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Subject: Re: [PATCH][next] drm: Replace zero-length array with flexible-array
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To:     Jani Nikula <jani.nikula@linux.intel.com>,
        Lucas Stach <l.stach@pengutronix.de>,
        Russell King <linux+etnaviv@armlinux.org.uk>,
        Christian Gmeiner <christian.gmeiner@gmail.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Patrik Jakobsson <patrik.r.jakobsson@gmail.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Rob Clark <robdclark@gmail.com>, Sean Paul <sean@poorly.run>,
        Dave Airlie <airlied@redhat.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Hans de Goede <hdegoede@redhat.com>,
        Eric Anholt <eric@anholt.net>,
        VMware Graphics <linux-graphics-maintainer@vmware.com>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
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References: <20200225140347.GA22864@embeddedor> <87a756sqdc.fsf@intel.com>
From:   "Gustavo A. R. Silva" <gustavo@embeddedor.com>
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Message-ID: <138ff691-94b3-1ce5-e7fa-e6d7c436bf8e@embeddedor.com>
Date:   Tue, 3 Mar 2020 12:20:16 -0600
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On 2/25/20 08:17, Jani Nikula wrote:
> On Tue, 25 Feb 2020, "Gustavo A. R. Silva" <gustavo@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@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