net/udp: return a local route address as the bound-to address

If the socket is bound to ANY and connected to a loopback address,
getsockname() has to return the loopback address. Without this fix,
getsockname() returns ANY.

PiperOrigin-RevId: 283647781

2 files changed, 45 insertions, 1 deletions

diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index 24cb88c13..4b161e404 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -1134,9 +1134,14 @@ func (e *endpoint) GetLocalAddress() (tcpip.FullAddress, *tcpip.Error) {
 	e.mu.RLock()
 	defer e.mu.RUnlock()
 
+	addr := e.ID.LocalAddress
+	if e.state == StateConnected {
+		addr = e.route.LocalAddress
+	}
+
 	return tcpip.FullAddress{
 		NIC:  e.RegisterNICID,
-		Addr: e.ID.LocalAddress,
+		Addr: addr,
 		Port: e.ID.LocalPort,
 	}, nil
 }
diff --git a/test/syscalls/linux/udp_socket_test_cases.cc b/test/syscalls/linux/udp_socket_test_cases.cc
index b6090ac66..63b92d6a7 100644
--- a/test/syscalls/linux/udp_socket_test_cases.cc
+++ b/test/syscalls/linux/udp_socket_test_cases.cc
@@ -527,6 +527,45 @@ TEST_P(UdpSocketTest, DisconnectAfterBind) {
               SyscallFailsWithErrno(ENOTCONN));
 }
 
+TEST_P(UdpSocketTest, BindToAnyConnnectToLocalhost) {
+  struct sockaddr_storage baddr = {};
+  auto port = *Port(reinterpret_cast<struct sockaddr_storage*>(addr_[1]));
+  if (GetParam() == AddressFamily::kIpv4) {
+    auto addr_in = reinterpret_cast<struct sockaddr_in*>(&baddr);
+    addr_in->sin_family = AF_INET;
+    addr_in->sin_port = port;
+    addr_in->sin_addr.s_addr = htonl(INADDR_ANY);
+  } else {
+    auto addr_in = reinterpret_cast<struct sockaddr_in6*>(&baddr);
+    addr_in->sin6_family = AF_INET6;
+    addr_in->sin6_port = port;
+    addr_in->sin6_scope_id = 0;
+    addr_in->sin6_addr = IN6ADDR_ANY_INIT;
+  }
+  ASSERT_THAT(bind(s_, reinterpret_cast<sockaddr*>(&baddr), addrlen_),
+              SyscallSucceeds());
+  // Connect the socket.
+  ASSERT_THAT(connect(s_, addr_[0], addrlen_), SyscallSucceeds());
+
+  struct sockaddr_storage addr = {};
+  socklen_t addrlen = sizeof(addr);
+  EXPECT_THAT(getsockname(s_, reinterpret_cast<sockaddr*>(&addr), &addrlen),
+              SyscallSucceeds());
+
+  // If the socket is bound to ANY and connected to a loopback address,
+  // getsockname() has to return the loopback address.
+  if (GetParam() == AddressFamily::kIpv4) {
+    auto addr_out = reinterpret_cast<struct sockaddr_in*>(&addr);
+    EXPECT_EQ(addrlen, sizeof(*addr_out));
+    EXPECT_EQ(addr_out->sin_addr.s_addr, htonl(INADDR_LOOPBACK));
+  } else {
+    auto addr_out = reinterpret_cast<struct sockaddr_in6*>(&addr);
+    struct in6_addr loopback = IN6ADDR_LOOPBACK_INIT;
+    EXPECT_EQ(addrlen, sizeof(*addr_out));
+    EXPECT_EQ(memcmp(&addr_out->sin6_addr, &loopback, sizeof(in6_addr)), 0);
+  }
+}
+
 TEST_P(UdpSocketTest, DisconnectAfterBindToAny) {
   struct sockaddr_storage baddr = {};
   socklen_t addrlen;