net/tstun: move TUN failure diagnostics to OS-specific files

Mostly so the Linux one can use Linux-specific stuff in package
syscall and not use os/exec for uname for portability.

But also it helps deps a tiny bit on iOS.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/tun.go

@@ -7,10 +7,8 @@
 package tstun
 
 import (
-	"bytes"
 	"errors"
 	"os"
-	"os/exec"
 	"runtime"
 	"strconv"
 	"strings"
@@ -18,7 +16,6 @@ import (
 
 	"golang.zx2c4.com/wireguard/tun"
 	"tailscale.com/types/logger"
-	"tailscale.com/version/distro"
 )
 
 // tunMTU is the MTU we set on tailscale's TUN interface. wireguard-go
@@ -78,90 +75,18 @@ func New(logf logger.Logf, tunName string) (tun.Device, string, error) {
 	return dev, name, nil
 }
 
+// tunDiagnoseFailure, if non-nil, does OS-specific diagnostics of why
+// TUN failed to work.
+var tunDiagnoseFailure func(tunName string, logf logger.Logf)
+
 // Diagnose tries to explain a tuntap device creation failure.
 // It pokes around the system and logs some diagnostic info that might
 // help debug why tun creation failed. Because device creation has
 // already failed and the program's about to end, log a lot.
 func Diagnose(logf logger.Logf, tunName string) {
-	switch runtime.GOOS {
+	if tunDiagnoseFailure != nil {
-	case "linux":
+		tunDiagnoseFailure(tunName, logf)
-		diagnoseLinuxTUNFailure(tunName, logf)
+	} else {
-	case "darwin":
-		diagnoseDarwinTUNFailure(tunName, logf)
-	default:
 		logf("no TUN failure diagnostics for OS %q", runtime.GOOS)
 	}
 }
-
-func diagnoseDarwinTUNFailure(tunName string, logf logger.Logf) {
-	if os.Getuid() != 0 {
-		logf("failed to create TUN device as non-root user; use 'sudo tailscaled', or run under launchd with 'sudo tailscaled install-system-daemon'")
-	}
-	if tunName != "utun" {
-		logf("failed to create TUN device %q; try using tun device \"utun\" instead for automatic selection", tunName)
-	}
-}
-
-func diagnoseLinuxTUNFailure(tunName string, logf logger.Logf) {
-	kernel, err := exec.Command("uname", "-r").Output()
-	kernel = bytes.TrimSpace(kernel)
-	if err != nil {
-		logf("no TUN, and failed to look up kernel version: %v", err)
-		return
-	}
-	logf("Linux kernel version: %s", kernel)
-
-	modprobeOut, err := exec.Command("/sbin/modprobe", "tun").CombinedOutput()
-	if err == nil {
-		logf("'modprobe tun' successful")
-		// Either tun is currently loaded, or it's statically
-		// compiled into the kernel (which modprobe checks
-		// with /lib/modules/$(uname -r)/modules.builtin)
-		//
-		// So if there's a problem at this point, it's
-		// probably because /dev/net/tun doesn't exist.
-		const dev = "/dev/net/tun"
-		if fi, err := os.Stat(dev); err != nil {
-			logf("tun module loaded in kernel, but %s does not exist", dev)
-		} else {
-			logf("%s: %v", dev, fi.Mode())
-		}
-
-		// We failed to find why it failed. Just let our
-		// caller report the error it got from wireguard-go.
-		return
-	}
-	logf("is CONFIG_TUN enabled in your kernel? `modprobe tun` failed with: %s", modprobeOut)
-
-	switch distro.Get() {
-	case distro.Debian:
-		dpkgOut, err := exec.Command("dpkg", "-S", "kernel/drivers/net/tun.ko").CombinedOutput()
-		if len(bytes.TrimSpace(dpkgOut)) == 0 || err != nil {
-			logf("tun module not loaded nor found on disk")
-			return
-		}
-		if !bytes.Contains(dpkgOut, kernel) {
-			logf("kernel/drivers/net/tun.ko found on disk, but not for current kernel; are you in middle of a system update and haven't rebooted? found: %s", dpkgOut)
-		}
-	case distro.Arch:
-		findOut, err := exec.Command("find", "/lib/modules/", "-path", "*/net/tun.ko*").CombinedOutput()
-		if len(bytes.TrimSpace(findOut)) == 0 || err != nil {
-			logf("tun module not loaded nor found on disk")
-			return
-		}
-		if !bytes.Contains(findOut, kernel) {
-			logf("kernel/drivers/net/tun.ko found on disk, but not for current kernel; are you in middle of a system update and haven't rebooted? found: %s", findOut)
-		}
-	case distro.OpenWrt:
-		out, err := exec.Command("opkg", "list-installed").CombinedOutput()
-		if err != nil {
-			logf("error querying OpenWrt installed packages: %s", out)
-			return
-		}
-		for _, pkg := range []string{"kmod-tun", "ca-bundle"} {
-			if !bytes.Contains(out, []byte(pkg+" - ")) {
-				logf("Missing required package %s; run: opkg install %s", pkg, pkg)
-			}
-		}
-	}
-}

