf_internal (s=<optimized out>, format=<optimized out>, ap=ap@entry=0x7fffffffd148) at vfprintf.c:1649
1649 vfprintf.c: No such file or directory.
(gdb) run vlan20
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /sbin/ifconfig vlan20
vlan20: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
Program received signal SIGSEGV, Segmentation fault.
0x00000008010c88ec in _IO_vfprintf_internal (s=<optimized out>, format=<optimized out>, ap=ap@entry=0x7fffffffd148) at vfprintf.c:1649
1649 vfprintf.c: No such file or directory.
(gdb)