ipv4: Update fib_select_default to handle nexthop objects

A user reported [0] hitting the WARN_ON in fib_info_nh:

    [ 8633.839816] ------------[ cut here ]------------
    [ 8633.839819] WARNING: CPU: 0 PID: 1719 at include/net/nexthop.h:251 fib_select_path+0x303/0x381
    ...
    [ 8633.839846] RIP: 0010:fib_select_path+0x303/0x381
    ...
    [ 8633.839848] RSP: 0018:ffffb04d407f7d00 EFLAGS: 00010286
    [ 8633.839850] RAX: 0000000000000000 RBX: ffff9460b9897ee8 RCX: 00000000000000fe
    [ 8633.839851] RDX: 0000000000000000 RSI: 00000000ffffffff RDI: 0000000000000000
    [ 8633.839852] RBP: ffff946076049850 R08: 0000000059263a83 R09: ffff9460840e4000
    [ 8633.839853] R10: 0000000000000014 R11: 0000000000000000 R12: ffffb04d407f7dc0
    [ 8633.839854] R13: ffffffffa4ce3240 R14: 0000000000000000 R15: ffff9460b7681f60
    [ 8633.839857] FS:  00007fcac2e02700(0000) GS:ffff9460bdc00000(0000) knlGS:0000000000000000
    [ 8633.839858] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    [ 8633.839859] CR2: 00007f27beb77e28 CR3: 0000000077734000 CR4: 00000000000006f0
    [ 8633.839867] Call Trace:
    [ 8633.839871]  ip_route_output_key_hash_rcu+0x421/0x890
    [ 8633.839873]  ip_route_output_key_hash+0x5e/0x80
    [ 8633.839876]  ip_route_output_flow+0x1a/0x50
    [ 8633.839878]  __ip4_datagram_connect+0x154/0x310
    [ 8633.839880]  ip4_datagram_connect+0x28/0x40
    [ 8633.839882]  __sys_connect+0xd6/0x100
    ...

The WARN_ON is triggered in fib_select_default which is invoked when
there are multiple default routes. Update the function to use
fib_info_nhc and convert the nexthop checks to use fib_nh_common.

Add test case that covers the affected code path.

[0] https://github.com/FRRouting/frr/issues/6089

Fixes: 493ced1a ("ipv4: Allow routes to use nexthop objects")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_semantics.c

@@ -2014,7 +2014,7 @@ static void fib_select_default(const struct flowi4 *flp, struct fib_result *res)
 
 	hlist_for_each_entry_rcu(fa, fa_head, fa_list) {
 		struct fib_info *next_fi = fa->fa_info;
-		struct fib_nh *nh;
+		struct fib_nh_common *nhc;
 
 		if (fa->fa_slen != slen)
 			continue;
@@ -2037,8 +2037,8 @@ static void fib_select_default(const struct flowi4 *flp, struct fib_result *res)
 		    fa->fa_type != RTN_UNICAST)
 			continue;
 
-		nh = fib_info_nh(next_fi, 0);
-		if (!nh->fib_nh_gw4 || nh->fib_nh_scope != RT_SCOPE_LINK)
+		nhc = fib_info_nhc(next_fi, 0);
+		if (!nhc->nhc_gw_family || nhc->nhc_scope != RT_SCOPE_LINK)
 			continue;
 
 		fib_alias_accessed(fa);

tools/testing/selftests/net/fib_nexthops.sh

@@ -749,6 +749,29 @@ ipv4_fcnal_runtime()
 	run_cmd "ip netns exec me ping -c1 -w1 172.16.101.1"
 	log_test $? 0 "Ping - multipath"
 
+	run_cmd "$IP ro delete 172.16.101.1/32 nhid 122"
+
+	#
+	# multiple default routes
+	# - tests fib_select_default
+	run_cmd "$IP nexthop add id 501 via 172.16.1.2 dev veth1"
+	run_cmd "$IP ro add default nhid 501"
+	run_cmd "$IP ro add default via 172.16.1.3 dev veth1 metric 20"
+	run_cmd "ip netns exec me ping -c1 -w1 172.16.101.1"
+	log_test $? 0 "Ping - multiple default routes, nh first"
+
+	# flip the order
+	run_cmd "$IP ro del default nhid 501"
+	run_cmd "$IP ro del default via 172.16.1.3 dev veth1 metric 20"
+	run_cmd "$IP ro add default via 172.16.1.2 dev veth1 metric 20"
+	run_cmd "$IP nexthop replace id 501 via 172.16.1.3 dev veth1"
+	run_cmd "$IP ro add default nhid 501 metric 20"
+	run_cmd "ip netns exec me ping -c1 -w1 172.16.101.1"
+	log_test $? 0 "Ping - multiple default routes, nh second"
+
+	run_cmd "$IP nexthop delete nhid 501"
+	run_cmd "$IP ro del default"
+
 	#
 	# IPv4 with blackhole nexthops
 	#