linux-kernel - Re: [PATCH v3][next] drm/nouveau: fifo: Avoid -Wflex-array-member-not-at-end warning

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Message-ID: <fksv7eprcqnb55n5zllfyhk7ynx6zgbeuqtuoimbpgamguyodh@niwjdhicah2j>
Date: Mon, 15 Sep 2025 16:06:59 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Lyude Paul <lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, dri-devel@...ts.freedesktop.org, 
	nouveau@...ts.freedesktop.org, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3][next] drm/nouveau: fifo: Avoid
 -Wflex-array-member-not-at-end warning

Hi,

On Thu, Aug 14, 2025 at 03:01:07PM +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.
> 
> Use the new TRAILING_OVERLAP() helper to fix the following warning:
> 
> drivers/gpu/drm/nouveau/nvif/fifo.c:29:42: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> This helper creates a union between a flexible-array member (FAM)
> and a set of members that would otherwise follow it. This overlays
> the trailing members onto the FAM while preserving the original
> memory layout.
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

I took a look at the modified structure layout with the union from the
macro using pahole and found the layouts and sizes to be equivalent --
all the while fixing the warning you demonstrated.

Reviewed-by: Justin Stitt <justinstitt@...gle.com>

> ---
> Changes in v3:
>  - Use the new TRAILING_OVERLAP() helper.

There's really starting to be a lot of these helper macros!

> 
> Changes in v2:
>  - Adjust heap allocation.
> 
>  drivers/gpu/drm/nouveau/nvif/fifo.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/nouveau/nvif/fifo.c b/drivers/gpu/drm/nouveau/nvif/fifo.c
> index a463289962b2..b0ab80995d98 100644
> --- a/drivers/gpu/drm/nouveau/nvif/fifo.c
> +++ b/drivers/gpu/drm/nouveau/nvif/fifo.c
> @@ -25,13 +25,12 @@ static int
>  nvif_fifo_runlists(struct nvif_device *device)
>  {
>  	struct nvif_object *object = &device->object;
> -	struct {
> -		struct nv_device_info_v1 m;
> +	TRAILING_OVERLAP(struct nv_device_info_v1, m, data,
>  		struct {
>  			struct nv_device_info_v1_data runlists;
>  			struct nv_device_info_v1_data runlist[64];
>  		} v;
> -	} *a;
> +	) *a;
>  	int ret, i;
>  
>  	if (device->runlist)
> -- 
> 2.43.0
> 
>

Thanks
Justin