catch EPERM sendmsg errors for the very first packet on Linux

send_conn.go

@@ -28,6 +28,9 @@ type sconn struct {
 	packetInfoOOB []byte
 	// If GSO enabled, and we receive a GSO error for this remote address, GSO is disabled.
 	gotGSOError bool
+	// Used to catch the error sometimes returned by the first sendmsg call on Linux,
+	// see https://github.com/golang/go/issues/63322.
+	wroteFirstPacket bool
 }
 
 var _ sendConn = &sconn{}
@@ -56,7 +59,7 @@ func newSendConn(c rawConn, remote net.Addr, info packetInfo, logger utils.Logge
 }
 
 func (c *sconn) Write(p []byte, gsoSize uint16, ecn protocol.ECN) error {
-	_, err := c.WritePacket(p, c.remoteAddr, c.packetInfoOOB, gsoSize, ecn)
+	err := c.writePacket(p, c.remoteAddr, c.packetInfoOOB, gsoSize, ecn)
 	if err != nil && isGSOError(err) {
 		// disable GSO for future calls
 		c.gotGSOError = true
@@ -69,7 +72,7 @@ func (c *sconn) Write(p []byte, gsoSize uint16, ecn protocol.ECN) error {
 			if l > int(gsoSize) {
 				l = int(gsoSize)
 			}
-			if _, err := c.WritePacket(p[:l], c.remoteAddr, c.packetInfoOOB, 0, ecn); err != nil {
+			if err := c.writePacket(p[:l], c.remoteAddr, c.packetInfoOOB, 0, ecn); err != nil {
 				return err
 			}
 			p = p[l:]
@@ -79,6 +82,15 @@ func (c *sconn) Write(p []byte, gsoSize uint16, ecn protocol.ECN) error {
 	return err
 }
 
+func (c *sconn) writePacket(p []byte, addr net.Addr, oob []byte, gsoSize uint16, ecn protocol.ECN) error {
+	_, err := c.WritePacket(p, addr, oob, gsoSize, ecn)
+	if err != nil && !c.wroteFirstPacket && isPermissionError(err) {
+		_, err = c.WritePacket(p, addr, oob, gsoSize, ecn)
+	}
+	c.wroteFirstPacket = true
+	return err
+}
+
 func (c *sconn) capabilities() connCapabilities {
 	capabilities := c.rawConn.capabilities()
 	if capabilities.GSO {

send_conn_test.go

@@ -76,4 +76,29 @@ var _ = Describe("Connection (for sending packets)", func() {
 			Expect(c.capabilities().GSO).To(BeFalse())
 		})
 	}
+
+	if runtime.GOOS == "linux" {
+		It("doesn't fail if the very first sendmsg call fails", func() {
+			rawConn := NewMockRawConn(mockCtrl)
+			rawConn.EXPECT().LocalAddr()
+			rawConn.EXPECT().capabilities().AnyTimes()
+			c := newSendConn(rawConn, remoteAddr, packetInfo{}, utils.DefaultLogger)
+			Expect(c.capabilities().GSO).To(BeTrue())
+			gomock.InOrder(
+				rawConn.EXPECT().WritePacket([]byte("foobar"), remoteAddr, gomock.Any(), gomock.Any(), protocol.ECNCE).Return(0, errNotPermitted),
+				rawConn.EXPECT().WritePacket([]byte("foobar"), remoteAddr, gomock.Any(), uint16(0), protocol.ECNCE).Return(6, nil),
+			)
+			Expect(c.Write([]byte("foobar"), 0, protocol.ECNCE)).To(Succeed())
+		})
+
+		It("fails if the sendmsg calls fail multiple times", func() {
+			rawConn := NewMockRawConn(mockCtrl)
+			rawConn.EXPECT().LocalAddr()
+			rawConn.EXPECT().capabilities().AnyTimes()
+			c := newSendConn(rawConn, remoteAddr, packetInfo{}, utils.DefaultLogger)
+			Expect(c.capabilities().GSO).To(BeTrue())
+			rawConn.EXPECT().WritePacket([]byte("foobar"), remoteAddr, gomock.Any(), gomock.Any(), protocol.ECNCE).Return(0, errNotPermitted).Times(2)
+			Expect(c.Write([]byte("foobar"), 0, protocol.ECNCE)).To(MatchError(errNotPermitted))
+		})
+	}
 })

sys_conn_helper_linux.go

@@ -97,3 +97,14 @@ func isGSOError(err error) bool {
 	}
 	return false
 }
+
+// The first sendmsg call on a new UDP socket sometimes errors on Linux.
+// It's not clear why this happens.
+// See https://github.com/golang/go/issues/63322.
+func isPermissionError(err error) bool {
+	var serr *os.SyscallError
+	if errors.As(err, &serr) {
+		return serr.Syscall == "sendmsg" && serr.Err == unix.EPERM
+	}
+	return false
+}

sys_conn_helper_linux_test.go

@@ -13,7 +13,10 @@ import (
 	. "github.com/onsi/gomega"
 )
 
-var errGSO = &os.SyscallError{Err: unix.EIO}
+var (
+	errGSO          = &os.SyscallError{Err: unix.EIO}
+	errNotPermitted = &os.SyscallError{Syscall: "sendmsg", Err: unix.EPERM}
+)
 
 var _ = Describe("forcing a change of send and receive buffer sizes", func() {
 	It("forces a change of the receive buffer size", func() {

sys_conn_helper_nonlinux.go

@@ -7,3 +7,4 @@ func forceSetSendBuffer(c any, bytes int) error    { return nil }
 
 func appendUDPSegmentSizeMsg([]byte, uint16) []byte { return nil }
 func isGSOError(error) bool                         { return false }
+func isPermissionError(err error) bool              { return false }

sys_conn_helper_nonlinux_test.go

@@ -4,4 +4,7 @@ package quic
 
 import "errors"
 
-var errGSO = errors.New("fake GSO error")
+var (
+	errGSO          = errors.New("fake GSO error")
+	errNotPermitted = errors.New("fake not permitted error")
+)