noop upon IP_PMTUDISC_INTERFACE and IP_PMTUDISC_OMIT

This is what we did previously, and doing so avoids breaking some users.

Fixes #10760.

PiperOrigin-RevId: 663358270

pkg/sentry/socket/netstack/netstack.go

@@ -2625,6 +2625,8 @@ func setSockOptIP(t *kernel.Task, s socket.Socket, ep commonEndpoint, name int,
 			v = int32(tcpip.PMTUDiscoveryDo)
 		case linux.IP_PMTUDISC_PROBE:
 			v = int32(tcpip.PMTUDiscoveryProbe)
+		case linux.IP_PMTUDISC_INTERFACE, linux.IP_PMTUDISC_OMIT:
+			return nil // Noop.
 		default:
 			return syserr.ErrNotSupported
 		}

test/syscalls/linux/tcp_socket.cc

@@ -2755,6 +2755,20 @@ TEST_P(SimpleTcpSocketTest, SetTCPCorkOff) {
 }
 #endif  // __linux__
 
+TEST_P(SimpleTcpSocketTest, SetUnsupportedPMTUDISC) {
+  int fd;
+  ASSERT_THAT(fd = socket(GetParam(), SOCK_STREAM, IPPROTO_TCP),
+              SyscallSucceeds());
+
+  int set = IP_PMTUDISC_INTERFACE;
+  socklen_t length = sizeof(set);
+  EXPECT_THAT(setsockopt(fd, IPPROTO_IP, IP_MTU_DISCOVER, &set, length),
+              SyscallSucceeds());
+  set = IP_PMTUDISC_OMIT;
+  EXPECT_THAT(setsockopt(fd, IPPROTO_IP, IP_MTU_DISCOVER, &set, length),
+              SyscallSucceeds());
+}
+
 INSTANTIATE_TEST_SUITE_P(AllInetTests, SimpleTcpSocketTest,
                          ::testing::Values(AF_INET, AF_INET6));