Use network protocol default ttl instead of a hardcoded one.

PiperOrigin-RevId: 423886874

pkg/sentry/socket/netstack/netstack.go

@@ -1330,6 +1330,15 @@ func getSockOptICMPv6(t *kernel.Task, s socket.SocketOps, ep commonEndpoint, nam
 	return nil, syserr.ErrProtocolNotAvailable
 }
 
+func defaultTTL(t *kernel.Task, network tcpip.NetworkProtocolNumber) (primitive.Int32, tcpip.Error) {
+	var opt tcpip.DefaultTTLOption
+	stack := inet.StackFromContext(t)
+	if err := stack.(*Stack).Stack.NetworkProtocolOption(network, &opt); err != nil {
+		return 0, err
+	}
+	return primitive.Int32(opt), nil
+}
+
 // getSockOptIPv6 implements GetSockOpt when level is SOL_IPV6.
 func getSockOptIPv6(t *kernel.Task, s socket.SocketOps, ep commonEndpoint, name int, outPtr hostarch.Addr, outLen int) (marshal.Marshallable, *syserr.Error) {
 	if _, ok := ep.(tcpip.Endpoint); !ok {
@@ -1377,9 +1386,10 @@ func getSockOptIPv6(t *kernel.Task, s socket.SocketOps, ep commonEndpoint, name
 		// Fill in the default value, if needed.
 		vP := primitive.Int32(v)
 		if vP == -1 {
-			// TODO(https://github.com/google/gvisor/issues/6973): Retrieve the
-			// configured DefaultTTLOption of the IPv6 protocol.
-			vP = DefaultTTL
+			vP, err = defaultTTL(t, header.IPv6ProtocolNumber)
+			if err != nil {
+				return nil, syserr.TranslateNetstackError(err)
+			}
 		}
 
 		return &vP, nil
@@ -1540,9 +1550,10 @@ func getSockOptIP(t *kernel.Task, s socket.SocketOps, ep commonEndpoint, name in
 		// Fill in the default value, if needed.
 		vP := primitive.Int32(v)
 		if vP == 0 {
-			// TODO(https://github.com/google/gvisor/issues/6973): Retrieve the
-			// configured DefaultTTLOption of the IPv4 protocol.
-			vP = DefaultTTL
+			vP, err = defaultTTL(t, header.IPv4ProtocolNumber)
+			if err != nil {
+				return nil, syserr.TranslateNetstackError(err)
+			}
 		}
 
 		return &vP, nil

test/syscalls/linux/socket_ip_unbound.cc

@@ -96,6 +96,7 @@ TEST_P(IPUnboundSocketTest, ResetTtlToDefault) {
   EXPECT_THAT(getsockopt(socket->get(), IPPROTO_IP, IP_TTL, &get2, &get2_sz),
               SyscallSucceedsWithValue(0));
   EXPECT_EQ(get2_sz, sizeof(get2));
+  EXPECT_TRUE(get2 == 64 || get2 == 127);
   EXPECT_EQ(get2, get1);
 }
 

test/syscalls/linux/socket_ipv6_unbound.cc

@@ -29,6 +29,7 @@ namespace testing {
 namespace {
 
 constexpr int kDefaultHopLimit = 64;
+constexpr int kDefaultTtl = 64;
 
 using ::testing::ValuesIn;
 using IPv6UnboundSocketTest = SimpleSocketTest;
@@ -37,13 +38,18 @@ TEST_P(IPv6UnboundSocketTest, HopLimitDefault) {
   std::unique_ptr<FileDescriptor> socket =
       ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
 
+  const int set = -1;
+  ASSERT_THAT(setsockopt(socket->get(), IPPROTO_IPV6, IPV6_UNICAST_HOPS, &set,
+                         sizeof(set)),
+              SyscallSucceedsWithValue(0));
+
   int get = -1;
   socklen_t get_sz = sizeof(get);
   ASSERT_THAT(
       getsockopt(socket->get(), IPPROTO_IPV6, IPV6_UNICAST_HOPS, &get, &get_sz),
       SyscallSucceedsWithValue(0));
   ASSERT_EQ(get_sz, sizeof(get));
-  EXPECT_EQ(get, kDefaultHopLimit);
+  EXPECT_EQ(get, kDefaultTtl);
 }
 
 TEST_P(IPv6UnboundSocketTest, SetHopLimit) {