QmlEngine::invoke_method is broken with Qt ≥6.5

[alois31]

In Qt 6.5, there have been some changes around QMetaObject::invokeMethod. Compilation has been fixed by https://github.com/woboq/qmetaobject-rs/pull/284, however this change was incorrect. QMetaObject::invokeMethod apparently now checks the signature of the method, and prints an error message on mismatches. Since we pass in a bunch of extra arguments which are not properly initialized, this segfaults trying to dereference a null pointer, probably when trying to print the type name of the extra arguments. I'm not sure how to fix this short of providing implementing each arity separately. Even then, only QVariants can be passed with the current code, so I'm not sure how useful that is.

[ogoffart]

cc @AdrianEddy

[AdrianEddy]

Hmm, everything works fine for me, do you have a reproducible example?

[alois31]

A simple cargo test segfaults reproducibly for me in the mentioned codepath.

[AdrianEddy]

Indeed, thanks for noticing this.

@ogoffart I confirmed it works when the method is called explicitly with the correct number of arguments, but it's not pretty:

QVariant ret;
#if QT_VERSION >= QT_VERSION_CHECK(6,5,0)
    #define INVOKE_METHOD_WITH_ARGS(...) QMetaObject::invokeMethod(robjs.first(), name, qReturnArg(ret), __VA_ARGS__);
#else
    #define INVOKE_METHOD_WITH_ARGS(...) QMetaObject::invokeMethod(robjs.first(), name, Q_RETURN_ARG(QVariant, ret), __VA_ARGS__);
#endif
switch (args_size) {
    case 0: INVOKE_METHOD_WITH_ARGS() break;
    case 1: INVOKE_METHOD_WITH_ARGS(Q_ARG(QVariant, args_ptr[0])); break;
    case 2: INVOKE_METHOD_WITH_ARGS(Q_ARG(QVariant, args_ptr[0]), Q_ARG(QVariant, args_ptr[1])); break;
    case 3: INVOKE_METHOD_WITH_ARGS(Q_ARG(QVariant, args_ptr[0]), Q_ARG(QVariant, args_ptr[1]), Q_ARG(QVariant, args_ptr[2])); break;
    case 4: INVOKE_METHOD_WITH_ARGS(Q_ARG(QVariant, args_ptr[0]), Q_ARG(QVariant, args_ptr[1]), Q_ARG(QVariant, args_ptr[2]), Q_ARG(QVariant, args_ptr[3])); break;
    case 5: INVOKE_METHOD_WITH_ARGS(Q_ARG(QVariant, args_ptr[0]), Q_ARG(QVariant, args_ptr[1]), Q_ARG(QVariant, args_ptr[2]), Q_ARG(QVariant, args_ptr[3]), Q_ARG(QVariant, args_ptr[4])); break;
    case 6: INVOKE_METHOD_WITH_ARGS(Q_ARG(QVariant, args_ptr[0]), Q_ARG(QVariant, args_ptr[1]), Q_ARG(QVariant, args_ptr[2]), Q_ARG(QVariant, args_ptr[3]), Q_ARG(QVariant, args_ptr[4]), Q_ARG(QVariant, args_ptr[5])); break;
    case 7: INVOKE_METHOD_WITH_ARGS(Q_ARG(QVariant, args_ptr[0]), Q_ARG(QVariant, args_ptr[1]), Q_ARG(QVariant, args_ptr[2]), Q_ARG(QVariant, args_ptr[3]), Q_ARG(QVariant, args_ptr[4]), Q_ARG(QVariant, args_ptr[5]), Q_ARG(QVariant, args_ptr[6])); break;
    case 8: INVOKE_METHOD_WITH_ARGS(Q_ARG(QVariant, args_ptr[0]), Q_ARG(QVariant, args_ptr[1]), Q_ARG(QVariant, args_ptr[2]), Q_ARG(QVariant, args_ptr[3]), Q_ARG(QVariant, args_ptr[4]), Q_ARG(QVariant, args_ptr[5]), Q_ARG(QVariant, args_ptr[6]), Q_ARG(QVariant, args_ptr[7])); break;
    case 9: INVOKE_METHOD_WITH_ARGS(Q_ARG(QVariant, args_ptr[0]), Q_ARG(QVariant, args_ptr[1]), Q_ARG(QVariant, args_ptr[2]), Q_ARG(QVariant, args_ptr[3]), Q_ARG(QVariant, args_ptr[4]), Q_ARG(QVariant, args_ptr[5]), Q_ARG(QVariant, args_ptr[6]), Q_ARG(QVariant, args_ptr[7]), Q_ARG(QVariant, args_ptr[8])); break;
}
return ret;

Do you want me to submit this PR, or you have a better idea?

[alois31]

We should probably also assert that there are at most 9 arguments instead of writing extra arguments out of bounds and then silently not passing them on (the same is actually the case for the old codepath). It's a pity that invokeMethodImpl is private, otherwise we could properly pass on the variadics.