wgengine/magicsock: apply peer MTU sockopts through net/sockopts

And merge repeated don't fragment code paths between Linux and Darwin.

Updates tailscale/corp#48168

Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>
Change-Id: I5aef408f037c8def99522b4d820c853a6a6a6964

wgengine/magicsock/peermtu.go

@@ -10,6 +10,7 @@ import (
 
 	"golang.org/x/sys/unix"
 	"tailscale.com/disco"
+	"tailscale.com/net/sockopts"
 	"tailscale.com/net/tstun"
 )
 
@@ -18,16 +19,16 @@ import (
 // DontFragSetting returns true if at least one of the underlying sockets of
 // this connection is a UDP socket with the don't fragment bit set, otherwise it
 // returns false. It also returns an error if either connection returned an error
-// other than errUnsupportedConnType.
+// other than [sockopts.ErrNoSocket].
 func (c *Conn) DontFragSetting() (bool, error) {
 	df4, err4 := c.getDontFragment("udp4")
 	df6, err6 := c.getDontFragment("udp6")
 	df := df4 || df6
 	err := err4
-	if err4 != nil && err4 != errUnsupportedConnType {
+	if err4 != nil && !errors.Is(err4, sockopts.ErrNoSocket) {
 		err = err6
 	}
-	if err == errUnsupportedConnType {
+	if errors.Is(err, sockopts.ErrNoSocket) {
 		err = nil
 	}
 	return df, err
@@ -103,7 +104,7 @@ func (c *Conn) UpdatePMTUD() {
 	err4 := c.setDontFragment("udp4", enable)
 	err6 := c.setDontFragment("udp6", enable)
 	anySuccess := err4 == nil || err6 == nil
-	noFailures := (err4 == nil || err4 == errUnsupportedConnType) && (err6 == nil || err6 == errUnsupportedConnType)
+	noFailures := (err4 == nil || errors.Is(err4, sockopts.ErrNoSocket)) && (err6 == nil || errors.Is(err6, sockopts.ErrNoSocket))
 
 	if anySuccess && noFailures {
 		c.logf("magicsock: peermtu: peer MTU status updated to %v", newStatus)

wgengine/magicsock/peermtu_darwin.go

@@ -6,46 +6,18 @@
 package magicsock
 
 import (
-	"syscall"
-
 	"golang.org/x/sys/unix"
 )
 
+// dontFrag{Enabled,Disabled} are the values of the don't fragment sockopt.
+const (
+	dontFragEnabled  = 1
+	dontFragDisabled = 0
+)
+
 func getDontFragOpt(network string) int {
 	if network == "udp4" {
 		return unix.IP_DONTFRAG
 	}
 	return unix.IPV6_DONTFRAG
 }
-
-func (c *Conn) setDontFragment(network string, enable bool) error {
-	optArg := 1
-	if enable == false {
-		optArg = 0
-	}
-	var err error
-	rcErr := c.connControl(network, func(fd uintptr) {
-		err = syscall.SetsockoptInt(int(fd), getIPProto(network), getDontFragOpt(network), optArg)
-	})
-
-	if rcErr != nil {
-		return rcErr
-	}
-	return err
-}
-
-func (c *Conn) getDontFragment(network string) (bool, error) {
-	var v int
-	var err error
-	rcErr := c.connControl(network, func(fd uintptr) {
-		v, err = syscall.GetsockoptInt(int(fd), getIPProto(network), getDontFragOpt(network))
-	})
-
-	if rcErr != nil {
-		return false, rcErr
-	}
-	if v == 1 {
-		return true, err
-	}
-	return false, err
-}

wgengine/magicsock/peermtu_linux.go

@@ -9,41 +9,15 @@ import (
 	"syscall"
 )
 
