linux-kernel - [PATCH net] net: ipv4: fix incorrect MTU in broadcast routes

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Message-Id: <20250702141515.9414-1-oscmaes92@gmail.com>
Date: Wed,  2 Jul 2025 16:15:15 +0200
From: Oscar Maes <oscmaes92@...il.com>
To: netdev@...r.kernel.org
Cc: davem@...emloft.net,
	dsahern@...nel.org,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	horms@...nel.org,
	stable@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Oscar Maes <oscmaes92@...il.com>
Subject: [PATCH net] net: ipv4: fix incorrect MTU in broadcast routes

Currently, __mkroute_output overrules the MTU value configured for
broadcast routes.

This buggy behaviour can be reproduced with:

ip link set dev eth1 mtu 9000
ip route del broadcast 192.168.0.255 dev eth1 proto kernel scope link src 192.168.0.2
ip route add broadcast 192.168.0.255 dev eth1 proto kernel scope link src 192.168.0.2 mtu 1500

The maximum packet size should be 1500, but it is actually 8000:

ping -b 192.168.0.255 -s 8000

Fix __mkroute_output to allow MTU values to be configured for
for broadcast routes (to support a mixed-MTU local-area-network).

Signed-off-by: Oscar Maes <oscmaes92@...il.com>
---
 net/ipv4/route.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index fccb05fb3..a2a3b6482 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2585,7 +2585,6 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
 	do_cache = true;
 	if (type == RTN_BROADCAST) {
 		flags |= RTCF_BROADCAST | RTCF_LOCAL;
-		fi = NULL;
 	} else if (type == RTN_MULTICAST) {
 		flags |= RTCF_MULTICAST | RTCF_LOCAL;
 		if (!ip_check_mc_rcu(in_dev, fl4->daddr, fl4->saddr,
-- 
2.39.5