net/tstun/tun_linux.go

@@ -0,0 +1,82 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tstun
+
+import (
+	"bytes"
+	"os"
+	"os/exec"
+
+	"tailscale.com/types/logger"
+	"tailscale.com/version/distro"
+)
+
+func init() {
+	tunDiagnoseFailure = diagnoseLinuxTUNFailure
+}
+
+func diagnoseLinuxTUNFailure(tunName string, logf logger.Logf) {
+	kernel, err := exec.Command("uname", "-r").Output()
+	kernel = bytes.TrimSpace(kernel)
+	if err != nil {
+		logf("no TUN, and failed to look up kernel version: %v", err)
+		return
+	}
+	logf("Linux kernel version: %s", kernel)
+
+	modprobeOut, err := exec.Command("/sbin/modprobe", "tun").CombinedOutput()
+	if err == nil {
+		logf("'modprobe tun' successful")
+		// Either tun is currently loaded, or it's statically
+		// compiled into the kernel (which modprobe checks
+		// with /lib/modules/$(uname -r)/modules.builtin)
+		//
+		// So if there's a problem at this point, it's
+		// probably because /dev/net/tun doesn't exist.
+		const dev = "/dev/net/tun"
+		if fi, err := os.Stat(dev); err != nil {
+			logf("tun module loaded in kernel, but %s does not exist", dev)
+		} else {
+			logf("%s: %v", dev, fi.Mode())
+		}
+
+		// We failed to find why it failed. Just let our
+		// caller report the error it got from wireguard-go.
+		return
+	}
+	logf("is CONFIG_TUN enabled in your kernel? `modprobe tun` failed with: %s", modprobeOut)
+
+	switch distro.Get() {
+	case distro.Debian:
+		dpkgOut, err := exec.Command("dpkg", "-S", "kernel/drivers/net/tun.ko").CombinedOutput()
+		if len(bytes.TrimSpace(dpkgOut)) == 0 || err != nil {
+			logf("tun module not loaded nor found on disk")
+			return
+		}
+		if !bytes.Contains(dpkgOut, kernel) {
+			logf("kernel/drivers/net/tun.ko found on disk, but not for current kernel; are you in middle of a system update and haven't rebooted? found: %s", dpkgOut)
+		}
+	case distro.Arch:
+		findOut, err := exec.Command("find", "/lib/modules/", "-path", "*/net/tun.ko*").CombinedOutput()
+		if len(bytes.TrimSpace(findOut)) == 0 || err != nil {
+			logf("tun module not loaded nor found on disk")
+			return
+		}
+		if !bytes.Contains(findOut, kernel) {
+			logf("kernel/drivers/net/tun.ko found on disk, but not for current kernel; are you in middle of a system update and haven't rebooted? found: %s", findOut)
+		}
+	case distro.OpenWrt:
+		out, err := exec.Command("opkg", "list-installed").CombinedOutput()
+		if err != nil {
+			logf("error querying OpenWrt installed packages: %s", out)
+			return
+		}
+		for _, pkg := range []string{"kmod-tun", "ca-bundle"} {
+			if !bytes.Contains(out, []byte(pkg+" - ")) {
+				logf("Missing required package %s; run: opkg install %s", pkg, pkg)
+			}
+		}
+	}
+}

net/tstun/tun_macos.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build darwin && !ios
+// +build darwin,!ios
+
+package tstun
+
+import (
+	"os"
+
+	"tailscale.com/types/logger"
+)
+
+func init() {
+	tunDiagnoseFailure = diagnoseDarwinTUNFailure
+}
+
+func diagnoseDarwinTUNFailure(tunName string, logf logger.Logf) {
+	if os.Getuid() != 0 {
+		logf("failed to create TUN device as non-root user; use 'sudo tailscaled', or run under launchd with 'sudo tailscaled install-system-daemon'")
+	}
+	if tunName != "utun" {
+		logf("failed to create TUN device %q; try using tun device \"utun\" instead for automatic selection", tunName)
+	}
+}