route: treat short sockaddr lengths as unspecified

Previously, we enforced minimum length requirements for sockaddr,
but the kernel can legitimately return shorter lengths. This change
treats any sockaddr with length less than the address offset as an
unspecified address (0.0.0.0 for IPv4 or :: for IPv6).

Fixes golang/go#71557

Author: hurricanehrndz

Diff for: route/address.go

@@ -178,13 +178,13 @@ func parseInetAddr(af int, b []byte) (Addr, error) {
 	)
 	switch af {
 	case syscall.AF_INET:
-		if len(b) < (off4+1) || len(b) < int(b[0]) {
+		if len(b) < int(b[0]) {
 			return nil, errInvalidAddr
 		}
 		sockAddrLen := int(b[0])
 		a := &Inet4Addr{}
 		// sockAddrLen of 0 is valid and represents 0.0.0.0
-		if sockAddrLen != 0 {
+		if sockAddrLen > off4 {
 			// Calculate how many bytes of the address to copy:
 			// either full IPv4 length or the available length.
 			n := off4 + ipv4Len
@@ -195,13 +195,13 @@ func parseInetAddr(af int, b []byte) (Addr, error) {
 		}
 		return a, nil
 	case syscall.AF_INET6:
-		if len(b) < (off6+1) || len(b) < int(b[0]) {
+		if len(b) < int(b[0]) {
 			return nil, errInvalidAddr
 		}
 		sockAddrLen := int(b[0])
 		a := &Inet6Addr{}
 		// sockAddrLen of 0 is valid and represents ::
-		if sockAddrLen != 0 {
+		if sockAddrLen > off6 {
 			n := off6 + ipv6Len
 			if sockAddrLen < n {
 				n = sockAddrLen

Diff for: route/address_darwin_test.go

@@ -96,6 +96,38 @@ var parseAddrsOnDarwinLittleEndianTests = []parseAddrsOnDarwinTest{
 			nil,
 		},
 	},
+	// sudo route -n add -inet6 -ifscope en11 -net :: -netmask :: fe80::2d0:4cff:fe10:15d2
+	// RTM_ADD: Add Route: len 152, pid: 81198, seq 1, errno 0, ifscope 13, flags:<UP,GATEWAY,DONE,STATIC,IFSCOPE>
+	// locks:  inits:
+	// sockaddrs: <DST,GATEWAY,NETMASK>
+	// :: fe80::2d0:4cff:fe10:15d2 ::
+	{
+		syscall.RTA_DST | syscall.RTA_GATEWAY | syscall.RTA_NETMASK,
+		parseKernelInetAddr,
+		[]byte{
+			0x1c, 0x1e, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+
+			0x1c, 0x1e, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x02, 0xd0, 0x4c, 0xff, 0xfe, 0x10, 0x15, 0xd2,
+			0x00, 0x00, 0x00, 0x00,
+
+			0x02, 0x1e, 0x00, 0x00,
+		},
+		[]Addr{
+			&Inet6Addr{},
+			&Inet6Addr{IP: [16]byte{0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0xd0, 0x4c, 0xff, 0xfe, 0x10, 0x15, 0xd2}},
+			&Inet6Addr{},
+			nil,
+			nil,
+			nil,
+			nil,
+			nil,
+		},
+	},
 	// golang/go#70528, the kernel can produce addresses of length 0
 	{
 		syscall.RTA_DST | syscall.RTA_GATEWAY | syscall.RTA_NETMASK,