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

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Message-ID: <202408221011.82876DA0C4@keescook>
Date: Thu, 22 Aug 2024 10:27:38 -0700
From: Kees Cook <kees@...nel.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Karol Herbst <kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>,
	Danilo Krummrich <dakr@...hat.com>,
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...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: Avoid -Wflex-array-member-not-at-end
 warning

On Wed, Aug 21, 2024 at 02:16:21PM -0600, Gustavo A. R. Silva wrote:
> 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 this, fix the following warning:
> 
> drivers/gpu/drm/nouveau/dispnv50/disp.c:779:47: 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/dispnv50/disp.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/gpu/drm/nouveau/dispnv50/disp.c b/drivers/gpu/drm/nouveau/dispnv50/disp.c
> index eed579a6c858..ddddc69640be 100644
> --- a/drivers/gpu/drm/nouveau/dispnv50/disp.c
> +++ b/drivers/gpu/drm/nouveau/dispnv50/disp.c
> @@ -774,11 +774,9 @@ nv50_hdmi_enable(struct drm_encoder *encoder, struct nouveau_crtc *nv_crtc,
>  	struct drm_hdmi_info *hdmi = &nv_connector->base.display_info.hdmi;
>  	union hdmi_infoframe infoframe = { 0 };
>  	const u8 rekey = 56; /* binary driver, and tegra, constant */
> +	DEFINE_RAW_FLEX(struct nvif_outp_infoframe_v0, args, data, 17);
> +	const u8 data_len = 17; /* same length as in DEFINE_RAW_FLEX above. */

To avoid repeating the open-coded "17", this could either be a define:

nv50_hdmi_enable(...)
{
...
#define data_len	17
	DEFINE_RAW_FLEX(struct nvif_outp_infoframe_v0, args, data, data_len);
...rest of function...
#undef data_len
}

or an ungainly but compile-time calculated value that exposes some
DEFINE_FLEX internals:

	const u8 data_len = (sizeof(args_u) - sizeof(*args)) / sizeof(*args->data);

(Maybe a helper is needed for that?)

#define STACK_FLEX_COUNT(name, member)	\
	((sizeof(name##_u) = sizeof(*(name))) / sizeof(*(name)->member))

> @@ -815,29 +813,29 @@ nv50_hdmi_enable(struct drm_encoder *encoder, struct nouveau_crtc *nv_crtc,
>  		return;
>  
>  	/* AVI InfoFrame. */
> -	args.infoframe.version = 0;
> -	args.infoframe.head = nv_crtc->index;
> +	args->version = 0;
> +	args->head = nv_crtc->index;

The stack variable (was before and is again) already zero-initialized,
so the "= 0" line shouldn't be needed.

But neither of these comments are show-stoppers, IMO.

Reviewed-by: Kees Cook <kees@...nel.org>

-- 
Kees Cook