Linux Virtualization - Mar 2020 - [PATCH][next] drm: Replace zero-length array with flexible-array member

On 2/25/20 08:17, Jani Nikula wrote:
> On Tue, 25 Feb 2020, "Gustavo A. R. Silva" <gustavo at
embeddedor.com> wrote:
>> The current codebase makes use of the zero-length array language
>> extension to the C90 standard, but the preferred mechanism to declare
>> variable-length types such as these ones is a flexible array
member[1][2],
>> introduced in C99:
>>
>> struct foo {
>>         int stuff;
>>         struct boo array[];
>> };
>>
>> By making use of the mechanism above, we will get a compiler warning
>> in case the flexible array does not occur last in the structure, which
>> will help us prevent some kind of undefined behavior bugs from being
>> inadvertently introduced[3] to the codebase from now on.
>>
>> Also, notice that, dynamic memory allocations won't be affected by
>> this change:
>>
>> "Flexible array members have incomplete type, and so the sizeof
operator
>> may not be applied. As a quirk of the original implementation of
>> zero-length arrays, sizeof evaluates to zero."[1]
>>
>> This issue was found with the help of Coccinelle.
>>
>> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
>> [2] https://github.com/KSPP/linux/issues/21
>> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined
behaviour")
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavo at embeddedor.com>
>> ---
>>  drivers/gpu/drm/etnaviv/etnaviv_gem.h         | 2 +-
>>  drivers/gpu/drm/gma500/intel_bios.h           | 2 +-
>>  drivers/gpu/drm/i915/display/intel_vbt_defs.h | 4 ++--
>>  drivers/gpu/drm/i915/gt/intel_lrc.c           | 2 +-
>>  drivers/gpu/drm/i915/i915_gpu_error.h         | 2 +-
> 
> Please split out the i915 changes to a separate patch.
> 
Sure thing. I can do that.
>>  drivers/gpu/drm/msm/msm_gem.h                 | 2 +-
>>  drivers/gpu/drm/qxl/qxl_cmd.c                 | 2 +-
>>  drivers/gpu/drm/vboxvideo/vboxvideo.h         | 2 +-
>>  drivers/gpu/drm/vc4/vc4_drv.h                 | 2 +-
>>  drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c    | 2 +-
>>  drivers/gpu/drm/vmwgfx/vmwgfx_surface.c       | 2 +-
>>  include/drm/bridge/mhl.h                      | 4 ++--
>>  include/drm/drm_displayid.h                   | 2 +-
>>  include/uapi/drm/i915_drm.h                   | 4 ++--
> 
> Not sure it's worth touching uapi headers. They're full of both [0]
and
> []. Again, please at least split it to a separate patch to be decided
> separately.
> 
Yeah, it's worth it; the purpose of these patches is to replace [0] with []
across
the whole tree.

Thanks
--
Gustavo

Hi Gustavo

Am 03.03.20 um 19:20 schrieb Gustavo A. R. Silva:
> 
> 
> On 2/25/20 08:17, Jani Nikula wrote:
>> On Tue, 25 Feb 2020, "Gustavo A. R. Silva" <gustavo at
embeddedor.com> wrote:
>>> The current codebase makes use of the zero-length array language
>>> extension to the C90 standard, but the preferred mechanism to
declare
>>> variable-length types such as these ones is a flexible array
member[1][2],
>>> introduced in C99:
>>>
>>> struct foo {
>>>         int stuff;
>>>         struct boo array[];
>>> };
>>>
>>> By making use of the mechanism above, we will get a compiler
warning
>>> in case the flexible array does not occur last in the structure,
which
>>> will help us prevent some kind of undefined behavior bugs from
being
>>> inadvertently introduced[3] to the codebase from now on.
>>>
>>> Also, notice that, dynamic memory allocations won't be affected
by
>>> this change:
>>>
>>> "Flexible array members have incomplete type, and so the
sizeof operator
>>> may not be applied. As a quirk of the original implementation of
>>> zero-length arrays, sizeof evaluates to zero."[1]
>>>
>>> This issue was found with the help of Coccinelle.
>>>
>>> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
>>> [2] https://github.com/KSPP/linux/issues/21
>>> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined
behaviour")
>>>
>>> Signed-off-by: Gustavo A. R. Silva <gustavo at
embeddedor.com>
>>> ---
>>>  drivers/gpu/drm/etnaviv/etnaviv_gem.h         | 2 +-
>>>  drivers/gpu/drm/gma500/intel_bios.h           | 2 +-
>>>  drivers/gpu/drm/i915/display/intel_vbt_defs.h | 4 ++--
>>>  drivers/gpu/drm/i915/gt/intel_lrc.c           | 2 +-
>>>  drivers/gpu/drm/i915/i915_gpu_error.h         | 2 +-
>>
>> Please split out the i915 changes to a separate patch.
>>
> 
> Sure thing. I can do that.
I think each driver deserves it's own patch. Makes backporting easier.

Best regards
Thomas
> 
>>>  drivers/gpu/drm/msm/msm_gem.h                 | 2 +-
>>>  drivers/gpu/drm/qxl/qxl_cmd.c                 | 2 +-
>>>  drivers/gpu/drm/vboxvideo/vboxvideo.h         | 2 +-
>>>  drivers/gpu/drm/vc4/vc4_drv.h                 | 2 +-
>>>  drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c    | 2 +-
>>>  drivers/gpu/drm/vmwgfx/vmwgfx_surface.c       | 2 +-
>>>  include/drm/bridge/mhl.h                      | 4 ++--
>>>  include/drm/drm_displayid.h                   | 2 +-
>>>  include/uapi/drm/i915_drm.h                   | 4 ++--
>>
>> Not sure it's worth touching uapi headers. They're full of both
[0] and
>> []. Again, please at least split it to a separate patch to be decided
>> separately.
>>
> 
> Yeah, it's worth it; the purpose of these patches is to replace [0]
with [] across
> the whole tree.
> 
> Thanks
> --
> Gustavo
> 
-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 N?rnberg, Germany
(HRB 36809, AG N?rnberg)
Gesch?ftsf?hrer: Felix Imend?rffer

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