From 85a45a9651d2b38e3616a7899800601656cf143e Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Fri, 28 Feb 2020 09:10:16 -0800
Subject: [PATCH] tun: fix data race on name field

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 tun/tun_linux.go | 44 +++++++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/tun/tun_linux.go b/tun/tun_linux.go
index 17b8822..2f97ebb 100644
--- a/tun/tun_linux.go
+++ b/tun/tun_linux.go
@@ -31,7 +31,6 @@ const (
 type NativeTun struct {
 	tunFile                 *os.File
 	index                   int32      // if index
-	name                    string     // name of interface
 	errors                  chan error // async error handling
 	events                  chan Event // device related events
 	nopi                    bool       // the device was passed IFF_NO_PI
@@ -39,6 +38,10 @@ type NativeTun struct {
 	netlinkCancel           *rwcancel.RWCancel
 	hackListenerClosed      sync.Mutex
 	statusListenersShutdown chan struct{}
+
+	nameOnce  sync.Once // guards calling initNameCache, which sets following fields
+	nameCache string    // name of interface
+	nameErr   error
 }
 
 func (tun *NativeTun) File() *os.File {
@@ -192,6 +195,11 @@ func getIFIndex(name string) (int32, error) {
 }
 
 func (tun *NativeTun) setMTU(n int) error {
+	name, err := tun.Name()
+	if err != nil {
+		return err
+	}
+
 	// open datagram socket
 	fd, err := unix.Socket(
 		unix.AF_INET,
@@ -206,9 +214,8 @@ func (tun *NativeTun) setMTU(n int) error {
 	defer unix.Close(fd)
 
 	// do ioctl call
-
 	var ifr [ifReqSize]byte
-	copy(ifr[:], tun.name)
+	copy(ifr[:], name)
 	*(*uint32)(unsafe.Pointer(&ifr[unix.IFNAMSIZ])) = uint32(n)
 	_, _, errno := unix.Syscall(
 		unix.SYS_IOCTL,
@@ -225,6 +232,11 @@ func (tun *NativeTun) setMTU(n int) error {
 }
 
 func (tun *NativeTun) MTU() (int, error) {
+	name, err := tun.Name()
+	if err != nil {
+		return 0, err
+	}
+
 	// open datagram socket
 	fd, err := unix.Socket(
 		unix.AF_INET,
@@ -241,7 +253,7 @@ func (tun *NativeTun) MTU() (int, error) {
 	// do ioctl call
 
 	var ifr [ifReqSize]byte
-	copy(ifr[:], tun.name)
+	copy(ifr[:], name)
 	_, _, errno := unix.Syscall(
 		unix.SYS_IOCTL,
 		uintptr(fd),
@@ -256,6 +268,15 @@ func (tun *NativeTun) MTU() (int, error) {
 }
 
 func (tun *NativeTun) Name() (string, error) {
+	tun.nameOnce.Do(tun.initNameCache)
+	return tun.nameCache, tun.nameErr
+}
+
+func (tun *NativeTun) initNameCache() {
+	tun.nameCache, tun.nameErr = tun.nameSlow()
+}
+
+func (tun *NativeTun) nameSlow() (string, error) {
 	sysconn, err := tun.tunFile.SyscallConn()
 	if err != nil {
 		return "", err
@@ -276,13 +297,11 @@ func (tun *NativeTun) Name() (string, error) {
 	if errno != 0 {
 		return "", errors.New("failed to get name of TUN device: " + errno.Error())
 	}
-	nullStr := ifr[:]
-	i := bytes.IndexByte(nullStr, 0)
-	if i != -1 {
-		nullStr = nullStr[:i]
+	name := ifr[:]
+	if i := bytes.IndexByte(name, 0); i != -1 {
+		name = name[:i]
 	}
-	tun.name = string(nullStr)
-	return tun.name, nil
+	return string(name), nil
 }
 
 func (tun *NativeTun) Write(buff []byte, offset int) (int, error) {
@@ -402,16 +421,15 @@ func CreateTUNFromFile(file *os.File, mtu int) (Device, error) {
 		statusListenersShutdown: make(chan struct{}),
 		nopi:                    false,
 	}
-	var err error
 
-	_, err = tun.Name()
+	name, err := tun.Name()
 	if err != nil {
 		return nil, err
 	}
 
 	// start event listener
 
-	tun.index, err = getIFIndex(tun.name)
+	tun.index, err = getIFIndex(name)
 	if err != nil {
 		return nil, err
 	}