net: Add driver helper functions to determine checksum offloadability

Add skb_csum_offload_chk driver helper function to determine if a
device with limited checksum offload capabilities is able to offload the
checksum for a given packet.

This patch includes:
  - The skb_csum_offload_chk function. Returns true if checksum is
    offloadable, else false. Optionally, in the case that the checksum
    is not offloable, the function can call skb_checksum_help to resolve
    the checksum. skb_csum_offload_chk also returns whether the checksum
    refers to an encapsulated checksum.
  - Definition of skb_csum_offl_spec structure that caller uses to
    indicate rules about what it can offload (e.g. IPv4/v6, TCP/UDP only,
    whether encapsulated checksums can be offloaded, whether checksum with
    IPv6 extension headers can be offloaded).
  - Ancilary functions called skb_csum_offload_chk_help,
    skb_csum_off_chk_help_cmn, skb_csum_off_chk_help_cmn_v4_only.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -138,6 +138,7 @@
 #include <linux/errqueue.h>
 #include <linux/hrtimer.h>
 #include <linux/netfilter_ingress.h>
+#include <linux/sctp.h>
 
 #include "net-sysfs.h"
 
@@ -2471,6 +2472,141 @@ out:
 }
 EXPORT_SYMBOL(skb_checksum_help);
 
+/* skb_csum_offload_check - Driver helper function to determine if a device
+ * with limited checksum offload capabilities is able to offload the checksum
+ * for a given packet.
+ *
+ * Arguments:
+ *   skb - sk_buff for the packet in question
+ *   spec - contains the description of what device can offload
+ *   csum_encapped - returns true if the checksum being offloaded is
+ *	      encpasulated. That is it is checksum for the transport header
+ *	      in the inner headers.
+ *   checksum_help - when set indicates that helper function should
+ *	      call skb_checksum_help if offload checks fail
+ *
+ * Returns:
+ *   true: Packet has passed the checksum checks and should be offloadable to
+ *	   the device (a driver may still need to check for additional
+ *	   restrictions of its device)
+ *   false: Checksum is not offloadable. If checksum_help was set then
+ *	   skb_checksum_help was called to resolve checksum for non-GSO
+ *	   packets and when IP protocol is not SCTP
+ */
+bool __skb_csum_offload_chk(struct sk_buff *skb,
+			    const struct skb_csum_offl_spec *spec,
+			    bool *csum_encapped,
+			    bool csum_help)
+{
+	struct iphdr *iph;
+	struct ipv6hdr *ipv6;
+	void *nhdr;
+	int protocol;
+	u8 ip_proto;
+
+	if (skb->protocol == htons(ETH_P_8021Q) ||
+	    skb->protocol == htons(ETH_P_8021AD)) {
+		if (!spec->vlan_okay)
+			goto need_help;
+	}
+
+	/* We check whether the checksum refers to a transport layer checksum in
+	 * the outermost header or an encapsulated transport layer checksum that
+	 * corresponds to the inner headers of the skb. If the checksum is for
+	 * something else in the packet we need help.
+	 */
+	if (skb_checksum_start_offset(skb) == skb_transport_offset(skb)) {
+		/* Non-encapsulated checksum */
+		protocol = eproto_to_ipproto(vlan_get_protocol(skb));
+		nhdr = skb_network_header(skb);
+		*csum_encapped = false;
+		if (spec->no_not_encapped)
+			goto need_help;
+	} else if (skb->encapsulation && spec->encap_okay &&
+		   skb_checksum_start_offset(skb) ==
+		   skb_inner_transport_offset(skb)) {
+		/* Encapsulated checksum */
+		*csum_encapped = true;
+		switch (skb->inner_protocol_type) {
+		case ENCAP_TYPE_ETHER:
+			protocol = eproto_to_ipproto(skb->inner_protocol);
+			break;
+		case ENCAP_TYPE_IPPROTO:
+			protocol = skb->inner_protocol;
+			break;
+		}
+		nhdr = skb_inner_network_header(skb);
+	} else {
+		goto need_help;
+	}
+
+	switch (protocol) {
+	case IPPROTO_IP:
+		if (!spec->ipv4_okay)
+			goto need_help;
+		iph = nhdr;
+		ip_proto = iph->protocol;
+		if (iph->ihl != 5 && !spec->ip_options_okay)
+			goto need_help;
+		break;
+	case IPPROTO_IPV6:
+		if (!spec->ipv6_okay)
+			goto need_help;
+		if (spec->no_encapped_ipv6 && *csum_encapped)
+			goto need_help;
+		ipv6 = nhdr;
+		nhdr += sizeof(*ipv6);
+		ip_proto = ipv6->nexthdr;
+		break;
+	default:
+		goto need_help;
+	}
+
+ip_proto_again:
+	switch (ip_proto) {
+	case IPPROTO_TCP:
+		if (!spec->tcp_okay ||
+		    skb->csum_offset != offsetof(struct tcphdr, check))
+			goto need_help;
+		break;
+	case IPPROTO_UDP:
+		if (!spec->udp_okay ||
+		    skb->csum_offset != offsetof(struct udphdr, check))
+			goto need_help;
+		break;
+	case IPPROTO_SCTP:
+		if (!spec->sctp_okay ||
+		    skb->csum_offset != offsetof(struct sctphdr, checksum))
+			goto cant_help;
+		break;
+	case NEXTHDR_HOP:
+	case NEXTHDR_ROUTING:
+	case NEXTHDR_DEST: {
+		u8 *opthdr = nhdr;
+
+		if (protocol != IPPROTO_IPV6 || !spec->ext_hdrs_okay)
+			goto need_help;
+
+		ip_proto = opthdr[0];
+		nhdr += (opthdr[1] + 1) << 3;
+
+		goto ip_proto_again;
+	}
+	default:
+		goto need_help;
+	}
+
+	/* Passed the tests for offloading checksum */
+	return true;
+
+need_help:
+	if (csum_help && !skb_shinfo(skb)->gso_size)
+		skb_checksum_help(skb);
+cant_help:
+	return false;
+}
+EXPORT_SYMBOL(__skb_csum_offload_chk);
+
 __be16 skb_network_protocol(struct sk_buff *skb, int *depth)
 {
 	__be16 type = skb->protocol;