+// dontFrag{Enabled,Disabled} are the values of the don't fragment sockopt.
+const (
+	dontFragEnabled  = syscall.IP_PMTUDISC_DO
+	dontFragDisabled = syscall.IP_PMTUDISC_DONT
+)
+
 func getDontFragOpt(network string) int {
 	if network == "udp4" {
 		return syscall.IP_MTU_DISCOVER
 	}
 	return syscall.IPV6_MTU_DISCOVER
 }
-
-func (c *Conn) setDontFragment(network string, enable bool) error {
-	optArg := syscall.IP_PMTUDISC_DO
-	if enable == false {
-		optArg = syscall.IP_PMTUDISC_DONT
-	}
-	var err error
-	rcErr := c.connControl(network, func(fd uintptr) {
-		err = syscall.SetsockoptInt(int(fd), getIPProto(network), getDontFragOpt(network), optArg)
-	})
-
-	if rcErr != nil {
-		return rcErr
-	}
-	return err
-}
-
-func (c *Conn) getDontFragment(network string) (bool, error) {
-	var v int
-	var err error
-	rcErr := c.connControl(network, func(fd uintptr) {
-		v, err = syscall.GetsockoptInt(int(fd), getIPProto(network), getDontFragOpt(network))
-	})
-
-	if rcErr != nil {
-		return false, rcErr
-	}
-	if v == syscall.IP_PMTUDISC_DO {
-		return true, err
-	}
-	return false, err
-}

wgengine/magicsock/peermtu_unix.go

@@ -7,6 +7,9 @@ package magicsock
 
 import (
 	"syscall"
+
+	"tailscale.com/net/sockopts"
+	"tailscale.com/types/nettype"
 )
 
 // getIPProto returns the value of the get/setsockopt proto argument necessary
@@ -19,24 +22,39 @@ func getIPProto(network string) int {
 	return syscall.IPPROTO_IPV6
 }
 
-// connControl allows the caller to run a system call on the socket underlying
-// Conn specified by the string network, which must be "udp4" or "udp6". If the
-// pconn type implements the syscall method, this function returns the value of
-// of the system call fn called with the fd of the socket as its arg (or the
-// error from rc.Control() if that fails). Otherwise it returns the error
-// errUnsupportedConnType.
-func (c *Conn) connControl(network string, fn func(fd uintptr)) error {
-	pconn := c.pconn4.pconn
+// connControl runs the system call fn on the fds of the sockets underlying the
+// Conn specified by the string network, which must be "udp4" or "udp6".
+// See [sockopts.ControlEach] for what it returns.
+func (c *Conn) connControl(network string, fn func(fd uintptr) error) error {
+	pconn := &c.pconn4
 	if network == "udp6" {
-		pconn = c.pconn6.pconn
+		pconn = &c.pconn6
 	}
-	sc, ok := pconn.(syscall.Conn)
-	if !ok {
-		return errUnsupportedConnType
+	return sockopts.ControlEach([]nettype.PacketConn{pconn.currentConn()}, fn)
+}
+
+// setDontFragment sets or clears the don't fragment bit on the network sockets.
+// Network must be "udp4" or "udp6".
+func (c *Conn) setDontFragment(network string, enable bool) error {
+	optArg := dontFragEnabled
+	if !enable {
+		optArg = dontFragDisabled
 	}
-	rc, err := sc.SyscallConn()
-	if err != nil {
-		return err
-	}
-	return rc.Control(fn)
+	return c.connControl(network, func(fd uintptr) error {
+		return syscall.SetsockoptInt(int(fd), getIPProto(network), getDontFragOpt(network), optArg)
+	})
+}
+
+// getDontFragment reports whether the network sockets have the don't fragment bit set.
+func (c *Conn) getDontFragment(network string) (bool, error) {
+	var set bool
+	err := c.connControl(network, func(fd uintptr) error {
+		v, err := syscall.GetsockoptInt(int(fd), getIPProto(network), getDontFragOpt(network))
+		if err != nil {
+			return err
+		}
+		set = v == dontFragEnabled
+		return nil
+	})
+	return set, err
 }