net: use skb->csum_not_inet to identify packets needing crc32c

skb->csum_not_inet carries the indication on which algorithm is needed to
compute checksum on skb in the transmit path, when skb->ip_summed is equal
to CHECKSUM_PARTIAL. If skb carries a SCTP packet and crc32c hasn't been
yet written in L4 header, skb->csum_not_inet is assigned to 1; otherwise,
assume Internet Checksum is needed and thus set skb->csum_not_inet to 0.

Suggested-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -189,12 +189,13 @@
  *
  *   NETIF_F_SCTP_CRC - This feature indicates that a device is capable of
  *     offloading the SCTP CRC in a packet. To perform this offload the stack
- *     will set ip_summed to CHECKSUM_PARTIAL and set csum_start and csum_offset
- *     accordingly. Note the there is no indication in the skbuff that the
- *     CHECKSUM_PARTIAL refers to an SCTP checksum, a driver that supports
- *     both IP checksum offload and SCTP CRC offload must verify which offload
- *     is configured for a packet presumably by inspecting packet headers; in
- *     case, skb_crc32c_csum_help is provided to compute CRC on SCTP packets.
+ *     will set set csum_start and csum_offset accordingly, set ip_summed to
+ *     CHECKSUM_PARTIAL and set csum_not_inet to 1, to provide an indication in
+ *     the skbuff that the CHECKSUM_PARTIAL refers to CRC32c.
+ *     A driver that supports both IP checksum offload and SCTP CRC32c offload
+ *     must verify which offload is configured for a packet by testing the
+ *     value of skb->csum_not_inet; skb_crc32c_csum_help is provided to resolve
+ *     CHECKSUM_PARTIAL on skbs where csum_not_inet is set to 1.
  *
  *   NETIF_F_FCOE_CRC - This feature indicates that a device is capable of
  *     offloading the FCOE CRC in a packet. To perform this offload the stack
@@ -557,6 +558,7 @@ typedef unsigned char *sk_buff_data_t;
  *	@wifi_acked_valid: wifi_acked was set
  *	@wifi_acked: whether frame was acked on wifi or not
  *	@no_fcs:  Request NIC to treat last 4 bytes as Ethernet FCS
+ *	@csum_not_inet: use CRC32c to resolve CHECKSUM_PARTIAL
  *	@dst_pending_confirm: need to confirm neighbour
   *	@napi_id: id of the NAPI struct this skb came from
  *	@secmark: security marking
@@ -685,7 +687,7 @@ struct sk_buff {
 	__u8			csum_valid:1;
 	__u8			csum_complete_sw:1;
 	__u8			csum_level:2;
-	__u8			__csum_bad_unused:1; /* one bit hole */
+	__u8			csum_not_inet:1;
 
 	__u8			dst_pending_confirm:1;
 #ifdef CONFIG_IPV6_NDISC_NODETYPE

net/core/dev.c

@@ -2649,6 +2649,7 @@ int skb_crc32c_csum_help(struct sk_buff *skb)
 						  crc32c_csum_stub));
 	*(__le32 *)(skb->data + offset) = crc32c_csum;
 	skb->ip_summed = CHECKSUM_NONE;
+	skb->csum_not_inet = 0;
 out:
 	return ret;
 }

net/sched/act_csum.c

@@ -350,6 +350,7 @@ static int tcf_csum_sctp(struct sk_buff *skb, unsigned int ihl,
 	sctph->checksum = sctp_compute_cksum(skb,
 					     skb_network_offset(skb) + ihl);
 	skb->ip_summed = CHECKSUM_NONE;
+	skb->csum_not_inet = 0;
 
 	return 1;
 }

net/sctp/offload.c

@@ -35,6 +35,7 @@
 static __le32 sctp_gso_make_checksum(struct sk_buff *skb)
 {
 	skb->ip_summed = CHECKSUM_NONE;
+	skb->csum_not_inet = 0;
 	return sctp_compute_cksum(skb, skb_transport_offset(skb));
 }
 

net/sctp/output.c

@@ -538,6 +538,7 @@ merge:
 	} else {
 chksum:
 		head->ip_summed = CHECKSUM_PARTIAL;
+		head->csum_not_inet = 1;
 		head->csum_start = skb_transport_header(head) - head->head;
 		head->csum_offset = offsetof(struct sctphdr, checksum);
 	}