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


From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v2 2/3] qobject: introduce QObjectCommon
Date: Wed, 28 Mar 2018 08:55:51 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

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.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org



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