[PATCH] drm/dp: Fix off-by-one in register cache size

[Gustavo A. R. Silva]

On Fri, Dec 03, 2021 at 12:43:33AM -0800, Kees Cook wrote:
> The pcon_dsc_dpcd array holds 13 registers (0x92 through 0x9E). Fix the
> math to calculate the max size. Found from a -Warray-bounds build:
> 
> drivers/gpu/drm/drm_dp_helper.c: In function 'drm_dp_pcon_dsc_bpp_incr':
> drivers/gpu/drm/drm_dp_helper.c:3130:28: error: array subscript 12 is outside array bounds of 'const u8[12]' {aka 'const unsigned char[12]'} [-Werror=array-bounds]
>  3130 |         buf = pcon_dsc_dpcd[DP_PCON_DSC_BPP_INCR - DP_PCON_DSC_ENCODER];
>       |               ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/gpu/drm/drm_dp_helper.c:3126:39: note: while referencing 'pcon_dsc_dpcd'
>  3126 | int drm_dp_pcon_dsc_bpp_incr(const u8 pcon_dsc_dpcd[DP_PCON_DSC_ENCODER_CAP_SIZE])
>       |                              ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Fixes: e2e16da398d9 ("drm/dp_helper: Add support for Configuring DSC for HDMI2.1 Pcon")

This should be tagged for -stable:

Cc: stable at vger.kernel.org

> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: Maxime Ripard <mripard at kernel.org>
> Cc: Thomas Zimmermann <tzimmermann at suse.de>
> Cc: David Airlie <airlied at linux.ie>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: dri-devel at lists.freedesktop.org
> Signed-off-by: Kees Cook <keescook at chromium.org>

Reviewed-by: Gustavo A. R. Silva <gustavoars at kernel.org>

Thanks
--
Gustavo

> ---
>  include/drm/drm_dp_helper.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
> index 30359e434c3f..472dac376284 100644
> --- a/include/drm/drm_dp_helper.h
> +++ b/include/drm/drm_dp_helper.h
> @@ -456,7 +456,7 @@ struct drm_panel;
>  #define DP_FEC_CAPABILITY_1			0x091   /* 2.0 */
>  
>  /* DP-HDMI2.1 PCON DSC ENCODER SUPPORT */
> -#define DP_PCON_DSC_ENCODER_CAP_SIZE        0xC	/* 0x9E - 0x92 */
> +#define DP_PCON_DSC_ENCODER_CAP_SIZE        0xD	/* 0x92 through 0x9E */
>  #define DP_PCON_DSC_ENCODER                 0x092
>  # define DP_PCON_DSC_ENCODER_SUPPORTED      (1 << 0)
>  # define DP_PCON_DSC_PPS_ENC_OVERRIDE       (1 << 1)
> -- 
> 2.30.2
>