Move ebpf code to its own package to avoid crash issues in Android (#1033)

* Move ebpf code to its own package to avoid crash issues in Android

Older versions of android crashes because of the bytecode files
Even when they aren't loaded as it was our case

* move c file to own folder

* fix lint

client/internal/wgproxy/ebpf/src/portreplace.c

@@ -0,0 +1,90 @@
+#include <stdbool.h>
+#include <linux/if_ether.h> // ETH_P_IP
+#include <linux/udp.h>
+#include <linux/ip.h>
+#include <netinet/in.h>
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+#define bpf_printk(fmt, ...)                                                   \
+  ({                                                                           \
+    char ____fmt[] = fmt;                                                      \
+    bpf_trace_printk(____fmt, sizeof(____fmt), ##__VA_ARGS__);                 \
+  })
+
+const __u32 map_key_proxy_port = 0;
+const __u32 map_key_wg_port = 1;
+
+struct bpf_map_def SEC("maps") xdp_port_map = {
+	.type = BPF_MAP_TYPE_ARRAY,
+	.key_size = sizeof(__u32),
+	.value_size = sizeof(__u16),
+	.max_entries = 10,
+};
+
+__u16 proxy_port = 0;
+__u16 wg_port = 0;
+
+bool read_port_settings() {
+    __u16 *value;
+    value = bpf_map_lookup_elem(&xdp_port_map, &map_key_proxy_port);
+    if(!value) {
+        return false;
+    }
+
+    proxy_port = *value;
+
+    value = bpf_map_lookup_elem(&xdp_port_map, &map_key_wg_port);
+    if(!value) {
+        return false;
+    }
+    wg_port = *value;
+
+    return true;
+}
+
+SEC("xdp")
+int xdp_prog_func(struct xdp_md *ctx) {
+    if(proxy_port == 0 || wg_port == 0) {
+        if(!read_port_settings()){
+            return XDP_PASS;
+        }
+        bpf_printk("proxy port: %d, wg port: %d", proxy_port, wg_port);
+    }
+
+	void *data     = (void *)(long)ctx->data;
+	void *data_end = (void *)(long)ctx->data_end;
+	struct ethhdr *eth = data;
+    struct iphdr  *ip   = (data + sizeof(struct ethhdr));
+    struct udphdr *udp = (data + sizeof(struct ethhdr) + sizeof(struct iphdr));
+
+    // return early if not enough data
+    if (data + sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct udphdr) > data_end){
+        return XDP_PASS;
+    }
+
+    // skip non IPv4 packages
+    if (eth->h_proto != htons(ETH_P_IP)) {
+       return XDP_PASS;
+    }
+
+    if (ip->protocol != IPPROTO_UDP) {
+       return XDP_PASS;
+    }
+
+    // 2130706433 = 127.0.0.1
+    if (ip->daddr != htonl(2130706433)) {
+        return XDP_PASS;
+    }
+
+    if (udp->source != htons(wg_port)){
+        return XDP_PASS;
+    }
+
+    __be16 new_src_port = udp->dest;
+    __be16 new_dst_port = htons(proxy_port);
+    udp->dest = new_dst_port;
+    udp->source = new_src_port;
+	return XDP_PASS;
+}
+char _license[] SEC("license") = "GPL";