VERDICT — DF-3395: reproduced (privileged rename silently wipes all vlan(4) config on the renamed parent)

Reproduction (DragonFly 6.5-DEVELOPMENT #0, X86_64_GENERIC)

# ifconfig vlan0 create; ifconfig vlan0 vlan 100 vlandev vtnet0
vlan0: ... mtu 1500 / vlan: 100 parent interface: vtnet0 / status: active

# ifconfig vtnet0 name vtnet0r          <- routine SIOCSIFNAME rename
vlan0: ... mtu 1500 / vlan: 0 parent interface: <none>

# ifconfig vtnet0r name vtnet0          <- rename back
vlan0: ... mtu 1500 / vlan: 0 parent interface: <none>   <- NOT restored

Root cause chain: - SIOCSIFNAME handler (sys/net/if.c:2190-2236) invokes EVENTHANDLER_INVOKE(ifnet_detach_event, ifp) at if.c:2206 for a rename, before installing the new name; ifnet_attach_event fires at :2232 after. - vlan registered on ifnet_detach_event (sys/net/vlan/if_vlan.c:377) — the event that IS fired (correct per DF-3344's lesson; lagg's bug was registering on the never-fired ifnet_departure_event). - vlan_ifdetach() (if_vlan.c:432) dispatches to netisr0; vlan_ifdetach_dispatch() (if_vlan.c:410-429) runs vlan_unconfig() on every vlan in the parent's trunk: parent pointer cleared, MTU reset, flags cleared, trunk freed. - Nothing on the vlan side re-attaches when ifnet_attach_event fires: the L2 configuration (parent + tag) is permanently destroyed.

Impact

Every vlan(4) riding the renamed interface is silently disconnected (traffic blackhole for all tagged subnets) until an administrator re-runs the full vlan configuration. Rename is a routine, non-destructive administrative gesture everywhere else in the system (addresses, routes, caps and media survive it). Privileged-only trigger (SIOCSIFNAME is caps_priv_check'd at if.c:2191), memory-safe teardown — hence Low.

Same collateral applies to other ifnet_detach_event consumers: e.g. a bridge member would be silently dropped on rename via bridge's detach handler.

Recommended fix

A rename is not a detach. Fire the announcement-only ifnet_departure_event (the key already exists in eventhandler space and has no state-tearing subscribers — see DF-3344) instead of ifnet_detach_event in the SIOCSIFNAME path, keeping ifnet_detach_event for real interface destruction in if_detach().

Fix status (batch validation)

Applied with the 36 sibling fix.diffs of this batch in findings/poc/_batch41/all_fixes.patch; single in-guest make -j6 nativekernel KERNCONF=X86_64_GENERIC completed RC=0 with zero errors under -Werror (full log: findings/poc/_batch41/fix_build8.log). Behavior A/B intentionally skipped for this Info-tier item.