[Bug 235920] ifconfig: unable to create another interface after renaming the previous one with the same name

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235920

            Bug ID: 235920
           Summary: ifconfig: unable to create another interface after
                    renaming the previous one with the same name
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: olevole at olevole.ru

Tested on: 12.0-RELEASE, 13-CURRENT (r344398)

When I try to create an interface (via ifconfig), then rename it (via ifconfig
name) and create another one with the same name, this causes an error. E.g:

1) Create and rename interface: 
ifconfig tap100 create name xxx
  or:
ifconfig tap100 create && ifconfig tap100 name xxx


2) check that the interface original interface is missing now:
ifconfig tap100
ifconfig: interface tap100 does not exist


3) great! we can create another one:
ifconfig tap100 create
ifconfig: SIOCIFCREATE2: File exists

As far as I can see, the problem is in the UNR(9) area (alloc_unr_specific)

The path to the error that I see:

a)
src/sbin/ifconfig/ifconfig.c:
  setifname() -> 
     ...
             if (ioctl(s, SIOCSIFNAME, (caddr_t)&ifr) < 0) {
                free(newname);
                err(1, "ioctl SIOCSIFNAME (set name)");
        }
     ...


b)
src/sys/net/if.c:
  ifioctl() ->
      ...
       case SIOCIFCREATE:
        case SIOCIFCREATE2:
                error = priv_check(td, PRIV_NET_IFCREATE);
                if (error == 0)
                        error = if_clone_create(ifr->ifr_name,
                            sizeof(ifr->ifr_name), cmd == SIOCIFCREATE2 ?
                            ifr_data_get_ptr(ifr) : NULL);
                goto out_noref;
      ...

c)
src/sys/net/if_clone.c:
  if_clone_create() ->
      ..
        return (if_clone_createif(ifc, name, len, params));
      ..

d)
src/sys/net/if_clone.c:
  if_clone_createif() ->
     ..
       err = ifc_simple_create(ifc, name, len, params);
     ..


e)
src/sys/net/if_clone.c:
  ifc_simple_create() ->
     ..
      err = ifc_alloc_unit(ifc, &unit);
     ..

f)
src/sys/net/if_clone.c:
  ifc_alloc_unit() ->
    ..
      if (alloc_unr_specific(ifc->ifc_unrhdr, *unit) == -1) {
                return (EEXIST);
        }
    ..


If we create an interface for the first time, this function ends with:
IF_CLONE_ADDREF(ifc);

When we create an interface a second time, we get:
return (EEXIST);

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