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My trigger is a second addrconf_ifdown() racing the pending > addrconf_dad_work from an earlier up, both inside a single "ip link set > lo up" call (that fires both NETDEV_UP and NETDEV_CHANGE through > addrconf_notify()). The route gets deleted, the notifier gets skipped, > and the async work later runs with ifp->rt NULL. Please share your reproducer. Earlier in the thread you said you're working on [1]. I asked Claude to translate the syz reproducer to bash and it came up with [2]. It does set an MTU below the minimum on the loopback device. I verified that the issue is reproduced without my patch and doesn't reproduce with my patch. syzbot was not able to test my patch because of some issue on its end. [1] https://syzkaller.appspot.com/bug?extid=57f410c9a4f8d7a441d6 [2] #!/bin/bash # syzbot 57f410c9a4f8d7a441d6 as iproute2. Run in a fresh netns so the # group-wide MTU change only reaches lo. ip netns add syz ns="ip netns exec syz" $ns ip link set dev lo up # 1st sendmsg: RTM_NEWLINK, no ifindex, IFLA_GROUP=0, IFLA_MTU=68 # Walks group 0 in registration order; lo is first, so it is already # at 68 when a later fallback tunnel (sit0, min_mtu 1280) makes the # whole request return -EINVAL. syzkaller ignores that, so do we. # -> rtnl_group_changelink() -> do_setlink() on every group-0 device $ns ip link set group 0 mtu 68 2>/dev/null $ns ip -d link show dev lo | grep -ow "mtu [0-9]*" # write(.../conf/all/keep_addr_on_down, "1") $ns sysctl -wq net.ipv6.conf.all.keep_addr_on_down=1 # "repeat":true — the whole program loops; the warning is a race between # the DAD work queued by fixup_permanent_addr() and the tail # addrconf_ifdown() in the same NETDEV_UP, so it needs a few iterations. for i in $(seq 1 20); do # 2nd sendmsg: RTM_NEWADDR 2001::fb/64 dev lo, no NLM_F_EXCL $ns ip -6 address replace 2001::fb/64 dev lo # SIOCSIFFLAGS lo 0x1, then SIOCSIFFLAGS lo 0x0 $ns ip link set dev lo up $ns ip link set dev lo down done echo "warnings: $(dmesg | grep -c 'missing its host route')" $ns ip -6 address show dev lo $ns ip -6 route show table local ip netns del syz