flow_offload: add control flag checking helpers

These helpers aim to help drivers, with checking
for the presence of unsupported control flags.

For drivers supporting at least one control flag:
  flow_rule_is_supp_control_flags()

For drivers using flow_rule_match_control(), but not using flags:
  flow_rule_has_control_flags()

For drivers not using flow_rule_match_control():
  flow_rule_match_has_control_flags()

While primarily aimed at FLOW_DISSECTOR_KEY_CONTROL
and flow_rule_match_control(), then the first two
can also be used with FLOW_DISSECTOR_KEY_ENC_CONTROL
and flow_rule_match_enc_control().

These helpers mirrors the existing check done in sfc:
  drivers/net/ethernet/sfc/tc.c +276

Only compile-tested.
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/flow_offload.h

@@ -449,6 +449,61 @@ static inline bool flow_rule_match_key(const struct flow_rule *rule,
 	return dissector_uses_key(rule->match.dissector, key);
 }
 
+/**
+ * flow_rule_is_supp_control_flags() - check for supported control flags
+ * @supp_flags: control flags supported by driver
+ * @ctrl_flags: control flags present in rule
+ * @extack: The netlink extended ACK for reporting errors.
+ *
+ * Return: true if only supported control flags are set, false otherwise.
+ */
+static inline bool flow_rule_is_supp_control_flags(const u32 supp_flags,
+						   const u32 ctrl_flags,
+						   struct netlink_ext_ack *extack)
+{
+	if (likely((ctrl_flags & ~supp_flags) == 0))
+		return true;
+
+	NL_SET_ERR_MSG_FMT_MOD(extack,
+			       "Unsupported match on control.flags %#x",
+			       ctrl_flags);
+
+	return false;
+}
+
+/**
+ * flow_rule_has_control_flags() - check for presence of any control flags
+ * @ctrl_flags: control flags present in rule
+ * @extack: The netlink extended ACK for reporting errors.
+ *
+ * Return: true if control flags are set, false otherwise.
+ */
+static inline bool flow_rule_has_control_flags(const u32 ctrl_flags,
+					       struct netlink_ext_ack *extack)
+{
+	return !flow_rule_is_supp_control_flags(0, ctrl_flags, extack);
+}
+
+/**
+ * flow_rule_match_has_control_flags() - match and check for any control flags
+ * @rule: The flow_rule under evaluation.
+ * @extack: The netlink extended ACK for reporting errors.
+ *
+ * Return: true if control flags are set, false otherwise.
+ */
+static inline bool flow_rule_match_has_control_flags(struct flow_rule *rule,
+						     struct netlink_ext_ack *extack)
+{
+	struct flow_match_control match;
+
+	if (!flow_rule_match_key(rule, FLOW_DISSECTOR_KEY_CONTROL))
+		return false;
+
+	flow_rule_match_control(rule, &match);
+
+	return flow_rule_has_control_flags(match.mask->flags, extack);
+}
+
 struct flow_stats {
 	u64	pkts;
 	u64	bytes;