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Sender: Guenter Roeck <groeck7@gmail.com>
From:   Guenter Roeck <linux@roeck-us.net>
To:     amd-gfx@lists.freedesktop.org
Cc:     regressions@lists.linux.dev,
        Harry Wentland <harry.wentland@amd.com>,
        Leo Li <sunpeng.li@amd.com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>,
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        Daniel Vetter <daniel@ffwll.ch>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        Guenter Roeck <linux@roeck-us.net>,
        Aurabindo Pillai <aurabindo.pillai@amd.com>,
        Hamza Mahfooz <hamza.mahfooz@amd.com>
Subject: [PATCH] drm/amd/display: fix array-bounds error in dc_stream_remove_writeback() [take 2]
Date:   Sun,  9 Oct 2022 23:05:12 -0700
Message-Id: <20221010060512.3064524-1-linux@roeck-us.net>
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Commit 5d8c3e836fc2 ("drm/amd/display: fix array-bounds error in
dc_stream_remove_writeback()") tried to fix an array bounds error seen
with gcc 12.0. Unfortunately, that results in another array bounds error,
seen with older versions of gcc.

Building csky:allmodconfig ... failed
--------------
Error log:
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c:
	In function 'dc_stream_remove_writeback':
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c:527:83:
	error: array subscript 1 is above array bounds of 'struct dc_writeback_info[1]' [-Werror=array-bounds]
  527 |                                 stream->writeback_info[j] = stream->writeback_info[i];
      |                                                             ~~~~~~~~~~~~~~~~~~~~~~^~~
In file included from drivers/gpu/drm/amd/amdgpu/../display/dc/dc.h:1269,
                 from drivers/gpu/drm/amd/amdgpu/../display/dc/inc/core_types.h:29,
                 from drivers/gpu/drm/amd/amdgpu/../display/dc/basics/dc_common.h:29,
                 from drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c:27:
drivers/gpu/drm/amd/amdgpu/../display/dc/dc_stream.h:241:34: note: while referencing 'writeback_info'
  241 |         struct dc_writeback_info writeback_info[MAX_DWB_PIPES];

We could check both i and j for overflow to fix the problem. That would,
however, be not make much sense since it is known and provable that j <= i.
Also, the check introduced with commit 5d8c3e836fc2 does not really add
value since it checks if j < MAX_DWB_PIPES. Since it is known that j <= i,
it would make more sense to check if i < MAX_DWB_PIPES. Unfortunately, that
does not help to solve the problem observed here: gcc still complains.

To solve the problem, replace the subsequent check for 'i != j' with
'j < i'. This is identical to the original check since we know that j <= i,
and it makes all versions of gcc happy. Drop the check introduced with
commit 5d8c3e836fc2 since it is not really useful and does not solve the
problem.

Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Fixes: 5d8c3e836fc2 ("drm/amd/display: fix array-bounds error in dc_stream_remove_writeback()")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
 drivers/gpu/drm/amd/display/dc/core/dc_stream.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/dc/core/dc_stream.c b/drivers/gpu/drm/amd/display/dc/core/dc_stream.c
index ae13887756bf..9f568d618cd0 100644
--- a/drivers/gpu/drm/amd/display/dc/core/dc_stream.c
+++ b/drivers/gpu/drm/amd/display/dc/core/dc_stream.c
@@ -520,9 +520,9 @@ bool dc_stream_remove_writeback(struct dc *dc,
 	}
 
 	/* remove writeback info for disabled writeback pipes from stream */
-	for (i = 0, j = 0; i < stream->num_wb_info && j < MAX_DWB_PIPES; i++) {
+	for (i = 0, j = 0; i < stream->num_wb_info; i++) {
 		if (stream->writeback_info[i].wb_enabled) {
-			if (i != j)
+			if (j < i)
 				/* trim the array */
 				stream->writeback_info[j] = stream->writeback_info[i];
 			j++;
-- 
2.36.2