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 43083C1975A for ; Thu, 12 Mar 2020 14:55:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1E52E20663 for ; Thu, 12 Mar 2020 14:55:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727554AbgCLOzM (ORCPT ); Thu, 12 Mar 2020 10:55:12 -0400 Received: from outgoing-auth-1.mit.edu ([18.9.28.11]:49924 "EHLO outgoing.mit.edu" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727340AbgCLOzM (ORCPT ); Thu, 12 Mar 2020 10:55:12 -0400 Received: from callcc.thunk.org (pool-72-93-95-157.bstnma.fios.verizon.net [72.93.95.157]) (authenticated bits=0) (User authenticated as tytso@ATHENA.MIT.EDU) by outgoing.mit.edu (8.14.7/8.12.4) with ESMTP id 02CEsum4018507 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 12 Mar 2020 10:54:57 -0400 Received: by callcc.thunk.org (Postfix, from userid 15806) id 2D5B1420E5E; Thu, 12 Mar 2020 10:54:56 -0400 (EDT) Date: Thu, 12 Mar 2020 10:54:56 -0400 From: "Theodore Y. Ts'o" To: "Gustavo A. R. Silva" Cc: Andreas Dilger , linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH][next] ext4: dir: Replace zero-length array with flexible-array member Message-ID: <20200312145456.GH7159@mit.edu> References: <20200309154838.GA31559@embeddedor> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200309154838.GA31559@embeddedor> Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Mon, Mar 09, 2020 at 10:48:38AM -0500, 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, thanks. - Ted