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From:   Alex Deucher <alexdeucher@gmail.com>
Date:   Mon, 10 Oct 2022 16:16:06 -0400
Message-ID: <CADnq5_N5wLSnACnpuqVCMm7LGc20cdGdRhtnwRjUBq7vbrZn8A@mail.gmail.com>
Subject: Re: [PATCH] drm/amd/display: fix array-bounds error in
 dc_stream_remove_writeback() [take 2]
To:     Guenter Roeck <linux@roeck-us.net>
Cc:     amd-gfx@lists.freedesktop.org, regressions@lists.linux.dev,
        Leo Li <sunpeng.li@amd.com>, Xinhui.Pan@amd.com,
        Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        Aurabindo Pillai <aurabindo.pillai@amd.com>,
        Hamza Mahfooz <hamza.mahfooz@amd.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>
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Precedence: bulk

Applied.  Thanks.

Alex

On Mon, Oct 10, 2022 at 2:05 AM Guenter Roeck <linux@roeck-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@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
>