net/netns: add Android implementation, allowing registration of JNI hook

Updates #2102
Updates #1809

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

net/netns/netns_android.go

@@ -0,0 +1,64 @@
+// 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.
+
+// +build android
+
+package netns
+
+import (
+	"fmt"
+	"sync"
+	"syscall"
+)
+
+var (
+	androidProtectFuncMu sync.Mutex
+	androidProtectFunc   func(fd int) error
+)
+
+// SetAndroidProtectFunc register a func that Android provides that JNI calls into
+// https://developer.android.com/reference/android/net/VpnService#protect(int)
+// which is documented as:
+//
+// "Protect a socket from VPN connections. After protecting, data sent
+// through this socket will go directly to the underlying network, so
+// its traffic will not be forwarded through the VPN. This method is
+// useful if some connections need to be kept outside of VPN. For
+// example, a VPN tunnel should protect itself if its destination is
+// covered by VPN routes. Otherwise its outgoing packets will be sent
+// back to the VPN interface and cause an infinite loop. This method
+// will fail if the application is not prepared or is revoked."
+//
+// A nil func disables the use the hook.
+//
+// This indirection is necessary because this is the supported, stable
+// interface to use on Android, and doing the sockopts to set the
+// fwmark return errors on Android. The actual implementation of
+// VpnService.protect ends up doing an IPC to another process on
+// Android, asking for the fwmark to be set.
+func SetAndroidProtectFunc(f func(fd int) error) {
+	androidProtectFuncMu.Lock()
+	defer androidProtectFuncMu.Unlock()
+	androidProtectFunc = f
+}
+
+// control marks c as necessary to dial in a separate network namespace.
+//
+// It's intentionally the same signature as net.Dialer.Control
+// and net.ListenConfig.Control.
+func control(network, address string, c syscall.RawConn) error {
+	var sockErr error
+	err := c.Control(func(fd uintptr) {
+		androidProtectFuncMu.Lock()
+		f := androidProtectFunc
+		androidProtectFuncMu.Unlock()
+		if f != nil {
+			sockErr = f(int(fd))
+		}
+	})
+	if err != nil {
+		return fmt.Errorf("RawConn.Control on %T: %w", c, err)
+	}
+	return sockErr
+}

net/netns/netns_linux.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build linux,!android
+
 package netns
 
 import (