On Thu, 09 Dec 2021, Kees Cook keescook@chromium.org wrote:
On Thu, Dec 09, 2021 at 05:20:45PM -0500, Harry Wentland wrote:
On 2021-12-09 01:23, Kees Cook wrote:
On Wed, Dec 08, 2021 at 01:19:28PM +0200, Jani Nikula wrote:
On Fri, 03 Dec 2021, Kees Cook keescook@chromium.org wrote:
The link_status array was not large enough to read the Adjust Request Post Cursor2 register. Adjust the size to include it. Found with a -Warray-bounds build:
drivers/gpu/drm/drm_dp_helper.c: In function 'drm_dp_get_adjust_request_post_cursor': drivers/gpu/drm/drm_dp_helper.c:59:27: error: array subscript 10 is outside array bounds of 'const u8[6]' {aka 'const unsigned char[6]'} [-Werror=array-bounds] 59 | return link_status[r - DP_LANE0_1_STATUS]; | ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~ drivers/gpu/drm/drm_dp_helper.c:147:51: note: while referencing 'link_status' 147 | u8 drm_dp_get_adjust_request_post_cursor(const u8 link_status[DP_LINK_STATUS_SIZE], | ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Fixes: 79465e0ffeb9 ("drm/dp: Add helper to get post-cursor adjustments") Signed-off-by: Kees Cook keescook@chromium.org
Using DP_ADJUST_REQUEST_POST_CURSOR2 has been deprecated since DP 1.3 published in 2014, and Tegra is the only user of drm_dp_get_adjust_request_post_cursor().
I see POST_CURSOR2 is used here too:
drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
Looks like we read and parse that in the admgpu driver without using drm_dp_get_adjust_request_post_cursor.
Right, and probably that could be switched to use it, but I'm not sure what the impact of the larger link_status read is.
I don't have a strong feeling but I liked your original patch a bit better. I'm not sure what it means when part of a spec is deprecated. Once a spec is written display vendors might implement it. We should make sure that displays like that are always handled in a sane manner.
Jani, Dave, any guidance here? I'm fine with whatever, but the current code is for sure broken. ;)
Post Cursor2 was completely optional for the transmitter even before it was deprecated.
And now we'd be adding 5 bytes extra to all link status reads. To fix the only user of drm_dp_get_adjust_request_post_cursor() that apparently has never worked as intended. I'm just not convinced.
I was trying to look through the implications of DP_LINK_STATUS_SIZE increase, and at least drm_dp_dpcd_read_phy_link_status() comes across as something probably needing attention.
BR, Jani.