Re: [Qemu-devel] [PATCH v2 2/3] qobject: introduce QObjectCommon

From: Paolo Bonzini <pbonzini@redhat.com>
To: "Eric Blake" <eblake@redhat.com>,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: qemu-devel <qemu-devel@nongnu.org>,
	"Armbruster, Markus" <armbru@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v2 2/3] qobject: introduce QObjectCommon
Date: Wed, 28 Mar 2018 16:11:28 +0200	[thread overview]
Message-ID: <6fca462f-7aca-f743-6f43-8475b6501014@redhat.com> (raw)
In-Reply-To: <ac132600-ac94-c9b7-b280-f2ff2ebebf74@redhat.com>

On 28/03/2018 15:55, Eric Blake wrote:
> On 03/28/2018 08:48 AM, Marc-André Lureau wrote:
> 
>>>> +#define QOBJECT(x) \
>>>> +   container_of(&(x)->base, QObject, base)
>>>
>>>
>>> If I understand correctly, this still causes clang complaints when
>>> called as
>>> QOBJECT(NULL).  As long as we are touching this, should we improve this
>>> macro to be friendly to NULL conversion?
>>
>> I don't see much need for allowing NULL (literally) to be passed to
>> QOBJECT(). If it's a null pointer, as long as it has the right type,
>> it should be fine, shouldn't it?
> 
> Not with clang ubsan (okay, the failure is at runtime, not compile time):
> 
> https://lists.gnu.org/archive/html/qemu-devel/2018-03/msg05143.html
> https://lists.gnu.org/archive/html/qemu-devel/2018-03/msg05148.html
> 
> Even when the offset is 0, the mere fact that you are computing an
> offset relative to a NULL pointer is undefined behavior.

Are there cases where we are passing NULL to qobject_{inc,dec}ref?  They
currently have an "if" in they're body, but my opinion is they ought to
crash and burn...

Paolo