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