Re: [PATCH][next] iommufd/iommufd_private.h: Avoid -Wflex-array-member-not-at-end warning

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 11/11/25 03:02, Nicolin Chen wrote:
> On Mon, Nov 10, 2025 at 08:35:31PM +0900, Gustavo A. R. Silva wrote:
>> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
>> getting ready to enable it, globally.
>>
>> Move the conflicting declaration to the end of the corresponding
>> structure. Notice that struct iommufd_vevent is a flexible
>> structure, this is a structure that contains a flexible-array
>> member.
>>
>> Fix the following warning:
>>
>> drivers/iommu/iommufd/iommufd_private.h:621:31: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> IIUIC, there might be data corruption due to this? If so, I think

Yep. Also, after taking a look at the commit you mention, the counted_by annotation
in struct iommufd_vevent is wrong in commit e8e1ef9b77a7 ("iommufd/viommu: Add
iommufd_viommu_report_event helper"). The counter, in this case vevent->data_len
must always be initialized before the first reference to the flexible array:

diff --git a/drivers/iommu/iommufd/driver.c b/drivers/iommu/iommufd/driver.c
index 6f1010da221c..21d4a35538f6 100644
--- a/drivers/iommu/iommufd/driver.c
+++ b/drivers/iommu/iommufd/driver.c
@@ -161,8 +161,8 @@ int iommufd_viommu_report_event(struct iommufd_viommu *viommu,
                 vevent = &veventq->lost_events_header;
                 goto out_set_header;
         }
-       memcpy(vevent->event_data, event_data, data_len);
         vevent->data_len = data_len;
+       memcpy(vevent->event_data, event_data, data_len);
         veventq->num_events++;

  out_set_header:

I'll turn this into a small patch series to fix the above issue as well.

Thanks
-Gustavo