Ensure correctness of saved receive window

When the scaled receive window size > 65535 (max uint16), we advertise
the scaled value as 65535, but are not adjusting the saved receive
window value when doing so. This would keep our current window
calculation logic to be incorrect, as the saved receive window value
is different from what was advertised.

Fixes #4903

PiperOrigin-RevId: 347771340

1 files changed, 15 insertions, 5 deletions

diff --git a/test/packetimpact/tests/tcp_zero_receive_window_test.go b/test/packetimpact/tests/tcp_zero_receive_window_test.go
index b494e179b..d06690705 100644
--- a/test/packetimpact/tests/tcp_zero_receive_window_test.go
+++ b/test/packetimpact/tests/tcp_zero_receive_window_test.go
@@ -15,7 +15,6 @@
 package tcp_zero_receive_window_test
 
 import (
-	"context"
 	"flag"
 	"fmt"
 	"testing"
@@ -49,12 +48,24 @@ func TestZeroReceiveWindow(t *testing.T) {
 			samplePayload := &testbench.Payload{Bytes: testbench.GenerateRandomPayload(t, payloadLen)}
 			// Expect the DUT to eventually advertise zero receive window.
 			// The test would timeout otherwise.
-			for {
+			for readOnce := false; ; {
 				conn.Send(t, testbench.TCP{Flags: testbench.Uint8(header.TCPFlagAck | header.TCPFlagPsh)}, samplePayload)
 				gotTCP, err := conn.Expect(t, testbench.TCP{Flags: testbench.Uint8(header.TCPFlagAck)}, time.Second)
 				if err != nil {
 					t.Fatalf("expected packet was not received: %s", err)
 				}
+				// Read once to trigger the subsequent window update from the
+				// DUT to grow the right edge of the receive window from what
+				// was advertised in the SYN-ACK. This ensures that we test
+				// for the full default buffer size (1MB on gVisor at the time
+				// of writing this comment), thus testing for cases when the
+				// scaled receive window size ends up > 65535 (0xffff).
+				if !readOnce {
+					if got := dut.Recv(t, acceptFd, int32(payloadLen), 0); len(got) != payloadLen {
+						t.Fatalf("got dut.Recv(t, %d, %d, 0) = %d, want %d", acceptFd, payloadLen, len(got), payloadLen)
+					}
+					readOnce = true
+				}
 				windowSize := *gotTCP.WindowSize
 				t.Logf("got window size = %d", windowSize)
 				if windowSize == 0 {
@@ -94,10 +105,9 @@ func TestNonZeroReceiveWindow(t *testing.T) {
 				if err != nil {
 					t.Fatalf("expected packet was not received: %s", err)
 				}
-				if ret, _, err := dut.RecvWithErrno(context.Background(), t, acceptFd, int32(payloadLen), 0); ret == -1 {
-					t.Fatalf("dut.RecvWithErrno(ctx, t, %d, %d, 0) = %d,_, %s", acceptFd, payloadLen, ret, err)
+				if got := dut.Recv(t, acceptFd, int32(payloadLen), 0); len(got) != payloadLen {
+					t.Fatalf("got dut.Recv(t, %d, %d, 0) = %d, want %d", acceptFd, payloadLen, len(got), payloadLen)
 				}
-
 				if *gotTCP.WindowSize == 0 {
 					t.Fatalf("expected non-zero receive window.")
 				}