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 up
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@vger.kernel.org
Fixes: 597cfe4f ("nexthop: Add support for IPv4 nexthops")
Reported-by: Edwin Brossette <edwin.brossette@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20220713114853.29406-1-nicolas.dichtel@6wind.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>