netstack: zero window probes must have a payload

Note that this implements Linux-like behavior that differs from RFC 9293
3.8.6.1. We do not send the next byte in the stream as a probe. We instead use
a previously ACKed sequence number with a junk payload that the receiver must
respond to.

Fixes #10243.

PiperOrigin-RevId: 622966365

pkg/tcpip/transport/tcp/snd.go

@@ -20,6 +20,7 @@ import (
 	"sort"
 	"time"
 
+	"gvisor.dev/gvisor/pkg/buffer"
 	"gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
@@ -904,12 +905,25 @@ func (s *sender) maybeSendSegment(seg *segment, limit int, end seqnum.Value) (se
 	return true
 }
 
+// zeroProbeJunk is data sent during zero window probes. Its value is
+// irrelevant; since the sequence number has already been acknowledged it will
+// be discarded. It's only here to avoid allocating.
+var zeroProbeJunk = []byte{0}
+
 // +checklocks:s.ep.mu
 func (s *sender) sendZeroWindowProbe() {
 	s.unackZeroWindowProbes++
-	// Send a zero window probe with sequence number pointing to
-	// the last acknowledged byte.
-	s.sendEmptySegment(header.TCPFlagAck, s.SndUna-1)
+
+	// Send a zero window probe with sequence number pointing to the last
+	// acknowledged byte. Note that, like Linux, this isn't quite what RFC
+	// 9293 3.8.6.1 describes: we don't send the next byte in the stream,
+	// we re-send an ACKed byte to goad the receiver into responding.
+	pkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
+		Payload: buffer.MakeWithData(zeroProbeJunk),
+	})
+	defer pkt.DecRef()
+	s.sendSegmentFromPacketBuffer(pkt, header.TCPFlagAck, s.SndUna-1)
+
 	// Rearm the timer to continue probing.
 	s.resendTimer.enable(s.RTO)
 }

pkg/tcpip/transport/tcp/test/e2e/tcp_test.go

@@ -2914,7 +2914,7 @@ func TestZeroWindowSend(t *testing.T) {
 	defer b.Release()
 	iss := seqnum.Value(context.TestInitialSequenceNumber).Add(1)
 	checker.IPv4(t, b,
-		checker.PayloadLen(header.TCPMinimumSize),
+		checker.PayloadLen(header.TCPMinimumSize+1),
 		checker.TCP(
 			checker.DstPort(context.TestPort),
 			checker.TCPSeqNum(uint32(c.IRS)),