netdev - [PATCH net-next V4 6/6] can: gw: use can_gw_hops instead of sk_buff::csum

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Message-Id: <20260128-can_skb_ext-v1-6-330f60fd5d7e@hartkopp.net>
Date: Wed, 28 Jan 2026 23:43:45 +0100
From: Oliver Hartkopp via B4 Relay <devnull+socketcan.hartkopp.net@...nel.org>
To: netdev@...r.kernel.org, linux-can@...r.kernel.org
Cc: Oliver Hartkopp <socketcan@...tkopp.net>, 
 Marc Kleine-Budde <mkl@...gutronix.de>
Subject: [PATCH net-next V4 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: Marc Kleine-Budde <mkl@...gutronix.de>
Signed-off-by: Oliver Hartkopp <socketcan@...tkopp.net>
---
 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 ad89a1913b3422f77fdf91b5552380282b2db09a..2c134c9e5159d659b83881c51b3814debbc5f9c8 100644
--- a/net/can/gw.c
+++ b/net/can/gw.c
@@ -69,12 +69,12 @@ MODULE_ALIAS(CAN_GW_NAME);
 
 #define CGW_MIN_HOPS 1
 #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) "-"
 		 __stringify(CGW_MAX_HOPS) " hops, "
 		 "default: " __stringify(CGW_DEFAULT_HOPS) ")");
@@ -478,23 +478,12 @@ static void can_can_gw_rcv(struct sk_buff *skb, void *data)
 		return;
 
 	/* 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;
 	}
 
@@ -530,15 +519,15 @@ static void can_can_gw_rcv(struct sk_buff *skb, void *data)
 		gwj->dropped_frames++;
 		return;
 	}
 
 	/* 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;
 
 	/* pointer to modifiable CAN frame */
 	cf = (struct canfd_frame *)nskb->data;

-- 
2.47.3