On Mon, 22 Mar 2021, Arnd Bergmann arnd@kernel.org 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.
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@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