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Avoid undefined behaviour when doing range checks for integers #107

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Avoid undefined behaviour when doing range checks for integers #107

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9 changes: 5 additions & 4 deletions string.go
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Original file line number Diff line number Diff line change
Expand Up @@ -79,6 +79,9 @@ func formatHelper(l *State, fs string, argCount int) string {
ArgumentError(l, arg, "no value")
}
f := scanFormat(l, fs[i:])
const uint_max = ^uint(0)
const int_max = int(uint_max >> 1)
const int_min = -int_max - 1
switch i += len(f) - 2; fs[i] {
case 'c':
// Ensure each character is represented by a single byte, while preserving format modifiers.
Expand All @@ -91,18 +94,16 @@ func formatHelper(l *State, fs string, argCount int) string {
fallthrough
case 'd':
n := CheckNumber(l, arg)
ArgumentCheck(l, float64(int_min) <= n && n < float64(int_max), arg, "not a number in proper range")
ni := int(n)
diff := n - float64(ni)
ArgumentCheck(l, -1 < diff && diff < 1, arg, "not a number in proper range")
fmt.Fprintf(&b, f, ni)
case 'u': // The fmt package doesn't support %u.
f = f[:len(f)-1] + "d"
fallthrough
case 'o', 'x', 'X':
n := CheckNumber(l, arg)
ArgumentCheck(l, 0 <= n && n < float64(uint_max), arg, "not a non-negative number in proper range")
ni := uint(n)
diff := n - float64(ni)
ArgumentCheck(l, -1 < diff && diff < 1, arg, "not a non-negative number in proper range")
fmt.Fprintf(&b, f, ni)
case 'e', 'E', 'f', 'g', 'G':
fmt.Fprintf(&b, f, CheckNumber(l, arg))
Expand Down