price: reject negative inputs in Parse

price/main.go

@@ -12,6 +12,7 @@ import (
 	"math/bits"
 	"regexp"
 	"strconv"
+	"strings"
 
 	"github.com/stellar/go-stellar-sdk/xdr"
 )
@@ -23,7 +24,7 @@ var (
 	// to `big.Rat.SetString` triggering long calculations.
 	// Note: {1,20} because the biggest amount you can use in Stellar is:
 	// len("922337203685.4775807") = 20.
-	validAmountSimple = regexp.MustCompile("^-?[.0-9]{1,20}$")
+	validAmountSimple = regexp.MustCompile("^[.0-9]{1,20}$")
 	// ErrDivisionByZero is returned when a price operation would result in a division by 0
 	ErrDivisionByZero = errors.New("division by 0")
 	// ErrOverflow is returned when a price operation would result in an integer overflow
@@ -52,6 +53,9 @@ func MustParse(v string) xdr.Price {
 // continuedFraction calculates and returns the best rational approximation of
 // the given real number.
 func continuedFraction(price string) (xdrPrice xdr.Price, err error) {
+	if strings.HasPrefix(price, "-") {
+		return xdrPrice, ErrNoNegatives
+	}
 	if !validAmountSimple.MatchString(price) {
 		return xdrPrice, fmt.Errorf("invalid price format: %s", price)
 	}

price/main_test.go

@@ -72,6 +72,41 @@ func TestParse(t *testing.T) {
 	}
 }
 
+func TestParseRejectsNegatives(t *testing.T) {
+	inputs := []string{
+		"-1",
+		"-0.5",
+		"-0",
+		"-0.0",
+		"-2147483647",
+		"-2147483648",
+		// These wrap on the int64 -> int32 narrowing and used to come back
+		// as valid positive prices.
+		"-2147483649",
+		"-2147483647.9",
+		"-4294967295",
+		"-4294967296",
+	}
+	for _, in := range inputs {
+		_, err := Parse(in)
+		assert.ErrorIs(t, err, ErrNoNegatives, "input %q", in)
+	}
+}
+
+// Every price Parse returns without error must also pass xdr.Price.Validate.
+func TestParseOutputIsValid(t *testing.T) {
+	for _, v := range Tests {
+		if !v.V {
+			continue
+		}
+		o, err := Parse(v.S)
+		if !assert.NoError(t, err, "input %q", v.S) {
+			continue
+		}
+		assert.NoError(t, o.Validate(), "input %q", v.S)
+	}
+}
+
 func TestStringFromFloat64(t *testing.T) {
 
 	tests := map[float64]string{