tape: Safely cast when dynamically encoding/decoding

tape/dynamic.go

@@ -171,10 +171,12 @@ func decodeAnyOrError(decoder *Decoder, destination reflect.Value, tag Tag) (n i
 		if length > uint64(MaxStructureLength) {
 			return 0, ErrTooLong
 		}
+		lengthCast, err := Uint64ToIntSafe(length)
+		if err != nil { return n, err }
 		oneTag, nn, err := decoder.ReadTag()
 		n += nn; if err != nil { return n, err }
-		if destination.Cap() < int(length) {
-			destination.Grow(int(length) - destination.Cap())
+		if destination.Cap() < lengthCast {
+			destination.Grow(lengthCast - destination.Cap())
 		}
 		// skip the rest of the array if the one tag doesn't
 		// match up with the destination
@@ -187,7 +189,7 @@ func decodeAnyOrError(decoder *Decoder, destination reflect.Value, tag Tag) (n i
 			break
 		}
 		if err != nil { return n, err }
-		destination.SetLen(int(length))
+		destination.SetLen(lengthCast)
 		for index := range length {
 			nn, err := decodeAny(decoder, destination.Index(int(index)), oneTag)
 			n += nn

tape/error.go

@@ -2,7 +2,8 @@ package tape
 
 // Error enumerates common errors in this package.
 type Error string; const (
-	ErrTooLong Error = "data structure too long"
+	ErrTooLong  Error = "data structure too long"
+	ErrTooLarge Error = "number too large"
 )
 
 // Error implements the error interface.

tape/limits.go

@@ -11,3 +11,16 @@ package tape
 // You shouldn't need to change this. If you do, it should only be set once at
 // the start of the program.
 var MaxStructureLength = 1024 * 1024
+
+// MaxInt is the maximum value an int can hold. This varies depending on the
+// system.
+const MaxInt int = int(^uint(0) >> 1)
+
+// Uint64ToIntSafe casts the input to an int if it can be done without overflow,
+// or returns an error otherwise.
+func Uint64ToIntSafe(input uint64) (int, error) {
+	if input > uint64(MaxInt) {
+		return 0, ErrTooLarge
+	}
+	return int(input), nil
+}