Return user_mss if user_mss is set and socket is unconnected.

Has the same effect as Linux commit 34dfde4ad87b ("tcp: Return user_mss for
TCP_MAXSEG in CLOSE/LISTEN state if user_mss set").

Updated test to accept both values (default and user set) so test works on
newer and older kernels.

PiperOrigin-RevId: 563844012

pkg/tcpip/transport/tcp/endpoint.go

@@ -2059,11 +2059,16 @@ func (e *endpoint) GetSockOptInt(opt tcpip.SockOptInt) (int, tcpip.Error) {
 		return v, nil
 
 	case tcpip.MaxSegOption:
-		// This is just stubbed out. Linux never returns the user_mss
-		// value as it either returns the defaultMSS or returns the
-		// actual current MSS. Netstack just returns the defaultMSS
-		// always for now.
+		// Linux only returns user_mss value if user_mss is set and the socket is
+		// unconnected. Otherwise Linux returns the actual current MSS. Netstack
+		// mimics the user_mss behavior, but otherwise just returns the defaultMSS
+		// for now.
 		v := header.TCPDefaultMSS
+		e.LockUser()
+		if state := e.EndpointState(); e.userMSS > 0 && (state.internal() || state == StateClose || state == StateListen) {
+			v = int(e.userMSS)
+		}
+		e.UnlockUser()
 		return v, nil
 
 	case tcpip.MTUDiscoverOption:

test/syscalls/linux/tcp_socket.cc

@@ -28,6 +28,7 @@
 #include <limits>
 #include <vector>
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/status/statusor.h"
 #include "absl/time/clock.h"
@@ -38,6 +39,8 @@
 #include "test/util/test_util.h"
 #include "test/util/thread_util.h"
 
+using ::testing::AnyOf;
+
 namespace gvisor {
 namespace testing {
 
@@ -1857,17 +1860,19 @@ TEST_P(SimpleTcpSocketTest, SetMaxSeg) {
                          sizeof(kTCPMaxSeg)),
               SyscallSucceedsWithValue(0));
 
-  // Linux actually never returns the user_mss value. It will always return the
-  // default MSS value defined above for an unconnected socket and always return
-  // the actual current MSS for a connected one.
   int optval;
   socklen_t optlen = sizeof(optval);
   ASSERT_THAT(getsockopt(s.get(), IPPROTO_TCP, TCP_MAXSEG, &optval, &optlen),
               SyscallSucceedsWithValue(0));
   ASSERT_EQ(optlen, sizeof(optval));
 
-  EXPECT_EQ(kDefaultMSS, optval);
-  EXPECT_EQ(sizeof(optval), optlen);
+  // In older Linux versions, user_mss value was never actually returned. Linux
+  // would always return the default MSS value for an unconnected socket and
+  // always return the actual current MSS for a connected one. However, the
+  // behavior changed since 34dfde4ad87b ("tcp: Return user_mss for TCP_MAXSEG
+  // in CLOSE/LISTEN state if user_mss set"). With this change, user_mss is
+  // returned if set for unconnected sockets. So allow both.
+  EXPECT_THAT(optval, AnyOf(kDefaultMSS, kTCPMaxSeg));
 }
 
 TEST_P(SimpleTcpSocketTest, SetMaxSegFailsForInvalidMSSValues) {