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R. Silva" Cc: Eric Anholt , David Airlie , Daniel Vetter , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH][next] drm/vc4/vc4_drv.h: Replace zero-length array with flexible-array member Message-ID: <20200306105025.GW2363188@phenom.ffwll.local> Mail-Followup-To: "Gustavo A. R. Silva" , Eric Anholt , David Airlie , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org References: <20200305105707.GA19261@embeddedor> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200305105707.GA19261@embeddedor> X-Operating-System: Linux phenom 5.3.0-3-amd64 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 05, 2020 at 04:57:07AM -0600, Gustavo A. R. Silva 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 Applied to drm-misc-next, thanks for your patch. -Daniel > --- > drivers/gpu/drm/vc4/vc4_drv.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/gpu/drm/vc4/vc4_drv.h b/drivers/gpu/drm/vc4/vc4_drv.h > index f90c0d08e740..5ecb8b4a48a1 100644 > --- a/drivers/gpu/drm/vc4/vc4_drv.h > +++ b/drivers/gpu/drm/vc4/vc4_drv.h > @@ -65,7 +65,7 @@ struct vc4_perfmon { > * Note that counter values can't be reset, but you can fake a reset by > * destroying the perfmon and creating a new one. > */ > - u64 counters[0]; > + u64 counters[]; > }; > > struct vc4_dev { > -- > 2.25.0 > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch