From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B5239C3F2D2 for ; Fri, 28 Feb 2020 13:52:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 92B3F24677 for ; Fri, 28 Feb 2020 13:52:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727112AbgB1NwI (ORCPT ); Fri, 28 Feb 2020 08:52:08 -0500 Received: from coyote.holtmann.net ([212.227.132.17]:59831 "EHLO mail.holtmann.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725943AbgB1NwI (ORCPT ); Fri, 28 Feb 2020 08:52:08 -0500 Received: from marcel-macpro.fritz.box (p4FEFC5A7.dip0.t-ipconnect.de [79.239.197.167]) by mail.holtmann.org (Postfix) with ESMTPSA id 9149CCECF9; Fri, 28 Feb 2020 15:01:32 +0100 (CET) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 13.0 \(3608.60.0.2.5\)) Subject: Re: [PATCH][next] 6lowpan: Replace zero-length array with flexible-array member From: Marcel Holtmann In-Reply-To: <20200228134758.GA29760@embeddedor> Date: Fri, 28 Feb 2020 14:52:05 +0100 Cc: Alexander Aring , Jukka Rissanen , "David S. Miller" , Jakub Kicinski , linux-bluetooth@vger.kernel.org, linux-wpan@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Content-Transfer-Encoding: 7bit Message-Id: <170C625F-CF47-4474-970B-EB31FA49E9A4@holtmann.org> References: <20200228134758.GA29760@embeddedor> To: "Gustavo A. R. Silva" X-Mailer: Apple Mail (2.3608.60.0.2.5) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Gustavo, > 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 > --- > include/net/6lowpan.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) patch has been applied to bluetooth-next tree. Regards Marcel