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Date: Mon, 10 Oct 2022 16:16:06 -0400
From: Alex Deucher <alexdeucher@...il.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: amd-gfx@...ts.freedesktop.org, regressions@...ts.linux.dev,
Leo Li <sunpeng.li@....com>, Xinhui.Pan@....com,
Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
Aurabindo Pillai <aurabindo.pillai@....com>,
Hamza Mahfooz <hamza.mahfooz@....com>,
Alex Deucher <alexander.deucher@....com>,
Christian König <christian.koenig@....com>
Subject: Re: [PATCH] drm/amd/display: fix array-bounds error in
dc_stream_remove_writeback() [take 2]
Applied. Thanks.
Alex
On Mon, Oct 10, 2022 at 2:05 AM Guenter Roeck <linux@...ck-us.net> wrote:
>
> 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@....com>
> Cc: Hamza Mahfooz <hamza.mahfooz@....com>
> Fixes: 5d8c3e836fc2 ("drm/amd/display: fix array-bounds error in dc_stream_remove_writeback()")
> Signed-off-by: Guenter Roeck <linux@...ck-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
>
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