net: fix IPMask.String not to crash on all-0xff mask

R=r
CC=golang-dev
https://golang.org/cl/438042

src/pkg/net/ip.go

@@ -51,6 +51,20 @@ func IPv4(a, b, c, d byte) IP {
 	return p
 }
 
+// IPv4Mask returns the IP mask (in 16-byte form) of the
+// IPv4 mask a.b.c.d.
+func IPv4Mask(a, b, c, d byte) IPMask {
+	p := make(IPMask, IPv6len)
+	for i := 0; i < 12; i++ {
+		p[i] = 0xff
+	}
+	p[12] = a
+	p[13] = b
+	p[14] = c
+	p[15] = d
+	return p
+}
+
 // Well-known IPv4 addresses
 var (
 	IPv4bcast     = IPv4(255, 255, 255, 255) // broadcast
@@ -103,9 +117,9 @@ func (ip IP) To16() IP {
 
 // Default route masks for IPv4.
 var (
-	classAMask = IPMask(IPv4(0xff, 0, 0, 0))
-	classBMask = IPMask(IPv4(0xff, 0xff, 0, 0))
-	classCMask = IPMask(IPv4(0xff, 0xff, 0xff, 0))
+	classAMask = IPv4Mask(0xff, 0, 0, 0)
+	classBMask = IPv4Mask(0xff, 0xff, 0, 0)
+	classCMask = IPv4Mask(0xff, 0xff, 0xff, 0)
 )
 
 // DefaultMask returns the default IP mask for the IP address ip.
@@ -229,18 +243,19 @@ func (ip IP) String() string {
 // If mask is a sequence of 1 bits followed by 0 bits,
 // return the number of 1 bits.
 func simpleMaskLength(mask IPMask) int {
-	var i int
-	for i = 0; i < len(mask); i++ {
-		if mask[i] != 0xFF {
-			break
-		}
-	}
-	n := 8 * i
-	v := mask[i]
+	var n int
+	for i, v := range mask {
+		if v == 0xff {
+			n += 8
+			continue
+		}
+		// found non-ff byte
+		// count 1 bits
 		for v&0x80 != 0 {
 			n++
 			v <<= 1
 		}
+		// rest must be 0 bits
 		if v != 0 {
 			return -1
 		}
@@ -249,6 +264,8 @@ func simpleMaskLength(mask IPMask) int {
 				return -1
 			}
 		}
+		break
+	}
 	return n
 }
 
@@ -266,8 +283,8 @@ func (mask IPMask) String() string {
 		}
 	case 16:
 		n := simpleMaskLength(mask)
-		if n >= 0 {
-			return itod(uint(n))
+		if n >= 12*8 {
+			return itod(uint(n - 12*8))
 		}
 	}
 	return IP(mask).String()