fix: network address of length zero are valid

Using the route command I replicated the original bug and observed the
correct marshaling of the route message, confirming what was stated
in chapter 18 of UNIX® Network Programming Volume 1, Third Edition: The
Sockets Networking API:

The socket address structures are variable-length, but this code assumes
that each has an sa_len field specifying its length. There are two
complications that must be handled. First, the two masks, the network
mask and the cloning mask, can be returned in a socket address structure
with an sa_len of 0, but this really occupies the size of an unsigned
long. (Chapter 19 of TCPv2 discusses the cloning feature of the 4.4BSD
routing table). This value represents a mask of all zero bits, which we
printed as 0.0.0.0 for the network mask of the default route in our
earlier example.

There are other references in the book which also state sa_len of 0 is
valid.

Author: hurricanehrndz

route/address.go

@@ -171,45 +171,55 @@ func (a *Inet6Addr) marshal(b []byte) (int, error) {
 // parseInetAddr parses b as an internet address for IPv4 or IPv6.
 func parseInetAddr(af int, b []byte) (Addr, error) {
 	const (
-		off4 = 4 // offset of in_addr
-		off6 = 8 // offset of in6_addr
+		off4    = 4  // offset of in_addr
+		off6    = 8  // offset of in6_addr
+		ipv4Len = 4  // length of IPv4 address in bytes
+		ipv6Len = 16 // length of IPv6 address in bytes
 	)
 	switch af {
 	case syscall.AF_INET:
-		if len(b) < (off4+1) || len(b) < int(b[0]) || b[0] == 0 {
+		if len(b) < (off4+1) || len(b) < int(b[0]) {
 			return nil, errInvalidAddr
 		}
 		sockAddrLen := int(b[0])
-		a := &Inet4Addr{}
-		n := off4 + 4
-		if sockAddrLen < n {
-			n = sockAddrLen
+		a := &Inet4Addr{IP: [ipv4Len]byte{}}
+		// sockAddrLen of 0 is valid and represents 0.0.0.0
+		if sockAddrLen != 0 {
+			// Calculate how many bytes of the address to copy:
+			// either full IPv4 length or the available length
+			n := off4 + ipv4Len
+			if sockAddrLen < n {
+				n = sockAddrLen
+			}
+			copy(a.IP[:], b[off4:n])
 		}
-		copy(a.IP[:], b[off4:n])
 		return a, nil
 	case syscall.AF_INET6:
-		if len(b) < (off6+1) || len(b) < int(b[0]) || b[0] == 0 {
+		if len(b) < (off6+1) || len(b) < int(b[0]) {
 			return nil, errInvalidAddr
 		}
 		sockAddrLen := int(b[0])
-		n := off6 + 16
-		if sockAddrLen < n {
-			n = sockAddrLen
-		}
-		a := &Inet6Addr{}
-		if sockAddrLen == sizeofSockaddrInet6 {
-			a.ZoneID = int(nativeEndian.Uint32(b[24:28]))
-		}
-		copy(a.IP[:], b[off6:n])
-		if a.IP[0] == 0xfe && a.IP[1]&0xc0 == 0x80 || a.IP[0] == 0xff && (a.IP[1]&0x0f == 0x01 || a.IP[1]&0x0f == 0x02) {
-			// KAME based IPv6 protocol stack usually
-			// embeds the interface index in the
-			// interface-local or link-local address as
-			// the kernel-internal form.
-			id := int(bigEndian.Uint16(a.IP[2:4]))
-			if id != 0 {
-				a.ZoneID = id
-				a.IP[2], a.IP[3] = 0, 0
+		a := &Inet6Addr{IP: [ipv6Len]byte{}}
+		// sockAddrLen of 0 is valid and represents ::
+		if sockAddrLen != 0 {
+			n := off6 + ipv6Len
+			if sockAddrLen < n {
+				n = sockAddrLen
+			}
+			if sockAddrLen == sizeofSockaddrInet6 {
+				a.ZoneID = int(nativeEndian.Uint32(b[24:28]))
+			}
+			copy(a.IP[:], b[off6:n])
+			if a.IP[0] == 0xfe && a.IP[1]&0xc0 == 0x80 || a.IP[0] == 0xff && (a.IP[1]&0x0f == 0x01 || a.IP[1]&0x0f == 0x02) {
+				// KAME based IPv6 protocol stack usually
+				// embeds the interface index in the
+				// interface-local or link-local address as
+				// the kernel-internal form.
+				id := int(bigEndian.Uint16(a.IP[2:4]))
+				if id != 0 {
+					a.ZoneID = id
+					a.IP[2], a.IP[3] = 0, 0
+				}
 			}
 		}
 		return a, nil
@@ -404,16 +414,12 @@ func parseAddrs(attrs uint, fn func(int, []byte) (int, Addr, error), b []byte) (
 				}
 				b = b[l:]
 			case syscall.AF_INET, syscall.AF_INET6:
-				// #70528: if the sockaddrlen is 0, no address to parse inside,
-				// skip over the record.
-				if b[0] > 0 {
-					af = int(b[1])
-					a, err := parseInetAddr(af, b)
-					if err != nil {
-						return nil, err
-					}
-					as[i] = a
+				af = int(b[1])
+				a, err := parseInetAddr(af, b)
+				if err != nil {
+					return nil, err
 				}
+				as[i] = a
 				l := roundup(int(b[0]))
 				if len(b) < l {
 					return nil, errMessageTooShort

route/address_darwin_test.go

@@ -112,7 +112,7 @@ var parseAddrsOnDarwinLittleEndianTests = []parseAddrsOnDarwinTest{
 			0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x00, 0x00,
 		},
 		[]Addr{
-			nil,
+			&Inet6Addr{IP: [16]byte{}},
 			&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,