netdev - Re: [PATCH net 1/2] ip: fix dflt addr selection for connected nexthop

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Date:   Tue, 12 Jul 2022 10:11:43 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        David Ahern <dsahern@...nel.org>
Cc:     netdev@...r.kernel.org, stable@...r.kernel.org,
        Edwin Brossette <edwin.brossette@...nd.com>
Subject: Re: [PATCH net 1/2] ip: fix dflt addr selection for connected
 nexthop

On Wed, 2022-07-06 at 18:05 +0200, Nicolas Dichtel wrote:
> When a nexthop is added, without a gw address, the default scope was set
> to 'host'. Thus, when a source address is selected, 127.0.0.1 may be chosen
> but rejected when the route is used.
> 
> When using a route without a nexthop id, the scope can be configured in the
> route, thus the problem doesn't exist.
> 
> To explain more deeply: when a user creates a nexthop, it cannot specify
> the scope. To create it, the function nh_create_ipv4() calls fib_check_nh()
> with scope set to 0. fib_check_nh() calls fib_check_nh_nongw() wich was
> setting scope to 'host'. Then, nh_create_ipv4() calls
> fib_info_update_nhc_saddr() with scope set to 'host'. The src addr is
> chosen before the route is inserted.
> 
> When a 'standard' route (ie without a reference to a nexthop) is added,
> fib_create_info() calls fib_info_update_nhc_saddr() with the scope set by
> the user. iproute2 set the scope to 'link' by default.
> 
> Here is a way to reproduce the problem:
> ip netns add foo
> ip -n foo link set lo up
> ip netns add bar
> ip -n bar link set lo up
> sleep 1
> 
> ip -n foo link add name eth0 type dummy
> ip -n foo link set eth0 up
> ip -n foo address add 192.168.0.1/24 dev eth0
> 
> ip -n foo link add name veth0 type veth peer name veth1 netns bar
> ip -n foo link set veth0 address 00:09:c0:26:05:82
> ip -n foo link set veth0 arp off

It looks like the 'arp off'/fixed mac address is not relevant for the
test case, could you please drop it, so that the example and the self-
test are more clean?

> ip -n foo link set veth0 up
> ip -n bar link set veth1 address 00:09:c0:26:05:82
> ip -n bar link set veth1 arp off
> ip -n bar link set veth1 up
> 
> ip -n bar address add 192.168.1.1/32 dev veth1
> ip -n bar route add default dev veth1
> 
> ip -n foo nexthop add id 1 dev veth0
> ip -n foo route add 192.168.1.1 nhid 1
> 
> Try to get/use the route:
> > $ ip -n foo route get 192.168.1.1
> > RTNETLINK answers: Invalid argument
> > $ ip netns exec foo ping -c1 192.168.1.1
> > ping: connect: Invalid argument
> 
> Try without nexthop group (iproute2 sets scope to 'link' by dflt):
> ip -n foo route del 192.168.1.1
> ip -n foo route add 192.168.1.1 dev veth0
> 
> Try to get/use the route:
> > $ ip -n foo route get 192.168.1.1
> > 192.168.1.1 dev veth0 src 192.168.0.1 uid 0
> >     cache
> > $ ip netns exec foo ping -c1 192.168.1.1
> > PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data.
> > 64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=0.039 ms
> > 
> > --- 192.168.1.1 ping statistics ---
> > 1 packets transmitted, 1 received, 0% packet loss, time 0ms
> > rtt min/avg/max/mdev = 0.039/0.039/0.039/0.000 ms
> 
> CC: stable@...r.kernel.org
> Fixes: 597cfe4fc339 ("nexthop: Add support for IPv4 nexthops")

Why that commit? It looks like fib_check_nh() used SCOPE_HOST for nongw
next hop since well before ?!?

Otherwise LGTM.

Paolo