From 73414d12c2381b3c30edfaef796b8a151f751e26 Mon Sep 17 00:00:00 2001
From: Trivikram Kamat <16024985+trivikr@users.noreply.github.com>
Date: Sat, 24 Oct 2026 23:54:24 -0700
Subject: [PATCH] ffi: reject arguments after callback in registerCallback

Passing the signature after the callback silently created a `void ()`
callback, so native callers got no arguments forwarded and an
undefined return value.

Signed-off-by: Trivikram Kamat <16024985+trivikr@users.noreply.github.com>
Assisted-by: claude:opus-5.5
---
 src/node_ffi.cc                | 4 ++++
 test/ffi/test-ffi-callbacks.js | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/src/node_ffi.cc b/src/node_ffi.cc
index fe110eeae8e..ea46a1d72db 100644
--- a/src/node_ffi.cc
+++ b/src/node_ffi.cc
@@ -1081,6 +1081,10 @@ void DynamicLibrary::RegisterCallback(const FunctionCallbackInfo<Value>& args) {
   }
 
   if (args[0]->IsFunction()) {
+    if (args.Length() > 1) {
+      THROW_ERR_INVALID_ARG_TYPE(env, "Callback must be the last argument");
+      return;
+    }
     fn = args[0].As<Function>();
   } else {
     if (!args[0]->IsObject() || args[0]->IsArray()) {
diff --git a/test/ffi/test-ffi-callbacks.js b/test/ffi/test-ffi-callbacks.js
index 2dca67d6605..70dc23d2cc2 100644
--- a/test/ffi/test-ffi-callbacks.js
+++ b/test/ffi/test-ffi-callbacks.js
@@ -63,6 +63,10 @@ test('ffi callback ref and unref APIs work', () => {
       return 0;
     },
   );
+  assert.throws(() => lib.registerCallback(() => {}, { arguments: ['i32'], return: 'i32' }), {
+    code: 'ERR_INVALID_ARG_TYPE',
+    message: 'Callback must be the last argument',
+  });
 
   try {
     lib.unrefCallback(voidCallback);