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Subject: Re: [PATCH][next] drm: Replace zero-length array with flexible-array
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To:     "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        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>,
        Maxime Ripard <mripard@kernel.org>
Cc:     etnaviv@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org, intel-gfx@lists.freedesktop.org,
        linux-arm-msm@vger.kernel.org, freedreno@lists.freedesktop.org,
        virtualization@lists.linux-foundation.org,
        spice-devel@lists.freedesktop.org
References: <20200225140347.GA22864@embeddedor> <87a756sqdc.fsf@intel.com>
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From:   Thomas Zimmermann <tzimmermann@suse.de>
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Date:   Mon, 9 Mar 2020 08:49:45 +0100
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From: Thomas Zimmermann <tzimmermann@suse.de>
To: "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
 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>,
 Maxime Ripard <mripard@kernel.org>
Cc: etnaviv@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
 linux-kernel@vger.kernel.org, intel-gfx@lists.freedesktop.org,
 linux-arm-msm@vger.kernel.org, freedreno@lists.freedesktop.org,
 virtualization@lists.linux-foundation.org, spice-devel@lists.freedesktop.org
Message-ID: <67c46a2b-8127-a4b7-c825-8cba6f28db8c@suse.de>
Subject: Re: [PATCH][next] drm: Replace zero-length array with flexible-array
 member
References: <20200225140347.GA22864@embeddedor> <87a756sqdc.fsf@intel.com>
 <138ff691-94b3-1ce5-e7fa-e6d7c436bf8e@embeddedor.com>
In-Reply-To: <138ff691-94b3-1ce5-e7fa-e6d7c436bf8e@embeddedor.com>

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Hi Gustavo

Am 03.03.20 um 19:20 schrieb Gustavo A. R. Silva:
>=20
>=20
> On 2/25/20 08:17, Jani Nikula wrote:
>> On Tue, 25 Feb 2020, "Gustavo A. R. Silva" <gustavo@embeddedor.com> wr=
ote:
>>> 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, whic=
h
>>> 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 opera=
tor
>>> 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.
>>
>=20
> Sure thing. I can do that.

I think each driver deserves it's own patch. Makes backporting easier.

Best regards
Thomas

>=20
>>>  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] an=
d
>> []. Again, please at least split it to a separate patch to be decided
>> separately.
>>
>=20
> Yeah, it's worth it; the purpose of these patches is to replace [0] wit=
h [] across
> the whole tree.
>=20
> Thanks
> --
> Gustavo
>=20

--=20
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 N=C3=BCrnberg, Germany
(HRB 36809, AG N=C3=BCrnberg)
Gesch=C3=A4ftsf=C3=BChrer: Felix Imend=C3=B6rffer


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