On Mon, 22 Mar 2021, Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> 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.
Ugh. drm_dp_channel_eq_ok() does not actually require more than
DP_LINK_STATUS_SIZE - 2 elements in the link_status. It's some other
related functions that do, and in most cases it's convenient to read all
those DP_LINK_STATUS_SIZE bytes.
However, here the case is slightly different for DP MST, and the change
causes reserved DPCD addresses to be read. Not sure it matters, but
really I think the problem is what drm_dp_channel_eq_ok() advertizes.
I also don't like the array notation with sizes in function parameters
in general, because I think it's misleading. Would gcc-11 warn if a
function actually accesses the memory out of bounds of the size?
Anyway. I don't think we're going to get rid of the array notation
anytime soon, if ever, no matter how much I dislike it, so I think the
right fix would be to at least state the correct required size in
drm_dp_channel_eq_ok().
BR,
Jani.
>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> 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
--
Jani Nikula, Intel Open Source Graphics Center
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