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=-8.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham 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 0E83DC38A2A for ; Thu, 7 May 2020 19:43:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E4632208CA for ; Thu, 7 May 2020 19:43:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728354AbgEGTn0 (ORCPT ); Thu, 7 May 2020 15:43:26 -0400 Received: from mga03.intel.com ([134.134.136.65]:49363 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726644AbgEGTnZ (ORCPT ); Thu, 7 May 2020 15:43:25 -0400 IronPort-SDR: IOuQnjpxDtYVnnUmRhGQOW+sEpA5+TnFcGsBqjH/6QjQC6O8FHQMUfQ3naPFakyHkJcumlqEjN 5lWFBzb+RWGw== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 May 2020 12:43:24 -0700 IronPort-SDR: 08o7ATRyAi9mAFigoZ+HjkcEM0EvLpHu/JpG6yj4ChlRR5OzDycoxW1qDCEYcRqx3HsKOKC2Eg BbbaDCcdYEgQ== X-IronPort-AV: E=Sophos;i="5.73,364,1583222400"; d="scan'208";a="251602971" Received: from agluck-desk2.sc.intel.com (HELO agluck-desk2.amr.corp.intel.com) ([10.3.52.68]) by fmsmga008-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 May 2020 12:43:24 -0700 Date: Thu, 7 May 2020 12:43:23 -0700 From: "Luck, Tony" To: "Gustavo A. R. Silva" Cc: Fenghua Yu , linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] treewide: Replace zero-length array with flexible-array Message-ID: <20200507194323.GA9073@agluck-desk2.amr.corp.intel.com> References: <20200507185334.GA14456@embeddedor> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200507185334.GA14456@embeddedor> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 07, 2020 at 01:53:34PM -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] > > sizeof(flexible-array-member) triggers a warning because flexible array > members have incomplete type[1]. There are some instances of code in > which the sizeof operator is being incorrectly/erroneously applied to > zero-length arrays and the result is zero. Such instances may be hiding > some bugs. So, this work (flexible-array member conversions) will also > help to get completely rid of those sorts of issues. > > 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 I'm fine with this specific change ... so Acked-by: Tony Luck for anyone picking up all of these treewide: patches (if they don't, then I can apply this to the ia64 tree) But a question ... is sizeof still ok on a structure that contains a flexible-array-member? E.g. I'd expect: struct foo { int stuff; struct boo array[]; }; printk("size of foo = %d\n", sizeof(struct foo)); To not give any complilation warnings and to tell me that the size of the structure is 4 bytes. Is that still true and OK? -Tony