Add bpf_tunnel_info and helpers

* Update compat.h
* Move bpf_clone_redirect to signed int, matching the implementation.
  This allows error codes to be compared to < 0.
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

src/cc/compat/linux/bpf.h

@@ -258,6 +258,17 @@ enum bpf_func_id {
 	BPF_FUNC_get_cgroup_classid,
 	BPF_FUNC_skb_vlan_push, /* bpf_skb_vlan_push(skb, vlan_proto, vlan_tci) */
 	BPF_FUNC_skb_vlan_pop,  /* bpf_skb_vlan_pop(skb) */
+
+	/**
+	 * bpf_skb_[gs]et_tunnel_key(skb, struct bpf_tunnel_key *key, int size)
+	 * retrieve or populate tunnel metadata
+	 * @skb: pointer to skb
+	 * @key: pointer to 'struct bpf_tunnel_key'
+	 * @size: size of 'struct bpf_tunnel_key'
+	 * Retrun: 0 on success
+	 */
+	BPF_FUNC_skb_get_tunnel_key,
+	BPF_FUNC_skb_set_tunnel_key,
 	__BPF_FUNC_MAX_ID,
 };
 
@@ -280,4 +291,9 @@ struct __sk_buff {
 	__u32 cb[5];
 };
 
+struct bpf_tunnel_key {
+	__u32 tunnel_id;
+	__u32 remote_ipv4;
+};
+
 #endif /* _UAPI__LINUX_BPF_H__ */

src/cc/export/helpers.h

@@ -64,7 +64,7 @@ static int (*bpf_trace_printk_)(const char *fmt, u64 fmt_size, ...) =
 	(void *) BPF_FUNC_trace_printk;
 int bpf_trace_printk(const char *fmt, ...) asm("llvm.bpf.extra");
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(4,2,0)
-static u64 (*bpf_clone_redirect)(void *ctx, u64 ifindex, u64 flags) =
+static int (*bpf_clone_redirect)(void *ctx, u64 ifindex, u64 flags) =
 	(void *) BPF_FUNC_clone_redirect;
 static u64 (*bpf_get_smp_processor_id)(void) =
 	(void *) BPF_FUNC_get_smp_processor_id;
@@ -83,6 +83,10 @@ static u64 (*bpf_skb_vlan_pop)(void *ctx) =
 static void bpf_tail_call_(u64 map_fd, void *ctx, int index) {
   ((void (*)(void *, u64, int))BPF_FUNC_tail_call)(ctx, map_fd, index);
 }
+static int (*bpf_skb_get_tunnel_key)(void *ctx, void *to, u32 size) =
+  (void *) BPF_FUNC_skb_get_tunnel_key;
+static int (*bpf_skb_set_tunnel_key)(void *ctx, void *from, u32 size) =
+  (void *) BPF_FUNC_skb_set_tunnel_key;
 #endif
 
 /* llvm builtin functions that eBPF C program may use to