Subject: batman-adv: Don't increase MTU when set by user
From: Sven Eckelmann <[email-masked]>
Date: Wed, 19 Jul 2023 10:15:06 +0200

If the user set an MTU value, it usually means that there are special
requirements for the MTU. But if an interface gots activated, the MTU was
always recalculated and then the user set value was overwritten.

The only reason why this user set value has to be overwritten, is when the
MTU has to be decreased because batman-adv is not able to transfer packets
with the user specified size.

Fixes: 88861ea9acb7 ("[batman-adv] replacing if up/down timer with kernel notifications")
Signed-off-by: Sven Eckelmann <[email-masked]>
Origin: upstream, https://git.open-mesh.org/batman-adv.git/commit/2745af592150b758ee96abf9329dd5f42cf22c25

--- a/net/batman-adv/hard-interface.c
+++ b/net/batman-adv/hard-interface.c
@@ -626,7 +626,19 @@ out:
  */
 void batadv_update_min_mtu(struct net_device *soft_iface)
 {
-	dev_set_mtu(soft_iface, batadv_hardif_min_mtu(soft_iface));
+	struct batadv_priv *bat_priv = netdev_priv(soft_iface);
+	int limit_mtu;
+	int mtu;
+
+	mtu = batadv_hardif_min_mtu(soft_iface);
+
+	if (bat_priv->mtu_set_by_user)
+		limit_mtu = bat_priv->mtu_set_by_user;
+	else
+		limit_mtu = ETH_DATA_LEN;
+
+	mtu = min(mtu, limit_mtu);
+	dev_set_mtu(soft_iface, mtu);
 
 	/* Check if the local translate table should be cleaned up to match a
 	 * new (and smaller) MTU.
--- a/net/batman-adv/soft-interface.c
+++ b/net/batman-adv/soft-interface.c
@@ -154,11 +154,14 @@ static int batadv_interface_set_mac_addr
 
 static int batadv_interface_change_mtu(struct net_device *dev, int new_mtu)
 {
+	struct batadv_priv *bat_priv = netdev_priv(dev);
+
 	/* check ranges */
 	if (new_mtu < 68 || new_mtu > batadv_hardif_min_mtu(dev))
 		return -EINVAL;
 
 	dev->mtu = new_mtu;
+	bat_priv->mtu_set_by_user = new_mtu;
 
 	return 0;
 }
--- a/net/batman-adv/types.h
+++ b/net/batman-adv/types.h
@@ -1547,6 +1547,12 @@ struct batadv_priv {
 	struct net_device *soft_iface;
 
 	/**
+	 * @mtu_set_by_user: MTU was set once by user
+	 * protected by rtnl_lock
+	 */
+	int mtu_set_by_user;
+
+	/**
 	 * @bat_counters: mesh internal traffic statistic counters (see
 	 *  batadv_counters)
 	 */