libexpr: Check primop arity earlier

src/libexpr/eval.cc

@@ -723,6 +723,23 @@ void EvalState::addConstant(const std::string & name, Value * v, Constant info)
 }
 
 
+void PrimOp::check()
+{
+    if (arity > maxPrimOpArity) {
+        throw Error("primop arity must not exceed %1%", maxPrimOpArity);
+    }
+}
+
+
+void Value::mkPrimOp(PrimOp * p)
+{
+    p->check();
+    clearValue();
+    internalType = tPrimOp;
+    primOp = p;
+}
+
+
 Value * EvalState::addPrimOp(PrimOp && primOp)
 {
     /* Hack to make constants lazy: turn them into a application of
@@ -1692,8 +1709,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 /* We have all the arguments, so call the primop with
                    the previous and new arguments. */
 
-                assert(arity < 64);
-                Value * vArgs[64];
+                Value * vArgs[maxPrimOpArity];
                 auto n = argsDone;
                 for (Value * arg = &vCur; arg->isPrimOpApp(); arg = arg->primOpApp.left)
                     vArgs[--n] = arg->primOpApp.right;

src/libexpr/eval.hh

@@ -18,6 +18,12 @@
 
 namespace nix {
 
+/**
+ * We put a limit on primop arity because it lets us use a fixed size array on
+ * the stack. 16 is already an impractical number of arguments. Use an attrset
+ * argument for such overly complicated functions.
+ */
+constexpr size_t maxPrimOpArity = 64;
 
 class Store;
 class EvalState;
@@ -71,6 +77,12 @@ struct PrimOp
      * Optional experimental for this to be gated on.
      */
     std::optional<ExperimentalFeature> experimentalFeature;
+
+    /**
+     * Validity check to be performed by functions that introduce primops,
+     * such as RegisterPrimOp() and Value::mkPrimOp().
+     */
+    void check();
 };
 
 /**

src/libexpr/tests/value/print.cc

@@ -114,7 +114,8 @@ TEST_F(ValuePrintingTests, vLambda)
 TEST_F(ValuePrintingTests, vPrimOp)
 {
     Value vPrimOp;
-    vPrimOp.mkPrimOp(nullptr);
+    PrimOp primOp{};
+    vPrimOp.mkPrimOp(&primOp);
 
     test(vPrimOp, "<PRIMOP>");
 }

src/libexpr/value.hh

@@ -354,13 +354,7 @@ public:
         // Value will be overridden anyways
     }
 
-    inline void mkPrimOp(PrimOp * p)
-    {
-        clearValue();
-        internalType = tPrimOp;
-        primOp = p;
-    }
-
+    void mkPrimOp(PrimOp * p);
 
     inline void mkPrimOpApp(Value * l, Value * r)
     {