Re: RTM_DELROUTE not sent anymore when deleting (last) nexthop of routes in 6.1

[Ido Schimmel <idosch@idosch.org>]

On Thu, Nov 24, 2022 at 10:20:00AM +0100, Jonas Gorski wrote:
> Hello,
> 
> when an IPv4 route gets removed because its nexthop was deleted, the
> kernel does not send a RTM_DELROUTE netlink notifications anymore in
> 6.1. A bisect lead me to 61b91eb33a69 ("ipv4: Handle attempt to delete
> multipath route when fib_info contains an nh reference"), and
> reverting it makes it work again.
> 
> It can be reproduced by doing the following and listening to netlink
> (e.g. via ip monitor)
> 
> ip a a 172.16.1.1/24 dev veth1
> ip nexthop add id 100 via 172.16.1.2 dev veth1
> ip route add 172.16.101.0/24 nhid 100
> ip nexthop del id 100
> 
> where the nexthop del will trigger a RTM_DELNEXTHOP message, but no
> RTM_DELROUTE, but the route is gone afterwards anyways.

I tried the reproducer and I get the same notifications in ip monitor
regardless of whether 61b91eb33a69 is reverted or not.

Looking at the code and thinking about it, I don't think we ever
generated RTM_DELROUTE notifications when IPv4 routes were flushed (to
avoid a notification storm).

Are you running an upstream kernel?

Thanks

> 
> Doing the same thing with IPv6 still works as expected
> 
> ip a a 2001:db8:91::1/64 dev veth1
> ip nexthop add id 100 via 2001:db8:91::2 dev veth1
> ip route add 2001:db8:101::/64 nhid 100
> ip nexthop del id 100
> 
> Here the kernel will send out both the RTM_DELNEXTHOP and the
> RTM_DELROUTE netlink messages.
> 
> Unfortunately my net-foo is not good enough to propose a fix.
> 
> Best regards
> Jonas