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Re: [Qemu-devel] [PATCH v6 1/5] qobject: ensure base is at offset 0
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH v6 1/5] qobject: ensure base is at offset 0 |
Date: |
Fri, 27 Apr 2018 10:14:40 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) |
Marc-André Lureau <address@hidden> writes:
> All QObject types have the base QObject as their first field. This
> allows the simplification of qobject_to().
>
> 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)
In my opinion, type casts between QObject * and subtypes are plain
wrong. I intend to apply my "[PATCH] qobject: Use qobject_to() instead
of type cast" first, and drop the paragraph, if you don't mind.
> Signed-off-by: Marc-André Lureau <address@hidden>
> Reviewed-by: Eric Blake <address@hidden>
> ---
> include/qapi/qmp/qobject.h | 5 ++---
> qobject/qobject.c | 9 +++++++++
> 2 files changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/include/qapi/qmp/qobject.h b/include/qapi/qmp/qobject.h
> index e022707578..5206ff9ee1 100644
> --- a/include/qapi/qmp/qobject.h
> +++ b/include/qapi/qmp/qobject.h
> @@ -61,9 +61,8 @@ struct QObject {
> QEMU_BUILD_BUG_MSG(QTYPE__MAX != 7,
> "The QTYPE_CAST_TO_* list needs to be extended");
>
> -#define qobject_to(type, obj) ({ \
> - QObject *_tmp = qobject_check_type(obj, glue(QTYPE_CAST_TO_, type)); \
> - _tmp ? container_of(_tmp, type, base) : (type *)NULL; })
> +#define qobject_to(type, obj) \
> + ((type *)qobject_check_type(obj, glue(QTYPE_CAST_TO_, type)))
>
> /* Initialize an object to default values */
> static inline void qobject_init(QObject *obj, QType type)
> diff --git a/qobject/qobject.c b/qobject/qobject.c
> index 23600aa1c1..87649c5be5 100644
> --- a/qobject/qobject.c
> +++ b/qobject/qobject.c
> @@ -16,6 +16,15 @@
> #include "qapi/qmp/qlist.h"
> #include "qapi/qmp/qstring.h"
>
> +QEMU_BUILD_BUG_MSG(
> + offsetof(QNull, base) != 0 ||
> + offsetof(QNum, base) != 0 ||
> + offsetof(QString, base) != 0 ||
> + offsetof(QDict, base) != 0 ||
> + offsetof(QList, base) != 0 ||
> + offsetof(QBool, base) != 0,
> + "base qobject must be at offset 0");
> +
> static void (*qdestroy[QTYPE__MAX])(QObject *) = {
> [QTYPE_NONE] = NULL, /* No such object exists */
> [QTYPE_QNULL] = NULL, /* qnull_ is indestructible */
As mentioned elsewhere in this thread, the coding errors this assertion
can catch are vanishingly unlikely. I could flip a coin to decide
whether to keep it. Instead I'm asking you :)
With the commit message rephrased not to give anyone ideas about type
casts, and with or without the assertion:
Reviewed-by: Markus Armbruster <address@hidden>
Re: [Qemu-devel] [PATCH v6 1/5] qobject: ensure base is at offset 0,
Markus Armbruster <=
[Qemu-devel] [PATCH v6 2/5] qobject: use a QObjectBase_ struct, Marc-André Lureau, 2018/04/19
[Qemu-devel] [PATCH v6 5/5] qobject: modify qobject_ref() to assert on NULL, Marc-André Lureau, 2018/04/19
[Qemu-devel] [PATCH v6 4/5] qobject: modify qobject_ref() to return obj, Marc-André Lureau, 2018/04/19