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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 46B03C4361B for ; Tue, 15 Dec 2020 22:19:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1389F2246B for ; Tue, 15 Dec 2020 22:19:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728211AbgLOWSt (ORCPT ); Tue, 15 Dec 2020 17:18:49 -0500 Received: from mga18.intel.com ([134.134.136.126]:56856 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728268AbgLOWSm (ORCPT ); Tue, 15 Dec 2020 17:18:42 -0500 IronPort-SDR: MK/mrGW9fa0xueslO2SWEKllol1j2JQNzmR9VaHKuD+VJeMbQeyTeivUm/Ms7PMrjdvrs48Ex8 bEjjN+cWu+LQ== X-IronPort-AV: E=McAfee;i="6000,8403,9836"; a="162709816" X-IronPort-AV: E=Sophos;i="5.78,422,1599548400"; d="scan'208";a="162709816" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Dec 2020 14:18:02 -0800 IronPort-SDR: OTPMfezoXkijYV96Qp4Hrr3Sq0p1I1eHad9SLSQFxZBzchi75qaEprNjwFIDn6dl72eKs7HUq9 Xo1uXn5BO5Xg== X-IronPort-AV: E=Sophos;i="5.78,422,1599548400"; d="scan'208";a="368490793" Received: from jekeller-mobl1.amr.corp.intel.com (HELO [10.209.14.218]) ([10.209.14.218]) by orsmga008-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Dec 2020 14:18:02 -0800 Subject: Re: sparse attribute packed on structures To: Luc Van Oostenryck Cc: linux-sparse@vger.kernel.org References: <0e8d816a-0849-c761-e0dc-93c3e5674e62@intel.com> <20201215205618.scarwvtzzhhtdwvk@ltop.local> From: Jacob Keller Organization: Intel Corporation Message-ID: Date: Tue, 15 Dec 2020 14:17:59 -0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: <20201215205618.scarwvtzzhhtdwvk@ltop.local> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-sparse@vger.kernel.org On 12/15/2020 12:56 PM, Luc Van Oostenryck wrote: > On Tue, Dec 15, 2020 at 10:15:35AM -0800, Jacob Keller wrote: >> Hi, >> >> I'm looking into an issue with sparse not calculating the size of a >> packed structure correctly, causing some static assertions to fail due >> to an incorrect size. >> >> With a structure like this: >> >> struct a { >> uint32_t a; >> uint8_t b; >> uint8_t c; >> } __attribute__ ((packed)); >> >> The packed attribute doesn't seem to get applied to the whole structure. >> Thus, the sparse sizeof evaluation for this results in 8 bytes (64 >> bits), when GCC would produce a structure of size 6 bytes (48 bits). >> >> If I use something instead like this: >> >> struct a { >> uint32_t a __attribute__ ((packed)); >> uint8_t b __attribute__ ((packed)); >> uint8_t c __attribute__ ((packed)); >> } __attribute__ ((packed)); >> >> Then the size is calculated correctly. >> >> I saw that there is support in parse.c for parsing attribute packed, but >> it doesn't seem to have a way to propagate from a structure down to its >> members. >> >> I thought it would be relatively straight forward to implement by adding >> a MOD_PACKED, but that doesn't seem to actually get assigned to the >> struct symbol, so when I tried that it didn't work. >> >> I would very much like to help get structure size packing to work properly. >> >> The following diff is what I tried initially, but it doesn't actually >> work as expected. I'm not sure what is wrong, or what is the best method >> to actually get the packed modifier to save into the structure symbol so >> that it can be checked when determining the structure size. >> >> Help would be appreciated. > > Hi, > > There is at least 3 issues with the packed attribute: > 1) at parsing time, types attributes are not applied to the > corresponding symbol, Yea, this is what I couldn't figure out. > 2) the size calculation must take the attribute in account, This is what I got working IFF I hacked the symbol to say MOD_PACKED. But I couldn't figure out (1) so I got stuck. > 3) the linearization of memory access must be adapted to be able> to access unaligned members otherwise the check access complain > loudly. > Yea I figured something would break here, I was basically mostly interested in handling the size. > Sorry, I don't have much time for this now but at first sight your patch > seems on the right track. I can look at it more closely this WE but > meanwhile I've pushed a branch 'packed' on > git://git.kernel.org/pub/scm/devel/sparse/sparse-dev.git > > This branch contains an unfinished patches but it should more or less > handle the points 1) & 2) and circumvent point 3) by disabling access > checking for bitfields. > Ok thanks! I'll take a look at the packed branch, that should help me. Perhaps I can take a stab at (3) based on whats in that branch. > I hope this will help you, > -- Luc > Great, Thanks! - Jake