[can-next 5/5] can: gw: use new can_gw_hops variable instead of re-using csum_start

[Oliver Hartkopp <socketcan@hartkopp.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.

As we still have 16 bit left in the inner protocol space for ethernet/IP
encapsulation the time-to-live counter is moved there to remove the 'hack'
using the skb->csum_start variable.

Patch 5/5 to remove the private CAN bus skb headroom infrastructure.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
---
 include/linux/skbuff.h |  2 ++
 net/can/gw.c           | 23 ++++++-----------------
 2 files changed, 8 insertions(+), 17 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index eccd0b3898a0..7ef0b8e24a30 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -863,10 +863,11 @@ enum skb_tstamp_type {
  *	@vlan_all: vlan fields (proto & tci)
  *	@vlan_proto: vlan encapsulation protocol
  *	@vlan_tci: vlan tag control information
  *	@can_iif: ifindex of the first interface the CAN frame appeared on
  *	@can_framelen: cached echo CAN frame length for bql
+ *	@can_gw_hops: can-gw CAN frame time-to-live counter
  *	@inner_protocol: Protocol (encapsulation)
  *	@inner_ipproto: (aka @inner_protocol) stores ipproto when
  *		skb->inner_protocol_type == ENCAP_TYPE_IPPROTO;
  *	@inner_transport_header: Inner transport layer header (encapsulation)
  *	@inner_network_header: Network layer header (encapsulation)
@@ -1085,10 +1086,11 @@ struct sk_buff {
 
 		/* space for protocols without protocol/header encapsulation */
 		struct {
 			int	can_iif;
 			__u16	can_framelen;
+			__u16	can_gw_hops;
 		};
 	};
 
 	__be16			protocol;
 	__u16			transport_header;
diff --git a/net/can/gw.c b/net/can/gw.c
index 74d771a3540c..fca0566963a2 100644
--- a/net/can/gw.c
+++ b/net/can/gw.c
@@ -68,12 +68,12 @@ 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 short max_hops __read_mostly = CGW_DEFAULT_HOPS;
+module_param(max_hops, ushort, 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) ")");
@@ -472,23 +472,12 @@ 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 (skb->can_gw_hops >= max_hops) {
 		/* indicate deleted frames due to misconfiguration */
 		gwj->deleted_frames++;
 		return;
 	}
 
@@ -517,15 +506,15 @@ 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;
+	nskb->can_gw_hops = skb->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 && nskb->can_gw_hops == 1)
+		nskb->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