Fix bug involving .send and overwritten methods.

@casperisfine reporting a bug in this gist https://gist.github.com/casperisfine/d59e297fba38eb3905a3d7152b9e9350

After investigating I found it was caused by a combination of send and a c_func that we have overwritten in the JIT. For send calls, we need to do some stack manipulation before making the call. Because of the way exits works, we need to do that stack manipulation at the last possible moment. In this case, we weren't doing that stack manipulation at all. Unfortunately, with how the code is structured there isn't a great place to do that stack manipulation for our overridden C funcs.

Each overridden C func can return a boolean stating that it shouldn't be used. We would need to do the stack manipulation after all of those checks are done. We could pass a lambda(?) or separate out the logic for "can I run this override" from "now generate the code for it". Since we are coming up on a release, I went with the path of least resistence and just decided to not use these overrides if we are in a send call.

We definitely should revist this in the future.

bootstraptest/test_yjit.rb

@@ -45,6 +45,29 @@ def call_graph_root
   Object.send(:remove_const, :Foo)
 }
 
+assert_normal_exit %q{
+  # Test to ensure send on overriden c functions
+  # doesn't corrupt the stack
+  class Bar
+    def bar(x)
+      x
+    end
+  end
+
+  class Foo
+    def bar
+      Bar.new
+    end
+  end
+
+  foo = Foo.new
+  # before this change, this line would error
+  # because "s" would still be on the stack
+  # String.to_s is the overridden method here
+  p foo.bar.bar("s".__send__(:to_s))
+}
+
+
 assert_equal '[nil, nil, nil, nil, nil, nil]', %q{
   [NilClass, TrueClass, FalseClass, Integer, Float, Symbol].each do |klass|
     klass.class_eval("def foo = @foo")

yjit/src/codegen.rs

@@ -4179,7 +4179,7 @@ fn gen_send_cfunc(
     }
 
     // Delegate to codegen for C methods if we have it.
-    if kw_arg.is_null() {
+    if kw_arg.is_null() && flags & VM_CALL_OPT_SEND == 0 {
         let codegen_p = lookup_cfunc_codegen(unsafe { (*cme).def });
         if let Some(known_cfunc_codegen) = codegen_p {
             if known_cfunc_codegen(jit, ctx, asm, ocb, ci, cme, block, argc, recv_known_klass) {