Re: [Qemu-devel] [PATCH v5 1/5] qobject: ensure base is at offset 0

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 04/18/2018 11:45 AM, Markus Armbruster wrote:
>
>>> Might also be worth mentioning that this explicitly guarantees that
>>> existing casts work correctly (even though we'd prefer to get rid of
>>> such casts in any location except the qobject.h macros); Markus pointed out:
>>>
>>>>> Uh, there's another reason: existing type casts from QObject * to
>>>>> subtypes.  I just spotted one in tests/check-qdict.c:
>>>>>
>>>>>     dst = (QDict *)qdict_crumple(src, &error_abort);
>> 
>> As far as I'm concerned, that's the real reason.  The simplification
>> plus the check to make it safe seems like a wash.
>> 
>> The cast I spotted appears to be the only one, though:
>> 
>>     $ git-grep '(Q[A-Z][a-z]* \*)'
>>     hmp.c:    qmp_device_add((QDict *)qdict, NULL, &err);
>>     include/qapi/qmp/qobject.h:        return (QObject *)obj;
>>     qobject/qobject.c:static void (*qdestroy[QTYPE__MAX])(QObject *) = {
>>     tests/check-qdict.c:    dst = (QDict *)qdict_crumple(src, &error_abort);
>> 
>> The first two cast away const, the third isn't a type cast.  The fourth
>> one should use qobject_to() instead, regardless of this patch.
>> 
>> Do we want to force base to come first anyway?
>> 
>> Where does PATCH 2 exploit "base first"?
>
> It doesn't, but PATCH 4 does:
>
>>  /**
>>   * qobject_ref(): Increment QObject's reference count
>> + *
>> + * Returns: the same @obj. The type of @obj will be propagated to the
>> + * return type.
>>   */
>> -#define qobject_ref(obj) qobject_ref_impl(QOBJECT(obj))
>> +#define qobject_ref(obj) ((typeof(obj)) qobject_ref_impl(QOBJECT(obj)))

Easy enough to fix:

    #define qobject_ref(obj) ({                     \
        typeof(obj) _obj = obj;                     \
        qobject_ref_impl(QOBJECT(_obj));            \
        _obj;                                       \
    })

Look ma, no type casts!