Stub out support for TCP_MAXSEG.

Adds support to set/get the TCP_MAXSEG value but does not
really change the segment sizes emitted by netstack or
alter the MSS advertised by the endpoint. This is currently
being added only to unblock iperf3 on gVisor. Plumbing
this correctly requires a bit more work which will come
in separate CLs.

PiperOrigin-RevId: 257859112

1 files changed, 55 insertions, 0 deletions

diff --git a/test/syscalls/linux/tcp_socket.cc b/test/syscalls/linux/tcp_socket.cc
index 4597e91e3..8d77431f2 100644
--- a/test/syscalls/linux/tcp_socket.cc
+++ b/test/syscalls/linux/tcp_socket.cc
@@ -890,6 +890,61 @@ TEST_P(SimpleTcpSocketTest, SetCongestionControlFailsForUnsupported) {
   EXPECT_EQ(0, memcmp(got_cc, old_cc, sizeof(kTcpCaNameMax)));
 }
 
+TEST_P(SimpleTcpSocketTest, MaxSegDefault) {
+  FileDescriptor s =
+      ASSERT_NO_ERRNO_AND_VALUE(Socket(GetParam(), SOCK_STREAM, IPPROTO_TCP));
+
+  constexpr int kDefaultMSS = 536;
+  int tcp_max_seg;
+  socklen_t optlen = sizeof(tcp_max_seg);
+  ASSERT_THAT(
+      getsockopt(s.get(), IPPROTO_TCP, TCP_MAXSEG, &tcp_max_seg, &optlen),
+      SyscallSucceedsWithValue(0));
+
+  EXPECT_EQ(kDefaultMSS, tcp_max_seg);
+  EXPECT_EQ(sizeof(tcp_max_seg), optlen);
+}
+
+TEST_P(SimpleTcpSocketTest, SetMaxSeg) {
+  FileDescriptor s =
+      ASSERT_NO_ERRNO_AND_VALUE(Socket(GetParam(), SOCK_STREAM, IPPROTO_TCP));
+
+  constexpr int kDefaultMSS = 536;
+  constexpr int kTCPMaxSeg = 1024;
+  ASSERT_THAT(setsockopt(s.get(), IPPROTO_TCP, TCP_MAXSEG, &kTCPMaxSeg,
+                         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));
+
+  EXPECT_EQ(kDefaultMSS, optval);
+  EXPECT_EQ(sizeof(optval), optlen);
+}
+
+TEST_P(SimpleTcpSocketTest, SetMaxSegFailsForInvalidMSSValues) {
+  FileDescriptor s =
+      ASSERT_NO_ERRNO_AND_VALUE(Socket(GetParam(), SOCK_STREAM, IPPROTO_TCP));
+
+  {
+    constexpr int tcp_max_seg = 10;
+    ASSERT_THAT(setsockopt(s.get(), IPPROTO_TCP, TCP_MAXSEG, &tcp_max_seg,
+                           sizeof(tcp_max_seg)),
+                SyscallFailsWithErrno(EINVAL));
+  }
+  {
+    constexpr int tcp_max_seg = 75000;
+    ASSERT_THAT(setsockopt(s.get(), IPPROTO_TCP, TCP_MAXSEG, &tcp_max_seg,
+                           sizeof(tcp_max_seg)),
+                SyscallFailsWithErrno(EINVAL));
+  }
+}
+
 INSTANTIATE_TEST_SUITE_P(AllInetTests, SimpleTcpSocketTest,
                          ::testing::Values(AF_INET, AF_INET6));