netdev - selftest fib_nexthop_multiprefix failed due to route mismatch

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Date: Tue, 21 Nov 2023 14:40:35 +0800
From: Hangbin Liu <liuhangbin@...il.com>
To: David Ahern <dsahern@...nel.org>
Cc: netdev@...r.kernel.org
Subject: selftest fib_nexthop_multiprefix failed due to route mismatch

Hi David,

Recently when run fib_nexthop_multiprefix test I saw all IPv6 test failed.
e.g.

# ./fib_nexthop_multiprefix.sh
TEST: IPv4: host 0 to host 1, mtu 1300                              [ OK ]
TEST: IPv6: host 0 to host 1, mtu 1300                              [FAIL]

With -v it shows

COMMAND: ip netns exec h0 /usr/sbin/ping6 -s 1350 -c5 -w5 2001:db8:101::1
PING 2001:db8:101::1(2001:db8:101::1) 1350 data bytes
>From 2001:db8:100::64 icmp_seq=1 Packet too big: mtu=1300

--- 2001:db8:101::1 ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms

Route get
2001:db8:101::1 via 2001:db8:100::64 dev eth0 src 2001:db8:100::1 metric 1024 expires 599sec mtu 1300 pref medium
Searching for:
    2001:db8:101::1 from :: via 2001:db8:100::64 dev eth0 src 2001:db8:100::1 .* mtu 1300

TEST: IPv6: host 0 to host 1, mtu 1300                              [FAIL]

So we can get the Packet too big from 2001:db8:100::64 successfully. There
is no "from ::" anymore. I plan to fix this issue. But I can't find which
commit changed the behavior and the client could receive Packet too big
message with correct src address.

Do you have any hints?

Thanks
Hangbin