[PATCH net-next] openvswitch: add TTL decrement action

[Matteo Croce]

New action to decrement TTL instead of setting it to a fixed value.
This action will decrement the TTL and, in case of expired TTL, send the
packet to userspace via output_userspace() to take care of it.

Supports both IPv4 and IPv6 via the ttl and hop_limit fields, respectively.

Tested with a corresponding change in the userspace:

    # ovs-dpctl dump-flows
    in_port(2),eth(),eth_type(0x0800), packets:0, bytes:0, used:never, actions:dec_ttl,1
    in_port(1),eth(),eth_type(0x0800), packets:0, bytes:0, used:never, actions:dec_ttl,2
    in_port(1),eth(),eth_type(0x0806), packets:0, bytes:0, used:never, actions:2
    in_port(2),eth(),eth_type(0x0806), packets:0, bytes:0, used:never, actions:1

    # ping -c1 192.168.0.2 -t 42
    IP (tos 0x0, ttl 41, id 61647, offset 0, flags [DF], proto ICMP (1), length 84)
        192.168.0.1 > 192.168.0.2: ICMP echo request, id 386, seq 1, length 64
    # ping -c1 192.168.0.2 -t 120
    IP (tos 0x0, ttl 119, id 62070, offset 0, flags [DF], proto ICMP (1), length 84)
        192.168.0.1 > 192.168.0.2: ICMP echo request, id 388, seq 1, length 64
    # ping -c1 192.168.0.2 -t 1
    #

Co-authored-by: Bindiya Kurle <bindiyakurle@xxxxxxxxx>
Signed-off-by: Bindiya Kurle <bindiyakurle@xxxxxxxxx>
Signed-off-by: Matteo Croce <mcroce@xxxxxxxxxx>
---
 include/uapi/linux/openvswitch.h |  2 ++
 net/openvswitch/actions.c        | 46 ++++++++++++++++++++++++++++++++
 net/openvswitch/flow_netlink.c   |  6 +++++
 3 files changed, 54 insertions(+)

diff --git a/include/uapi/linux/openvswitch.h b/include/uapi/linux/openvswitch.h
index 1887a451c388..a3bdb1ecd1e7 100644
--- a/include/uapi/linux/openvswitch.h
+++ b/include/uapi/linux/openvswitch.h
@@ -890,6 +890,7 @@ struct check_pkt_len_arg {
  * @OVS_ACTION_ATTR_CHECK_PKT_LEN: Check the packet length and execute a set
  * of actions if greater than the specified packet length, else execute
  * another set of actions.
+ * @OVS_ACTION_ATTR_DEC_TTL: Decrement the IP TTL.
  *
  * 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
@@ -925,6 +926,7 @@ enum ovs_action_attr {
 	OVS_ACTION_ATTR_METER,        /* u32 meter ID. */
 	OVS_ACTION_ATTR_CLONE,        /* Nested OVS_CLONE_ATTR_*.  */
 	OVS_ACTION_ATTR_CHECK_PKT_LEN, /* Nested OVS_CHECK_PKT_LEN_ATTR_*. */
+	OVS_ACTION_ATTR_DEC_TTL,      /* Decrement ttl action */
 
 	__OVS_ACTION_ATTR_MAX,	      /* Nothing past this will be accepted
 				       * from userspace. */
diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
index 12936c151cc0..077b7f309c93 100644
--- a/net/openvswitch/actions.c
+++ b/net/openvswitch/actions.c
@@ -1174,6 +1174,43 @@ static int execute_check_pkt_len(struct datapath *dp, struct sk_buff *skb,
 			     nla_len(actions), last, clone_flow_key);
 }
 
+static int execute_dec_ttl(struct sk_buff *skb, struct sw_flow_key *key)
+{
+	int err;
+
+	if (skb->protocol == htons(ETH_P_IPV6)) {
+		struct ipv6hdr *nh = ipv6_hdr(skb);
+
+		err = skb_ensure_writable(skb, skb_network_offset(skb) +
+					  sizeof(*nh));
+		if (unlikely(err))
+			return err;
+
+		if (nh->hop_limit <= 1)
+			return -EHOSTUNREACH;
+
+		key->ip.ttl = --nh->hop_limit;
+	} else {
+		struct iphdr *nh = ip_hdr(skb);
+		u8 old_ttl;
+
+		err = skb_ensure_writable(skb, skb_network_offset(skb) +
+					  sizeof(*nh));
+		if (unlikely(err))
+			return err;
+
+		if (nh->ttl <= 1)
+			return -EHOSTUNREACH;
+
+		old_ttl = nh->ttl--;
+		csum_replace2(&nh->check, htons(old_ttl << 8),
+			      htons(nh->ttl << 8));
+		key->ip.ttl = nh->ttl;
+	}
+
+	return 0;
+}
+
 /* Execute a list of actions against 'skb'. */
 static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
 			      struct sw_flow_key *key,
@@ -1345,6 +1382,15 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
 
 			break;
 		}
+
+		case OVS_ACTION_ATTR_DEC_TTL:
+			err = execute_dec_ttl(skb, key);
+			if (err == -EHOSTUNREACH) {
+				output_userspace(dp, skb, key, a, attr,
+						 len, OVS_CB(skb)->cutlen);
+				OVS_CB(skb)->cutlen = 0;
+			}
+			break;
 		}
 
 		if (unlikely(err)) {
diff --git a/net/openvswitch/flow_netlink.c b/net/openvswitch/flow_netlink.c
index 65c2e3458ff5..d17f2d4b420f 100644
--- a/net/openvswitch/flow_netlink.c
+++ b/net/openvswitch/flow_netlink.c
@@ -79,6 +79,7 @@ static bool actions_may_change_flow(const struct nlattr *actions)
 		case OVS_ACTION_ATTR_SET_MASKED:
 		case OVS_ACTION_ATTR_METER:
 		case OVS_ACTION_ATTR_CHECK_PKT_LEN:
+		case OVS_ACTION_ATTR_DEC_TTL:
 		default:
 			return true;
 		}
@@ -3005,6 +3006,7 @@ static int __ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
 			[OVS_ACTION_ATTR_METER] = sizeof(u32),
 			[OVS_ACTION_ATTR_CLONE] = (u32)-1,
 			[OVS_ACTION_ATTR_CHECK_PKT_LEN] = (u32)-1,
+			[OVS_ACTION_ATTR_DEC_TTL] = 0,
 		};
 		const struct ovs_action_push_vlan *vlan;
 		int type = nla_type(a);
@@ -3233,6 +3235,10 @@ static int __ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
 			break;
 		}
 
+		case OVS_ACTION_ATTR_DEC_TTL:
+			/* Nothing to do.  */
+			break;
+
 		default:
 			OVS_NLERR(log, "Unknown Action type %d", type);
 			return -EINVAL;
-- 
2.23.0