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Subject: Re: [PATCH 11/11] [RFC] drm/i915/dp: fix array overflow warning
To:     Arnd Bergmann <arnd@kernel.org>, linux-kernel@vger.kernel.org,
        Martin Sebor <msebor@gcc.gnu.org>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>, imre.deak@intel.com
Cc:     Arnd Bergmann <arnd@arndb.de>, x86@kernel.org,
        Ning Sun <ning.sun@intel.com>,
        Kalle Valo <kvalo@codeaurora.org>,
        Simon Kelley <simon@thekelleys.org.uk>,
        James Smart <james.smart@broadcom.com>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        Anders Larsen <al@alarsen.net>, Tejun Heo <tj@kernel.org>,
        Serge Hallyn <serge@hallyn.com>,
        Imre Deak <imre.deak@intel.com>,
        linux-arm-kernel@lists.infradead.org,
        tboot-devel@lists.sourceforge.net, intel-gfx@lists.freedesktop.org,
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        =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= <ville.syrjala@linux.intel.com>,
        Manasi Navare <manasi.d.navare@intel.com>,
        Uma Shankar <uma.shankar@intel.com>,
        Ankit Nautiyal <ankit.k.nautiyal@intel.com>,
        Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>,
        Animesh Manna <animesh.manna@intel.com>,
        Sean Paul <seanpaul@chromium.org>
References: <20210322160253.4032422-1-arnd@kernel.org>
 <20210322160253.4032422-12-arnd@kernel.org>
From:   Hans de Goede <hdegoede@redhat.com>
Message-ID: <949db606-ac48-69ae-b0f7-b1cba6fc2d7f@redhat.com>
Date:   Tue, 30 Mar 2021 12:56:51 +0200
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Hi,

On 3/22/21 5:02 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> gcc-11 warns that intel_dp_check_mst_status() has a local array of
> fourteen bytes and passes the last four bytes into a function that
> expects a six-byte array:
> 
> drivers/gpu/drm/i915/display/intel_dp.c: In function ‘intel_dp_check_mst_status’:
> drivers/gpu/drm/i915/display/intel_dp.c:4556:22: error: ‘drm_dp_channel_eq_ok’ reading 6 bytes from a region of size 4 [-Werror=stringop-overread]
>  4556 |                     !drm_dp_channel_eq_ok(&esi[10], intel_dp->lane_count)) {
>       |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/gpu/drm/i915/display/intel_dp.c:4556:22: note: referencing argument 1 of type ‘const u8 *’ {aka ‘const unsigned char *’}
> In file included from drivers/gpu/drm/i915/display/intel_dp.c:38:
> include/drm/drm_dp_helper.h:1459:6: note: in a call to function ‘drm_dp_channel_eq_ok’
>  1459 | bool drm_dp_channel_eq_ok(const u8 link_status[DP_LINK_STATUS_SIZE],
>       |      ^~~~~~~~~~~~~~~~~~~~
> 
> Clearly something is wrong here, but I can't quite figure out what.
> Changing the array size to 16 bytes avoids the warning, but is
> probably the wrong solution here.

The drm displayport-helpers indeed expect a 6 bytes buffer, but they
usually only consume 4 bytes.

I don't think that changing the DP_DPRX_ESI_LEN is a good fix here,
since it is used in multiple places, but the esi array already gets
zero-ed out by its initializer, so we can just pass 2 extra 0 bytes
to give drm_dp_channel_eq_ok() call the 6 byte buffer its prototype
specifies by doing this:

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 897711d9d7d3..147962d4ad06 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -4538,7 +4538,11 @@ intel_dp_check_mst_status(struct intel_dp *intel_dp)
 	drm_WARN_ON_ONCE(&i915->drm, intel_dp->active_mst_links < 0);
 
 	for (;;) {
-		u8 esi[DP_DPRX_ESI_LEN] = {};
+		/*
+		 * drm_dp_channel_eq_ok() expects a 6 byte large buffer, but
+		 * the ESI info only contains 4 bytes, pass 2 extra 0 bytes.
+		 */
+		u8 esi[DP_DPRX_ESI_LEN + 2] = {};
 		bool handled;
 		int retry;
 

So i915 devs, would such a fix be acceptable ?

Regards,

Hans


> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 8c12d5375607..830e2515f119 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -65,7 +65,7 @@
>  #include "intel_vdsc.h"
>  #include "intel_vrr.h"
>  
> -#define DP_DPRX_ESI_LEN 14
> +#define DP_DPRX_ESI_LEN 16
>  
>  /* DP DSC throughput values used for slice count calculations KPixels/s */
>  #define DP_DSC_PEAK_PIXEL_RATE			2720000
>