net: openvswitch: add explicit drop action

From: Eric Garver <[e-mail]>

This adds an explicit drop action. This is used by OVS to drop packets
for which it cannot determine what to do. An explicit action in the
kernel allows passing the reason _why_ the packet is being dropped or
zero to indicate no particular error happened (i.e: OVS intentionally
dropped the packet).

Since the error codes coming from userspace mean nothing for the kernel,
we squash all of them into only two drop reasons:
- OVS_DROP_EXPLICIT_WITH_ERROR to indicate a non-zero value was passed
- OVS_DROP_EXPLICIT to indicate a zero value was passed (no error)

e.g. trace all OVS dropped skbs

# perf trace -e skb:kfree_skb --filter="reason >= 0x30000"
[..]
106.023 ping/2465 skb:kfree_skb(skbaddr: 0xffffa0e8765f2000, \
location:0xffffffffc0d9b462, protocol: 2048, reason: 196611)

reason: 196611 --> 0x30003 (OVS_DROP_EXPLICIT)

Also, this patch allows ovs-dpctl.py to add explicit drop actions as:
"drop" -> implicit empty-action drop
"drop(0)" -> explicit non-error action drop
"drop(42)" -> explicit error action drop

Signed-off-by: Eric Garver <[e-mail]>
Co-developed-by: Adrian Moreno <[e-mail]>
Signed-off-by: Adrian Moreno <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/uapi/linux/openvswitch.h

···
  * start of the packet or at the start of the l3 header depending on the value
  * of l3 tunnel flag in the tun_flags field of OVS_ACTION_ATTR_ADD_MPLS
  * argument.
+ * @OVS_ACTION_ATTR_DROP: Explicit drop action.
  *
  * Only a single header can be set with a single %OVS_ACTION_ATTR_SET.  Not all
  * fields within a header are modifiable, e.g. the IPv4 protocol and fragment
···
 	OVS_ACTION_ATTR_CHECK_PKT_LEN, /* Nested OVS_CHECK_PKT_LEN_ATTR_*. */
 	OVS_ACTION_ATTR_ADD_MPLS,     /* struct ovs_action_add_mpls. */
 	OVS_ACTION_ATTR_DEC_TTL,      /* Nested OVS_DEC_TTL_ATTR_*. */
+	OVS_ACTION_ATTR_DROP,         /* u32 error code. */
 
 	__OVS_ACTION_ATTR_MAX,	      /* Nothing past this will be accepted
 				       * from userspace. */

net/openvswitch/actions.c

···
 				return dec_ttl_exception_handler(dp, skb,
 								 key, a);
 			break;
+
+		case OVS_ACTION_ATTR_DROP: {
+			enum ovs_drop_reason reason = nla_get_u32(a)
+				? OVS_DROP_EXPLICIT_WITH_ERROR
+				: OVS_DROP_EXPLICIT;
+
+			ovs_kfree_skb_reason(skb, reason);
+			return 0;
+		}
 		}
 
 		if (unlikely(err)) {

net/openvswitch/drop.h

···
 #define OVS_DROP_REASONS(R)			\
 	R(OVS_DROP_LAST_ACTION)		        \
 	R(OVS_DROP_ACTION_ERROR)		\
+	R(OVS_DROP_EXPLICIT)			\
+	R(OVS_DROP_EXPLICIT_WITH_ERROR)		\
 	/* deliberate comment for trailing \ */
 
 enum ovs_drop_reason {

net/openvswitch/flow_netlink.c

···
 #include <net/tun_proto.h>
 #include <net/erspan.h>
 
+#include "drop.h"
 #include "flow_netlink.h"
 
 struct ovs_len_tbl {
···
 		case OVS_ACTION_ATTR_RECIRC:
 		case OVS_ACTION_ATTR_TRUNC:
 		case OVS_ACTION_ATTR_USERSPACE:
+		case OVS_ACTION_ATTR_DROP:
 			break;
 
 		case OVS_ACTION_ATTR_CT:
···
 	/* Whenever new actions are added, the need to update this
 	 * function should be considered.
 	 */
-	BUILD_BUG_ON(OVS_ACTION_ATTR_MAX != 23);
+	BUILD_BUG_ON(OVS_ACTION_ATTR_MAX != 24);
 
 	if (!actions)
 		return;
···
 			[OVS_ACTION_ATTR_CHECK_PKT_LEN] = (u32)-1,
 			[OVS_ACTION_ATTR_ADD_MPLS] = sizeof(struct ovs_action_add_mpls),
 			[OVS_ACTION_ATTR_DEC_TTL] = (u32)-1,
+			[OVS_ACTION_ATTR_DROP] = sizeof(u32),
 		};
 		const struct ovs_action_push_vlan *vlan;
 		int type = nla_type(a);
···
 			if (err)
 				return err;
 			skip_copy = true;
+			break;
+
+		case OVS_ACTION_ATTR_DROP:
+			if (!nla_is_last(a, rem))
+				return -EINVAL;
 			break;
 
 		default:

tools/testing/selftests/net/openvswitch/ovs-dpctl.py

···
         ("OVS_ACTION_ATTR_CHECK_PKT_LEN", "none"),
         ("OVS_ACTION_ATTR_ADD_MPLS", "none"),
         ("OVS_ACTION_ATTR_DEC_TTL", "none"),
+        ("OVS_ACTION_ATTR_DROP", "uint32"),
     )
 
     class ctact(nla):
···
                     print_str += "recirc(0x%x)" % int(self.get_attr(field[0]))
                 elif field[0] == "OVS_ACTION_ATTR_TRUNC":
                     print_str += "trunc(%d)" % int(self.get_attr(field[0]))
+                elif field[0] == "OVS_ACTION_ATTR_DROP":
+                    print_str += "drop(%d)" % int(self.get_attr(field[0]))
             elif field[1] == "flag":
                 if field[0] == "OVS_ACTION_ATTR_CT_CLEAR":
                     print_str += "ct_clear"
···
         while len(actstr) != 0:
             parsed = False
             if actstr.startswith("drop"):
-                # for now, drops have no explicit action, so we
-                # don't need to set any attributes.  The final
-                # act of the processing chain will just drop the packet
-                return
+                # If no reason is provided, the implicit drop is used (i.e no
+                # action). If some reason is given, an explicit action is used.
+                actstr, reason = parse_extract_field(
+                    actstr,
+                    "drop(",
+                    "([0-9]+)",
+                    lambda x: int(x, 0),
+                    False,
+                    None,
+                )
+                if reason is not None:
+                    self["attrs"].append(["OVS_ACTION_ATTR_DROP", reason])
+                    parsed = True
+                else:
+                    return
 
             elif parse_starts_block(actstr, "^(\d+)", False, True):
                 actstr, output = parse_extract_field(