derp/xdp,cmd/xdpderper: initial skeleton (#12390)

This commit introduces a userspace program for managing an experimental
eBPF XDP STUN server program. derp/xdp contains the eBPF pseudo-C along
with a Go pkg for loading it and exporting its metrics.
cmd/xdpderper is a package main user of derp/xdp.

Updates tailscale/corp#20689

Signed-off-by: Jordan Whited <jordan@tailscale.com>

derp/xdp/headers/update.go

@@ -0,0 +1,99 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"archive/tar"
+	"compress/gzip"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+const (
+	libbpfVersion = "1.4.3"
+	prefix        = "libbpf-" + libbpfVersion
+)
+
+var (
+	filesToExtract = map[string]struct{}{
+		prefix + "/LICENSE.BSD-2-Clause":  {},
+		prefix + "/src/bpf_endian.h":      {},
+		prefix + "/src/bpf_helper_defs.h": {},
+		prefix + "/src/bpf_helpers.h":     {},
+		prefix + "/src/bpf_tracing.h":     {},
+	}
+)
+
+var (
+	flagDest = flag.String("dest", ".", "destination directory")
+)
+
+// TODO(jwhited): go generate strategy for derp/xdp
+func main() {
+	flag.Parse()
+
+	f, err := os.CreateTemp("", "libbpf")
+	if err != nil {
+		log.Panic(err)
+	}
+	defer os.Remove(f.Name())
+
+	resp, err := http.Get(fmt.Sprintf("https://github.com/libbpf/libbpf/archive/refs/tags/v%s.tar.gz", libbpfVersion))
+	if err != nil {
+		log.Panic(err)
+	}
+	defer resp.Body.Close()
+
+	_, err = io.Copy(f, resp.Body)
+	if err != nil {
+		log.Panic(err)
+	}
+
+	_, err = f.Seek(0, 0)
+	if err != nil {
+		log.Panic(err)
+	}
+	g, err := gzip.NewReader(f)
+	if err != nil {
+		log.Panic(err)
+	}
+	defer g.Close()
+	t := tar.NewReader(g)
+
+	seen := make(map[string]bool, len(filesToExtract))
+	for {
+		h, err := t.Next()
+		if err != nil {
+			log.Panic(err)
+		}
+		if strings.Contains(h.Name, "..") {
+			continue
+		}
+		_, ok := filesToExtract[h.Name]
+		if ok {
+			if seen[h.Name] {
+				log.Panicf("saw %s more than once in archive", h.Name)
+			}
+			seen[h.Name] = true
+			p := filepath.Join(*flagDest, filepath.Base(h.Name))
+			e, err := os.Create(p)
+			if err != nil {
+				log.Panic(err)
+			}
+			_, err = io.Copy(e, t)
+			if err != nil {
+				log.Panic(err)
+			}
+			if len(seen) == len(filesToExtract) {
+				break
+			}
+		}
+	}
+}