route: fix parse of zero-length sockaddrs in RIBs

Zero-length sockaddrs were observed in RIBs within golang/go#70528.
These records are to be skipped, and an invariant for later slice
manipulation is to be enforced by a defensive check in parseAddr.

Fixes golang/go#70528

Author: raggi

route/address.go

@@ -176,7 +176,7 @@ 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) < (off4+1) || len(b) < int(b[0]) || int(b[0]) == 0 {
 			return nil, errInvalidAddr
 		}
 		sockAddrLen := int(b[0])
@@ -188,7 +188,7 @@ func parseInetAddr(af int, b []byte) (Addr, error) {
 		copy(a.IP[:], b[off4:n])
 		return a, nil
 	case syscall.AF_INET6:
-		if len(b) < (off6+1) || len(b) < int(b[0]) {
+		if len(b) < (off6+1) || len(b) < int(b[0]) || int(b[0]) == 0 {
 			return nil, errInvalidAddr
 		}
 		sockAddrLen := int(b[0])
@@ -404,12 +404,16 @@ func parseAddrs(attrs uint, fn func(int, []byte) (int, Addr, error), b []byte) (
 				}
 				b = b[l:]
 			case syscall.AF_INET, syscall.AF_INET6:
-				af = int(b[1])
-				a, err := parseInetAddr(af, b)
-				if err != nil {
-					return nil, err
+				// #70528: if the sockaddrlen is 0, no address to parse inside,
+				// skip over the record.
+				if int(b[0]) > 0 {
+					af = int(b[1])
+					a, err := parseInetAddr(af, b)
+					if err != nil {
+						return nil, err
+					}
+					as[i] = a
 				}
-				as[i] = a
 				l := roundup(int(b[0]))
 				if len(b) < l {
 					return nil, errMessageTooShort

route/address_darwin_test.go

@@ -86,9 +86,35 @@ var parseAddrsOnDarwinLittleEndianTests = []parseAddrsOnDarwinTest{
 			0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x00, 0x00,
 		},
 		[]Addr{
-			&Inet6Addr{IP: [16]byte{ 0xfd, 0x84, 0x1b, 0x4e, 0x62, 0x81 }},
-			&Inet6Addr{IP: [16]byte{ 0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xf2, 0x2f, 0x4b, 0xff, 0xfe, 0x09, 0x3b, 0xff }, ZoneID: 33},
-			&Inet6Addr{IP: [16]byte{ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,}},
+			&Inet6Addr{IP: [16]byte{0xfd, 0x84, 0x1b, 0x4e, 0x62, 0x81}},
+			&Inet6Addr{IP: [16]byte{0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xf2, 0x2f, 0x4b, 0xff, 0xfe, 0x09, 0x3b, 0xff}, ZoneID: 33},
+			&Inet6Addr{IP: [16]byte{0xff, 0xff, 0xff, 0xff, 0xff, 0xff}},
+			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,
+		parseKernelInetAddr,
+		[]byte{
+			0x00, 0x1e, 0x00, 0x00,
+
+			0x1c, 0x1e, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0xfe, 0x80, 0x00, 0x21, 0x00, 0x00, 0x00, 0x00,
+			0xf2, 0x2f, 0x4b, 0xff, 0xfe, 0x09, 0x3b, 0xff,
+			0x00, 0x00, 0x00, 0x00,
+
+			0x0e, 0x1e, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x00, 0x00,
+		},
+		[]Addr{
+			nil,
+			&Inet6Addr{IP: [16]byte{0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xf2, 0x2f, 0x4b, 0xff, 0xfe, 0x09, 0x3b, 0xff}, ZoneID: 33},
+			&Inet6Addr{IP: [16]byte{0xff, 0xff, 0xff, 0xff, 0xff, 0xff}},
 			nil,
 			nil,
 			nil,