linux-kernel - Re: [PATCH][next] drm: Replace zero-length array with flexible-array member

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Date:   Mon, 9 Mar 2020 08:49:45 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Lucas Stach <l.stach@...gutronix.de>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Patrik Jakobsson <patrik.r.jakobsson@...il.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        Dave Airlie <airlied@...hat.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Eric Anholt <eric@...olt.net>,
        VMware Graphics <linux-graphics-maintainer@...are.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>
Cc:     etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        linux-arm-msm@...r.kernel.org, freedreno@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org,
        spice-devel@...ts.freedesktop.org
Subject: Re: [PATCH][next] drm: Replace zero-length array with flexible-array
 member

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@...eddedor.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@...eddedor.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 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer



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