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=-10.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 71B2AC54E8D for ; Mon, 11 May 2020 15:26:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5B1CF206D5 for ; Mon, 11 May 2020 15:26:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730045AbgEKP0h (ORCPT ); Mon, 11 May 2020 11:26:37 -0400 Received: from www62.your-server.de ([213.133.104.62]:55710 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726410AbgEKP0h (ORCPT ); Mon, 11 May 2020 11:26:37 -0400 Received: from sslproxy06.your-server.de ([78.46.172.3]) by www62.your-server.de with esmtpsa (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256) (Exim 4.89_1) (envelope-from ) id 1jYAK8-0004PL-AQ; Mon, 11 May 2020 17:26:32 +0200 Received: from [178.195.186.98] (helo=pc-9.home) by sslproxy06.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1jYAK7-0008Vc-UJ; Mon, 11 May 2020 17:26:31 +0200 Subject: Re: [PATCH] libbpf: Replace zero-length array with flexible-array To: Yonghong Song , "Gustavo A. R. Silva" , Alexei Starovoitov Cc: Martin KaFai Lau , Song Liu , Andrii Nakryiko , John Fastabend , KP Singh , netdev@vger.kernel.org, bpf@vger.kernel.org, linux-kernel@vger.kernel.org References: <20200507185057.GA13981@embeddedor> <0ba4c222-b1e6-c003-56f1-6f43405066f0@fb.com> From: Daniel Borkmann Message-ID: <13960d59-3786-a58e-0be0-f9d91f8ee919@iogearbox.net> Date: Mon, 11 May 2020 17:26:27 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <0ba4c222-b1e6-c003-56f1-6f43405066f0@fb.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Authenticated-Sender: daniel@iogearbox.net X-Virus-Scanned: Clear (ClamAV 0.102.2/25809/Mon May 11 14:16:55 2020) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/9/20 9:16 PM, Yonghong Song wrote: > On 5/7/20 11:50 AM, 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 think this is probably for bpf-next. > > Acked-by: Yonghong Song Applied, thanks!