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

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Message-ID: <3d50e0a2-77fd-4c87-bf74-8ab8b6b15ce2@embeddedor.com>
Date: Tue, 11 Nov 2025 17:49:20 +0900
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Nicolin Chen <nicolinc@...dia.com>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Kevin Tian <kevin.tian@...el.com>,
 Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] iommufd/iommufd_private.h: Avoid
 -Wflex-array-member-not-at-end warning



On 11/11/25 16:20, Gustavo A. R. Silva wrote:
> 
> 
> 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

Okay, I didn't find evidence of data corruption. So, this patch can be applied to
a -next tree.

> 
> 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 will submit the above as a separate patch.

Thanks
-Gustavo