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

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Message-ID: <Z-a6wtoIAkDY2ERx@pollux>
Date: Fri, 28 Mar 2025 16:05:38 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Lyude Paul <lyude@...hat.com>, 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][next] drm/nouveau/outp: Avoid
 -Wflex-array-member-not-at-end warning

On Fri, Mar 28, 2025 at 08:45:32AM -0600, 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 `DEFINE_RAW_FLEX()` helper for an on-stack definition of
> a flexible structure where the size of the flexible-array member
> is known at compile-time, and refactor the rest of the code,
> accordingly.
> 
> So, with these changes, fix the following warning:
> 
> drivers/gpu/drm/nouveau/nvif/outp.c:199:45: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  drivers/gpu/drm/nouveau/nvif/outp.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/nouveau/nvif/outp.c b/drivers/gpu/drm/nouveau/nvif/outp.c
> index 6daeb7f0b09b..403cf16d5e84 100644
> --- a/drivers/gpu/drm/nouveau/nvif/outp.c
> +++ b/drivers/gpu/drm/nouveau/nvif/outp.c
> @@ -195,20 +195,18 @@ nvif_outp_dp_aux_pwr(struct nvif_outp *outp, bool enable)
>  int
>  nvif_outp_hda_eld(struct nvif_outp *outp, int head, void *data, u32 size)
>  {
> -	struct {
> -		struct nvif_outp_hda_eld_v0 mthd;
> -		u8 data[128];
> -	} args;
> +	DEFINE_RAW_FLEX(struct nvif_outp_hda_eld_v0, mthd, data, 128);
>  	int ret;
>  
> -	if (WARN_ON(size > ARRAY_SIZE(args.data)))
> +	if (WARN_ON(size > 128))

Seems a bit unfortunate that the size is duplicated here.

Can we have an accessor macro to derive the size for us?

	union {
		u8 bytes[struct_size_t(type, member, count)];
		type obj;
	} name##_u initializer;

Maybe a macro that returns the size difference between bytes and obj?

>  		return -EINVAL;
>  
> -	args.mthd.version = 0;
> -	args.mthd.head = head;
> +	mthd->version = 0;
> +	mthd->head = head;
>  
> -	memcpy(args.data, data, size);
> -	ret = nvif_mthd(&outp->object, NVIF_OUTP_V0_HDA_ELD, &args, sizeof(args.mthd) + size);
> +	memcpy(mthd->data, data, size);
> +	ret = nvif_mthd(&outp->object, NVIF_OUTP_V0_HDA_ELD, mthd,
> +			__struct_size(mthd) + size);
>  	NVIF_ERRON(ret, &outp->object, "[HDA_ELD head:%d size:%d]", head, size);
>  	return ret;
>  }
> -- 
> 2.43.0
>