[PATCH] conf: force 8 byte alignment for virObjectEvent

[Michal Privoznik]

On 6/3/20 12:22 PM, Daniel P. Berrangé wrote:
> We need to be able to cast from virObjectEventPtr to one of
> its many subclasses. Some of these subclasses have 8 byte
> alignment on 32-bit platforms, but virObjectEventPtr only
> has 4 byte alignment.
> 
> Previously the virObject base class had 8 byte alignment
> but this dropped to 4 byte when converted to inherit from
> GObject. This introduces cast alignment warnings on 32-bit:
> 
> ../../src/conf/domain_event.c: In function 'virDomainEventDispatchDefaultFunc':
> ../../src/conf/domain_event.c:1656:30: error: cast increases required alignment of target type [-Werror=cast-align]
>   1656 |             rtcChangeEvent = (virDomainEventRTCChangePtr)event;
>        |                              ^
> ../../src/conf/domain_event.c:1785:34: error: cast increases required alignment of target type [-Werror=cast-align]
>   1785 |             balloonChangeEvent = (virDomainEventBalloonChangePtr)event;
>        |                                  ^
> ../../src/conf/domain_event.c:1896:35: error: cast increases required alignment of target type [-Werror=cast-align]
>   1896 |             blockThresholdEvent = (virDomainEventBlockThresholdPtr)event;
>        |                                   ^
> ../../src/conf/domain_event.c: In function 'virDomainQemuMonitorEventDispatchFunc':
> ../../src/conf/domain_event.c:1974:24: error: cast increases required alignment of target type [-Werror=cast-align]
>   1974 |     qemuMonitorEvent = (virDomainQemuMonitorEventPtr)event;
>        |                        ^
> ../../src/conf/domain_event.c: In function 'virDomainQemuMonitorEventFilter':
> ../../src/conf/domain_event.c:2179:20: error: cast increases required alignment of target type [-Werror=cast-align]
>   2179 |     monitorEvent = (virDomainQemuMonitorEventPtr) event;
>        |                    ^
> 
> Forcing 8-byte alignment on virObjectEventPtr removes the
> alignment increase during casts to subclasses.
> 
> Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
> ---
> 
> Technically a build-breaker, but since we don't have any existing
> usage of __attribute__((aligned)), I wanted to get a second opinion
> on this approach.
> 
> One alternative approach would be to switch one of the current "int"
> fields in virObjectEvent to "long long".
> 
>   src/conf/object_event_private.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/conf/object_event_private.h b/src/conf/object_event_private.h
> index b31441c53a..126464a9a5 100644
> --- a/src/conf/object_event_private.h
> +++ b/src/conf/object_event_private.h
> @@ -42,7 +42,7 @@ typedef void
>                                 virConnectObjectEventGenericCallback cb,
>                                 void *cbopaque);
>   
> -struct _virObjectEvent {
> +struct  __attribute__((aligned(4))) _virObjectEvent {
>       virObject parent;
>       int eventID;
>       virObjectMeta meta;
> 

Reviewed-by: Michal Privoznik <mprivozn at redhat.com>

Michal