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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 26C7CC2D0DB for ; Tue, 28 Jan 2020 13:19:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0184E20CC7 for ; Tue, 28 Jan 2020 13:19:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726024AbgA1NT0 convert rfc822-to-8bit (ORCPT ); Tue, 28 Jan 2020 08:19:26 -0500 Received: from correo.us.es ([193.147.175.20]:48672 "EHLO mail.us.es" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725881AbgA1NT0 (ORCPT ); Tue, 28 Jan 2020 08:19:26 -0500 Received: from antivirus1-rhel7.int (unknown [192.168.2.11]) by mail.us.es (Postfix) with ESMTP id 17FA380B3A for ; Tue, 28 Jan 2020 14:19:25 +0100 (CET) Received: from antivirus1-rhel7.int (localhost [127.0.0.1]) by antivirus1-rhel7.int (Postfix) with ESMTP id 0622FDA878 for ; Tue, 28 Jan 2020 14:19:25 +0100 (CET) Received: by antivirus1-rhel7.int (Postfix, from userid 99) id 804EAFC5FE; Tue, 28 Jan 2020 14:19:16 +0100 (CET) Received: from antivirus1-rhel7.int (localhost [127.0.0.1]) by antivirus1-rhel7.int (Postfix) with ESMTP id 3AD36DA86B; Tue, 28 Jan 2020 14:18:54 +0100 (CET) Received: from 192.168.1.97 (192.168.1.97) by antivirus1-rhel7.int (F-Secure/fsigk_smtp/550/antivirus1-rhel7.int); Tue, 28 Jan 2020 14:18:54 +0100 (CET) X-Virus-Status: clean(F-Secure/fsigk_smtp/550/antivirus1-rhel7.int) Received: from us.es (unknown [90.77.255.23]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) (Authenticated sender: 1984lsi) by entrada.int (Postfix) with ESMTPSA id 15ABC4301DE0; Tue, 28 Jan 2020 14:18:54 +0100 (CET) Date: Tue, 28 Jan 2020 14:18:52 +0100 X-SMTPAUTHUS: auth mail.us.es From: Pablo Neira Ayuso To: Jeremy Sowden Cc: Netfilter Devel Subject: Re: [PATCH nf-next v4 00/10] netfilter: nft_bitwise: shift support Message-ID: <20200128131852.5j3tjzck532fy4qg@salvia> References: <20200115213216.77493-1-jeremy@azazel.net> <20200116144833.jeshvfqvjpbl6fez@salvia> <20200116145954.GC18463@azazel.net> <20200126111251.e4kncc54umrq7mea@salvia> <20200127111314.GA377617@azazel.net> <20200128100035.m4s54v5mfrlqvo4e@salvia> <20200128113139.GA437225@azazel.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: 8BIT In-Reply-To: <20200128113139.GA437225@azazel.net> User-Agent: NeoMutt/20170113 (1.7.2) X-Virus-Scanned: ClamAV using ClamSMTP Sender: netfilter-devel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netfilter-devel@vger.kernel.org On Tue, Jan 28, 2020 at 11:31:39AM +0000, Jeremy Sowden wrote: > On 2020-01-28, at 11:00:35 +0100, Pablo Neira Ayuso wrote: > > On Mon, Jan 27, 2020 at 11:13:14AM +0000, Jeremy Sowden wrote: > > > On 2020-01-26, at 12:12:51 +0100, Pablo Neira Ayuso wrote: > > > > I've been looking into (ab)using bitwise to implement add/sub. I > > > > would like to not add nft_arith for only this, and it seems to me > > > > much of your code can be reused. > > > > > > > > Do you think something like this would work? > > > > > > Absolutely. > > > > > > A couple of questions. What's the use-case? > > > > inc/dec ip ttl field. > > If it's just a simple addition or subtraction on one value, would > this make more sense? > > for (i = 0; i < words; i++) { > dst[i] = src[i] + delta; > delta = dst[i] < src[i] ? 1 : 0; > } This can be done through _INC / _DEC instead, however... > > > I find the combination of applying the delta to every u32 and having > > > a carry curious. Do you want to support bigendian arithmetic (i.e., > > > carrying to the left) as well? > > > > Userspace should convert to host endianess before doing arithmetics. > > Yes, but if the host is bigendian, the least significant bytes will be > on the right, and we need to carry to the left, don't we? > > for (i = words; i > 0; i--) { > dst[i - 1] = src[i - 1] + delta; > delta = dst[i - 1] < src[i - 1] ? 1 : 0; > } I think some simplified version of bignum add/subtract is needed, something like: for (i = len - 1; i >= 0; i--) { res[i] = a[i] + b[i] + carry; carry = res[i] < a[i] + b[i]; } where 'len' is in bytes. Values in a[] and b[] are u8 and data is represented in big endian.