[PATCH] gpu/drm/drm_framebuffer.c: Use Macro instead of actual number.

[彭昊]

Good idea!
> From: "Thomas Zimmermann"<tzimmermann at suse.de>
> Date:  Thu, Nov 2, 2023, 21:27
> Subject:  Re: [PATCH] gpu/drm/drm_framebuffer.c: Use Macro instead of actual number.
> To: "Peng Hao"<penghao at dingdao.com>, <maarten.lankhorst at linux.intel.com>, <mripard at kernel.org>, <airlied at gmail.com>, <daniel at ffwll.ch>
> Cc: <linux-kernel at vger.kernel.org>, <dri-devel at lists.freedesktop.org>
> Hi
> 
> Am 02.11.23 um 03:29 schrieb Peng Hao:
> > Use Macro DRM_FORMAT_MAX_PLANES instead of 4, to improve modifiability.
> > 
> > Signed-off-by: Peng Hao <penghao at dingdao.com>
> > ---
> >   drivers/gpu/drm/drm_framebuffer.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
> > index 2dd97473ca10..bf283dae9090 100644
> > --- a/drivers/gpu/drm/drm_framebuffer.c
> > +++ b/drivers/gpu/drm/drm_framebuffer.c
> > @@ -254,7 +254,7 @@ static int framebuffer_check(struct drm_device *dev,
> >   		}
> >   	}
> >   
> > -	for (i = info->num_planes; i < 4; i++) {
> > +	for (i = info->num_planes; i < DRM_FORMAT_MAX_PLANES; i++) {
> 
> This change makes the code more fragile. '4' is a fixed constant in the 
> UAPI struct, while DRM_FORMAT_MAX_PLANES is an internal constant. I 
> agree that both should reasonably have the same value. But (potentially) 
> changing the value of DRM_FORMAT_MAX_PLANES will break these loops with 
> a possible OOB access.
> 
> To make make this code more robust, it might be better to rewrite the 
> tests like this
> 
> for (i = num_planes; i < ARRAY_SIZE(r->modifier); +i) {
> 	// the test for modifier[i]
> }
> 
> if (r->flags & DRM_MODE_FB_MODIFIERS) {
>   	for (i < ARRAY_SIZE(handles)) {
> 		// test for handles[i]
> 	}
>   	for (i < ARRAY_SIZE(pitches)) {
> 		// test for pitches[i]
> 	}
>   	for (i < ARRAY_SIZE(offsets)) {
> 		// test for offsets[i]
> 	}
> }
> 
> Best regards
> Thomas
> 
> >   		if (r->modifier[i]) {
> >   			drm_dbg_kms(dev, "non-zero modifier for unused plane %d\n", i);
> >   			return -EINVAL;
> 
> -- 
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Frankenstrasse 146, 90461 Nuernberg, Germany
> GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
> HRB 36809 (AG Nuernberg)
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