[release-branch.go1.25] net: fix WriteMsgUDPAddrPort addr handling on IPv4 sockets

Accept IPv4-mapped IPv6 destination addresses on IPv4 UDP sockets.

Fixes #74999.

Change-Id: I4624b9b8f861aedcae29e51d5298d23ce1c0f2c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/689976
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
(cherry picked from commit bdb2d50fdf40706e7d7411f33ade80edac726707)
Reviewed-on: https://go-review.googlesource.com/c/go/+/695875
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

2 files changed, 37 insertions, 1 deletions

diff --git a/src/net/ipsock_posix.go b/src/net/ipsock_posix.go
index 2aeabd4487..52712f932f 100644
--- a/src/net/ipsock_posix.go
+++ b/src/net/ipsock_posix.go
@@ -237,8 +237,12 @@ func ipToSockaddr(family int, ip IP, port int, zone string) (syscall.Sockaddr, e
 func addrPortToSockaddrInet4(ap netip.AddrPort) (syscall.SockaddrInet4, error) {
 	// ipToSockaddrInet4 has special handling here for zero length slices.
 	// We do not, because netip has no concept of a generic zero IP address.
+	//
+	// addr is allowed to be an IPv4-mapped IPv6 address.
+	// As4 will unmap it to an IPv4 address.
+	// The error message is kept consistent with ipToSockaddrInet4.
 	addr := ap.Addr()
-	if !addr.Is4() {
+	if !addr.Is4() && !addr.Is4In6() {
 		return syscall.SockaddrInet4{}, &AddrError{Err: "non-IPv4 address", Addr: addr.String()}
 	}
 	sa := syscall.SockaddrInet4{
diff --git a/src/net/udpsock_test.go b/src/net/udpsock_test.go
index 6dacc81df6..7ad8a585b0 100644
--- a/src/net/udpsock_test.go
+++ b/src/net/udpsock_test.go
@@ -705,3 +705,35 @@ func TestIPv6WriteMsgUDPAddrPortTargetAddrIPVersion(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+// TestIPv4WriteMsgUDPAddrPortTargetAddrIPVersion verifies that
+// WriteMsgUDPAddrPort accepts IPv4 and IPv4-mapped IPv6 destination addresses,
+// and rejects IPv6 destination addresses on a "udp4" connection.
+func TestIPv4WriteMsgUDPAddrPortTargetAddrIPVersion(t *testing.T) {
+	if !testableNetwork("udp4") {
+		t.Skipf("skipping: udp4 not available")
+	}
+
+	conn, err := ListenUDP("udp4", &UDPAddr{IP: IPv4(127, 0, 0, 1)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+
+	daddr4 := netip.AddrPortFrom(netip.MustParseAddr("127.0.0.1"), 12345)
+	daddr4in6 := netip.AddrPortFrom(netip.MustParseAddr("::ffff:127.0.0.1"), 12345)
+	daddr6 := netip.AddrPortFrom(netip.MustParseAddr("::1"), 12345)
+	buf := make([]byte, 8)
+
+	if _, _, err = conn.WriteMsgUDPAddrPort(buf, nil, daddr4); err != nil {
+		t.Errorf("conn.WriteMsgUDPAddrPort(buf, nil, daddr4) failed: %v", err)
+	}
+
+	if _, _, err = conn.WriteMsgUDPAddrPort(buf, nil, daddr4in6); err != nil {
+		t.Errorf("conn.WriteMsgUDPAddrPort(buf, nil, daddr4in6) failed: %v", err)
+	}
+
+	if _, _, err = conn.WriteMsgUDPAddrPort(buf, nil, daddr6); err == nil {
+		t.Errorf("conn.WriteMsgUDPAddrPort(buf, nil, daddr6) should have failed, but got no error")
+	}
+}