linux-kernel - [PATCH can-next v2 6/6] can: gw: use can_gw_hops instead of sk_buff::csum

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Message-Id: <20260128-can-skb-ext-v2-6-fe64aa152c8a@pengutronix.de>
Date: Wed, 28 Jan 2026 10:28:58 +0100
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Marc Kleine-Budde <mkl@...gutronix.de>, 
 Vincent Mailhol <mailhol@...nel.org>, 
 Oliver Hartkopp <socketcan@...tkopp.net>, 
 Robin van der Gracht <robin@...tonic.nl>, 
 Oleksij Rempel <o.rempel@...gutronix.de>, kernel@...gutronix.de, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Simon Horman <horms@...nel.org>
Cc: linux-can@...r.kernel.org, linux-kernel@...r.kernel.org, 
 netdev@...r.kernel.org
Subject: [PATCH can-next v2 6/6] can: gw: use can_gw_hops instead of
 sk_buff::csum_start

From: Oliver Hartkopp <socketcan@...tkopp.net>

As CAN skbs don't use IP checksums the skb->csum_start variable was used to
store the can-gw CAN frame time-to-live counter together with
skb->ip_summed set to CHECKSUM_UNNECESSARY.

Remove the 'hack' using the skb->csum_start variable and move the content
to can_skb_ext::can_gw_hops of the CAN skb extensions.

The module parameter 'max_hops' has been reduced to a single byte to fit
can_skb_ext::can_gw_hops as the maximum value to be stored is 6.

Signed-off-by: Oliver Hartkopp <socketcan@...tkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@...gutronix.de>
---
 net/can/gw.c | 23 ++++++-----------------
 1 file changed, 6 insertions(+), 17 deletions(-)

diff --git a/net/can/gw.c b/net/can/gw.c
index ad89a1913b34..2c134c9e5159 100644
--- a/net/can/gw.c
+++ b/net/can/gw.c
@@ -71,8 +71,8 @@ MODULE_ALIAS(CAN_GW_NAME);
 #define CGW_MAX_HOPS 6
 #define CGW_DEFAULT_HOPS 1

-static unsigned int max_hops __read_mostly = CGW_DEFAULT_HOPS;
-module_param(max_hops, uint, 0444);
+static unsigned char max_hops __read_mostly = CGW_DEFAULT_HOPS;
+module_param(max_hops, byte, 0444);
 MODULE_PARM_DESC(max_hops,
 		 "maximum " CAN_GW_NAME " routing hops for CAN frames "
 		 "(valid values: " __stringify(CGW_MIN_HOPS) "-"
@@ -480,19 +480,8 @@ static void can_can_gw_rcv(struct sk_buff *skb, void *data)
 	/* Do not handle CAN frames routed more than 'max_hops' times.
 	 * In general we should never catch this delimiter which is intended
 	 * to cover a misconfiguration protection (e.g. circular CAN routes).
-	 *
-	 * The Controller Area Network controllers only accept CAN frames with
-	 * correct CRCs - which are not visible in the controller registers.
-	 * According to skbuff.h documentation the csum_start element for IP
-	 * checksums is undefined/unused when ip_summed == CHECKSUM_UNNECESSARY.
-	 * Only CAN skbs can be processed here which already have this property.
 	 */
-
-#define cgw_hops(skb) ((skb)->csum_start)
-
-	BUG_ON(skb->ip_summed != CHECKSUM_UNNECESSARY);
-
-	if (cgw_hops(skb) >= max_hops) {
+	if (csx->can_gw_hops >= max_hops) {
 		/* indicate deleted frames due to misconfiguration */
 		gwj->deleted_frames++;
 		return;
@@ -532,11 +521,11 @@ static void can_can_gw_rcv(struct sk_buff *skb, void *data)
 	}

 	/* put the incremented hop counter in the cloned skb */
-	cgw_hops(nskb) = cgw_hops(skb) + 1;
+	ncsx->can_gw_hops = csx->can_gw_hops + 1;

 	/* first processing of this CAN frame -> adjust to private hop limit */
-	if (gwj->limit_hops && cgw_hops(nskb) == 1)
-		cgw_hops(nskb) = max_hops - gwj->limit_hops + 1;
+	if (gwj->limit_hops && ncsx->can_gw_hops == 1)
+		ncsx->can_gw_hops = max_hops - gwj->limit_hops + 1;

 	nskb->dev = gwj->dst.dev;

-- 
2.51.0