flow_dissector: Add PPPoE dissectors

Allow to dissect PPPoE specific fields which are:
- session ID (16 bits)
- ppp protocol (16 bits)
- type (16 bits) - this is PPPoE ethertype, for now only
ETH_P_PPP_SES is supported, possible ETH_P_PPP_DISC
in the future

The goal is to make the following TC command possible:

# tc filter add dev ens6f0 ingress prio 1 protocol ppp_ses \
flower \
pppoe_sid 12 \
ppp_proto ip \
action drop

Note that only PPPoE Session is supported.

Signed-off-by: Wojciech Drewek <[e-mail]>
Acked-by: Guillaume Nault <[e-mail]>
Signed-off-by: Tony Nguyen <[e-mail]>

include/linux/ppp_defs.h

···
 #include <uapi/linux/ppp_defs.h>
 
 #define PPP_FCS(fcs, c) crc_ccitt_byte(fcs, c)
+
+/**
+ * ppp_proto_is_valid - checks if PPP protocol is valid
+ * @proto: PPP protocol
+ *
+ * Assumes proto is not compressed.
+ * Protocol is valid if the value is odd and the least significant bit of the
+ * most significant octet is 0 (see RFC 1661, section 2).
+ */
+static inline bool ppp_proto_is_valid(u16 proto)
+{
+	return !!((proto & 0x0101) == 0x0001);
+}
+
 #endif /* _PPP_DEFS_H_ */

include/net/flow_dissector.h

···
 	u8 num_of_vlans;
 };
 
+/**
+ * struct flow_dissector_key_pppoe:
+ * @session_id: pppoe session id
+ * @ppp_proto: ppp protocol
+ * @type: pppoe eth type
+ */
+struct flow_dissector_key_pppoe {
+	__be16 session_id;
+	__be16 ppp_proto;
+	__be16 type;
+};
+
 enum flow_dissector_key_id {
 	FLOW_DISSECTOR_KEY_CONTROL, /* struct flow_dissector_key_control */
 	FLOW_DISSECTOR_KEY_BASIC, /* struct flow_dissector_key_basic */
···
 	FLOW_DISSECTOR_KEY_CT, /* struct flow_dissector_key_ct */
 	FLOW_DISSECTOR_KEY_HASH, /* struct flow_dissector_key_hash */
 	FLOW_DISSECTOR_KEY_NUM_OF_VLANS, /* struct flow_dissector_key_num_of_vlans */
+	FLOW_DISSECTOR_KEY_PPPOE, /* struct flow_dissector_key_pppoe */
 
 	FLOW_DISSECTOR_KEY_MAX,
 };

net/core/flow_dissector.c

···
 	return result == BPF_OK;
 }
 
+static bool is_pppoe_ses_hdr_valid(struct pppoe_hdr hdr)
+{
+	return hdr.ver == 1 && hdr.type == 1 && hdr.code == 0;
+}
+
 /**
  * __skb_flow_dissect - extract the flow_keys struct and return it
  * @net: associated network namespace, derived from @skb if NULL
···
 			struct pppoe_hdr hdr;
 			__be16 proto;
 		} *hdr, _hdr;
+		u16 ppp_proto;
+
 		hdr = __skb_header_pointer(skb, nhoff, sizeof(_hdr), data, hlen, &_hdr);
 		if (!hdr) {
 			fdret = FLOW_DISSECT_RET_OUT_BAD;
 			break;
 		}
 
-		nhoff += PPPOE_SES_HLEN;
-		switch (hdr->proto) {
-		case htons(PPP_IP):
-			proto = htons(ETH_P_IP);
-			fdret = FLOW_DISSECT_RET_PROTO_AGAIN;
-			break;
-		case htons(PPP_IPV6):
-			proto = htons(ETH_P_IPV6);
-			fdret = FLOW_DISSECT_RET_PROTO_AGAIN;
-			break;
-		default:
+		if (!is_pppoe_ses_hdr_valid(hdr->hdr)) {
 			fdret = FLOW_DISSECT_RET_OUT_BAD;
 			break;
+		}
+
+		/* least significant bit of the most significant octet
+		 * indicates if protocol field was compressed
+		 */
+		ppp_proto = ntohs(hdr->proto);
+		if (ppp_proto & 0x0100) {
+			ppp_proto = ppp_proto >> 8;
+			nhoff += PPPOE_SES_HLEN - 1;
+		} else {
+			nhoff += PPPOE_SES_HLEN;
+		}
+
+		if (ppp_proto == PPP_IP) {
+			proto = htons(ETH_P_IP);
+			fdret = FLOW_DISSECT_RET_PROTO_AGAIN;
+		} else if (ppp_proto == PPP_IPV6) {
+			proto = htons(ETH_P_IPV6);
+			fdret = FLOW_DISSECT_RET_PROTO_AGAIN;
+		} else if (ppp_proto == PPP_MPLS_UC) {
+			proto = htons(ETH_P_MPLS_UC);
+			fdret = FLOW_DISSECT_RET_PROTO_AGAIN;
+		} else if (ppp_proto == PPP_MPLS_MC) {
+			proto = htons(ETH_P_MPLS_MC);
+			fdret = FLOW_DISSECT_RET_PROTO_AGAIN;
+		} else if (ppp_proto_is_valid(ppp_proto)) {
+			fdret = FLOW_DISSECT_RET_OUT_GOOD;
+		} else {
+			fdret = FLOW_DISSECT_RET_OUT_BAD;
+			break;
+		}
+
+		if (dissector_uses_key(flow_dissector,
+				       FLOW_DISSECTOR_KEY_PPPOE)) {
+			struct flow_dissector_key_pppoe *key_pppoe;
+
+			key_pppoe = skb_flow_dissector_target(flow_dissector,
+							      FLOW_DISSECTOR_KEY_PPPOE,
+							      target_container);
+			key_pppoe->session_id = hdr->hdr.sid;
+			key_pppoe->ppp_proto = htons(ppp_proto);
+			key_pppoe->type = htons(ETH_P_PPP_SES);
 		}
 		break;
 	}