Support feature/nullish coalesce

Sources/Fuzzilli/FuzzIL/JSTyper.swift

@@ -427,7 +427,8 @@ public struct JSTyper: Analyzer {
                  .UnRShift:
                 return maybeBigIntOr(.integer)
             case .LogicAnd,
-                 .LogicOr:
+                 .LogicOr,
+                 .NullCoalesce:
                 return state.type(of: inputs[0]) | state.type(of: inputs[1])
             }
         }

Sources/Fuzzilli/FuzzIL/JsOperations.swift

@@ -1403,6 +1403,8 @@ public enum BinaryOperator: String, CaseIterable {
     case RShift   = ">>"
     case Exp      = "**"
     case UnRShift = ">>>"
+    // Nullish coalescing operator (??)
+    case NullCoalesce = "??"
 
     var token: String {
         return self.rawValue

Sources/Fuzzilli/Lifting/JavaScriptExploreLifting.swift

@@ -247,8 +247,7 @@ struct JavaScriptExploreLifting {
         //
         function exploreObject(o) {
             if (o === null) {
-                // Can't do anything with null.
-                return NO_ACTION;
+                return exploreNullish(o);
             }
 
             // TODO: Add special handling for ArrayBuffers: most of the time, wrap these into a Uint8Array to be able to modify them.
@@ -405,6 +404,11 @@ struct JavaScriptExploreLifting {
             return action;
         }
 
+        function exploreNullish(v) {
+            // Best thing we can do with nullish values is a NullCoalescing (??) operation.
+            return new Action(OP_NULL_COALESCE, [exploredValueInput, Inputs.randomArgument()])
+        }
+
         // Explores the given value and returns an action to perform on it.
         function exploreValue(id, v) {
             if (isObject(v)) {
@@ -422,8 +426,7 @@ struct JavaScriptExploreLifting {
             } else if (isBoolean(v)) {
                 return exploreBoolean(v);
             } else if (isUndefined(v)) {
-                // Can't do anything with undefined.
-                return NO_ACTION;
+                return exploreNullish(v);
             } else {
                 throw "Unexpected value type: " + typeof v;
             }

Sources/Fuzzilli/Lifting/JavaScriptRuntimeAssistedMutatorLifting.swift

@@ -389,6 +389,7 @@ struct JavaScriptRuntimeAssistedMutatorLifting {
         const OP_LOGICAL_AND = 'LOGICAL_AND';
         const OP_LOGICAL_OR = 'LOGICAL_OR';
         const OP_LOGICAL_NOT = 'LOGICAL_NOT';
+        const OP_NULL_COALESCE = 'NULL_COALESCE';
 
         const OP_BITWISE_AND = 'BITWISE_AND';
         const OP_BITWISE_OR = 'BITWISE_OR';
@@ -511,6 +512,7 @@ struct JavaScriptRuntimeAssistedMutatorLifting {
           [OP_LOGICAL_AND]: (inputs) => inputs[0] && inputs[1],
           [OP_LOGICAL_OR]: (inputs) => inputs[0] || inputs[1],
           [OP_LOGICAL_NOT]: (inputs) => !inputs[0],
+          [OP_NULL_COALESCE]: (inputs) => inputs[0] ?? inputs[1],
           [OP_BITWISE_AND]: (inputs) => inputs[0] & inputs[1],
           [OP_BITWISE_OR]: (inputs) => inputs[0] | inputs[1],
           [OP_BITWISE_XOR]: (inputs) => inputs[0] ^ inputs[1],

Sources/Fuzzilli/Mutators/RuntimeAssistedMutator.swift

@@ -186,6 +186,7 @@ public class RuntimeAssistedMutator: Mutator {
         case LogicalAnd = "LOGICAL_AND"
         case LogicalOr = "LOGICAL_OR"
         case LogicalNot = "LOGICAL_NOT"
+        case NullCoalesce = "NULL_COALESCE"
         case BitwiseAnd = "BITWISE_AND"
         case BitwiseOr = "BITWISE_OR"
         case BitwiseXor = "BITWISE_XOR"
@@ -385,6 +386,8 @@ extension RuntimeAssistedMutator.Action {
             try translateBinaryOperation(.BitOr)
         case .BitwiseXor:
             try translateBinaryOperation(.Xor)
+        case .NullCoalesce:
+            try translateBinaryOperation(.NullCoalesce)    
         case .LeftShift:
             try translateBinaryOperation(.LShift)
         case .SignedRightShift: