ffi: fix use-after-free in PrepareFunction

PrepareFunction() looked up the function cache before parsing the
signature. Parsing can run user getters, and a getter that calls
lib.close() clears the cache, which invalidates the iterator. For a
function that was already cached, reading it afterwards used freed
memory and crashed the process.

Look up the cache after the signature is parsed. If the library was
closed during parsing, ResolveSymbol() now throws
ERR_FFI_LIBRARY_CLOSED.

Signed-off-by: Trivikram Kamat <16024985+trivikr@users.noreply.github.com>
Assisted-by: claude:opus-5.5
PR-URL: #66368
Fixes: #66367
Reviewed-By: Daeyeon Jeong <daeyeon.dev@gmail.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: trivikr

src/node_ffi.cc

@@ -144,12 +144,14 @@ Maybe<void*> DynamicLibrary::ResolveSymbol(Environment* env,
 Maybe<DynamicLibrary::PreparedFunction> DynamicLibrary::PrepareFunction(
     Environment* env, const std::string& name, Local<Object> signature) {
   std::shared_ptr<FFIFunction> fn;
-  auto existing = functions_.find(name);
   FunctionSignature parsed;
 
   if (!ParseFunctionSignature(env, name, signature).To(&parsed)) {
     return {};
   }
+  // Look up the cache only after parsing: the signature's getters run user
+  // code that may close the library, which clears `functions_`.
+  auto existing = functions_.find(name);
   auto [return_type, args, return_type_name, arg_type_names] =
       std::move(parsed);
 

test/ffi/test-ffi-dynamic-library.js

@@ -290,6 +290,21 @@ test('closed libraries reject subsequent operations', () => {
   assert.throws(() => lib.getSymbols(), /Library is closed/);
 });
 
+test('closing the library from a signature getter of a cached function', () => {
+  const lib = new ffi.DynamicLibrary(libraryPath);
+  lib.getFunction('add_i32', fixtureSymbols.add_i32);
+
+  assert.throws(() => {
+    lib.getFunction('add_i32', {
+      arguments: ['i32', 'i32'],
+      get return() {
+        lib.close();
+        return 'i32';
+      },
+    });
+  }, { code: 'ERR_FFI_LIBRARY_CLOSED' });
+});
+
 test('optimized fast calls reject calls after the library is closed', () => {
   const { lib, functions } = ffi.dlopen(libraryPath, {
     multiply_f64: fixtureSymbols.multiply_f64,