generate: Safely cast in the static decoder

generate/generate.go

@@ -602,7 +602,7 @@ func (this *Generator) generateDecodeValue(typ Type, typeName, valueSource, tagS
 		this.pop()
 		nn, err = this.iprintf("}\n")
 		n += nn; if err != nil { return n, err }
-		nn, err = this.iprintf("buffer := make([]byte, int(%s))\n", lengthVar)
+		nn, err = this.iprintf("buffer := make([]byte, %s)\n", lengthVar)
 		n += nn; if err != nil { return n, err }
 		nn, err = this.iprintf("nn, err = decoder.Read(buffer)\n")
 		n += nn; if err != nil { return n, err }
@@ -799,7 +799,7 @@ func (this *Generator) generateDecodeBranch(hash [16]byte, typ Type, typeName st
 			// problems
 
 			// read fields
-			nn, err = this.iprintf("for _ = range int(%s) {\n", lengthVar)
+			nn, err = this.iprintf("for _ = range %s {\n", lengthVar)
 			n += nn; if err != nil { return n, err }
 			this.push()
 				// read field header

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) {
+		if destination.Cap() < lengthCast {
-			destination.Grow(int(length) - destination.Cap())
+			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

@@ -3,6 +3,7 @@ package tape
 // Error enumerates common errors in this package.
 type Error string; const (
 	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
+}