netdev - [PATCH net-next] ipv4: Honor "ignore_routes_with

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Message-ID: <20250430100240.484636-1-idosch@nvidia.com>
Date: Wed, 30 Apr 2025 13:02:40 +0300
From: Ido Schimmel <idosch@...dia.com>
To: <netdev@...r.kernel.org>
CC: <davem@...emloft.net>, <kuba@...nel.org>, <pabeni@...hat.com>,
	<edumazet@...gle.com>, <dsahern@...nel.org>, <horms@...nel.org>,
	<willemb@...gle.com>, Ido Schimmel <idosch@...dia.com>
Subject: [PATCH net-next] ipv4: Honor "ignore_routes_with_linkdown" sysctl in nexthop selection

Commit 32607a332cfe ("ipv4: prefer multipath nexthop that matches source
address") changed IPv4 nexthop selection to prefer a nexthop whose
nexthop device is assigned the specified source address for locally
generated traffic.

While the selection honors the "fib_multipath_use_neigh" sysctl and will
not choose a nexthop with an invalid neighbour, it does not honor the
"ignore_routes_with_linkdown" sysctl and can choose a nexthop without a
carrier:

 $ sysctl net.ipv4.conf.all.ignore_routes_with_linkdown
 net.ipv4.conf.all.ignore_routes_with_linkdown = 1
 $ ip route show 198.51.100.0/24
 198.51.100.0/24
         nexthop via 192.0.2.2 dev dummy1 weight 1
         nexthop via 192.0.2.18 dev dummy2 weight 1 dead linkdown
 $ ip route get 198.51.100.1 from 192.0.2.17
 198.51.100.1 from 192.0.2.17 via 192.0.2.18 dev dummy2 uid 0

Solve this by skipping over nexthops whose assigned hash upper bound is
minus one, which is the value assigned to nexthops that do not have a
carrier when the "ignore_routes_with_linkdown" sysctl is set.

In practice, this probably does not matter a lot as the initial route
lookup for the source address would not choose a nexthop that does not
have a carrier in the first place, but the change does make the code
clearer.

Signed-off-by: Ido Schimmel <idosch@...dia.com>
---
 net/ipv4/fib_semantics.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 03959c60d128..dabe2b7044ab 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -2188,7 +2188,14 @@ void fib_select_multipath(struct fib_result *res, int hash,
 	saddr = fl4 ? fl4->saddr : 0;
 
 	change_nexthops(fi) {
-		if (use_neigh && !fib_good_nh(nexthop_nh))
+		int nh_upper_bound;
+
+		/* Nexthops without a carrier are assigned an upper bound of
+		 * minus one when "ignore_routes_with_linkdown" is set.
+		 */
+		nh_upper_bound = atomic_read(&nexthop_nh->fib_nh_upper_bound);
+		if (nh_upper_bound == -1 ||
+		    (use_neigh && !fib_good_nh(nexthop_nh)))
 			continue;
 
 		if (!found) {
@@ -2197,7 +2204,7 @@ void fib_select_multipath(struct fib_result *res, int hash,
 			found = !saddr || nexthop_nh->nh_saddr == saddr;
 		}
 
-		if (hash > atomic_read(&nexthop_nh->fib_nh_upper_bound))
+		if (hash > nh_upper_bound)
 			continue;
 
 		if (!saddr || nexthop_nh->nh_saddr == saddr) {
-- 
2.49.0