Avoid UB in conversions from floating point

When truncating floating point values to integer values, we need to
avoid undefined behavior if the argument does not fit into the target
type which is currently impossible using casts of primitive types.

Hence, this reimplements those conversions using arbitrary precision
integers and rationals from the num crate.

Cargo.toml

@@ -15,6 +15,8 @@ wasmi-validation = { version = "0.1", path = "validation", default-features = fa
 parity-wasm = { version = "0.31", default-features = false }
 memory_units = "0.3.0"
 libm = { version = "0.1.2", optional = true }
+num-rational = "0.2.2"
+num-traits = "0.2.8"
 
 [dev-dependencies]
 assert_matches = "1.1"
@@ -27,6 +29,8 @@ default = ["std"]
 std = [
     "parity-wasm/std",
     "wasmi-validation/std",
+    "num-rational/std",
+    "num-traits/std"
 ]
 # Enable for no_std support
 core = [

src/lib.rs

@@ -128,6 +128,9 @@ use std::error;
 #[cfg(not(feature = "std"))]
 extern crate libm;
 
+extern crate num_rational;
+extern crate num_traits;
+
 /// Error type which can be thrown by wasm code or by host environment.
 ///
 /// Under some conditions, wasm execution may produce a `Trap`, which immediately aborts execution.

src/value.rs

@@ -366,27 +366,18 @@ impl WrapInto<F32> for F64 {
 }
 
 macro_rules! impl_try_truncate_into {
-    ($from: ident, $into: ident) => {
+    (@primitive $from: ident, $into: ident, $to_primitive:path) => {
         impl TryTruncateInto<$into, TrapKind> for $from {
             fn try_truncate_into(self) -> Result<$into, TrapKind> {
                 // Casting from a float to an integer will round the float towards zero
-                // NOTE: currently this will cause Undefined Behavior if the rounded value cannot be represented by the
-                // target integer type. This includes Inf and NaN. This is a bug and will be fixed.
-                if self.is_nan() || self.is_infinite() {
-                    return Err(TrapKind::InvalidConversionToInt);
-                }
-
-                // range check
-                let result = self as $into;
-                if result as $from != self.trunc() {
-                    return Err(TrapKind::InvalidConversionToInt);
-                }
-
-                Ok(self as $into)
+                num_rational::BigRational::from_float(self)
+                    .map(|val| val.to_integer())
+                    .and_then(|val| $to_primitive(&val))
+                    .ok_or(TrapKind::InvalidConversionToInt)
             }
         }
     };
-    ($from:ident, $intermediate:ident, $into:ident) => {
+    (@wrapped $from:ident, $intermediate:ident, $into:ident) => {
         impl TryTruncateInto<$into, TrapKind> for $from {
             fn try_truncate_into(self) -> Result<$into, TrapKind> {
                 $intermediate::from(self).try_truncate_into()
@@ -395,22 +386,22 @@ macro_rules! impl_try_truncate_into {
     };
 }
 
-impl_try_truncate_into!(f32, i32);
-impl_try_truncate_into!(f32, i64);
-impl_try_truncate_into!(f64, i32);
-impl_try_truncate_into!(f64, i64);
-impl_try_truncate_into!(f32, u32);
-impl_try_truncate_into!(f32, u64);
-impl_try_truncate_into!(f64, u32);
-impl_try_truncate_into!(f64, u64);
-impl_try_truncate_into!(F32, f32, i32);
-impl_try_truncate_into!(F32, f32, i64);
-impl_try_truncate_into!(F64, f64, i32);
-impl_try_truncate_into!(F64, f64, i64);
-impl_try_truncate_into!(F32, f32, u32);
-impl_try_truncate_into!(F32, f32, u64);
-impl_try_truncate_into!(F64, f64, u32);
-impl_try_truncate_into!(F64, f64, u64);
+impl_try_truncate_into!(@primitive f32, i32, num_traits::cast::ToPrimitive::to_i32);
+impl_try_truncate_into!(@primitive f32, i64, num_traits::cast::ToPrimitive::to_i64);
+impl_try_truncate_into!(@primitive f64, i32, num_traits::cast::ToPrimitive::to_i32);
+impl_try_truncate_into!(@primitive f64, i64, num_traits::cast::ToPrimitive::to_i64);
+impl_try_truncate_into!(@primitive f32, u32, num_traits::cast::ToPrimitive::to_u32);
+impl_try_truncate_into!(@primitive f32, u64, num_traits::cast::ToPrimitive::to_u64);
+impl_try_truncate_into!(@primitive f64, u32, num_traits::cast::ToPrimitive::to_u32);
+impl_try_truncate_into!(@primitive f64, u64, num_traits::cast::ToPrimitive::to_u64);
+impl_try_truncate_into!(@wrapped F32, f32, i32);
+impl_try_truncate_into!(@wrapped F32, f32, i64);
+impl_try_truncate_into!(@wrapped F64, f64, i32);
+impl_try_truncate_into!(@wrapped F64, f64, i64);
+impl_try_truncate_into!(@wrapped F32, f32, u32);
+impl_try_truncate_into!(@wrapped F32, f32, u64);
+impl_try_truncate_into!(@wrapped F64, f64, u32);
+impl_try_truncate_into!(@wrapped F64, f64, u64);
 
 macro_rules! impl_extend_into {
     ($from:ident, $into:ident) => {

test.sh

@@ -5,7 +5,11 @@ set -eux
 EXTRA_ARGS=""
 
 if [ -n "${TARGET-}" ]; then
-    EXTRA_ARGS="--target=${TARGET} -- --test-threads=1"
+    # Tests build in debug mode are prohibitively
+    # slow when ran under emulation so that
+    # e.g. Travis CI will hit timeouts.
+    EXTRA_ARGS="--release --target=${TARGET}"
+    export RUSTFLAGS="--cfg debug_assertions"
 fi
 
 cd $(dirname $0)