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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 41FC2E82CB1 for ; Wed, 27 Sep 2023 16:52:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229473AbjI0Qwl (ORCPT ); Wed, 27 Sep 2023 12:52:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57220 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229437AbjI0Qwl (ORCPT ); Wed, 27 Sep 2023 12:52:41 -0400 Received: from ganesha.gnumonks.org (ganesha.gnumonks.org [IPv6:2001:780:45:1d:225:90ff:fe52:c662]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 80097DE for ; Wed, 27 Sep 2023 09:52:39 -0700 (PDT) Received: from [78.30.34.192] (port=40764 helo=gnumonks.org) by ganesha.gnumonks.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1qlXm6-00DZCn-VI; Wed, 27 Sep 2023 18:52:37 +0200 Date: Wed, 27 Sep 2023 18:52:34 +0200 From: Pablo Neira Ayuso To: Phil Sutter , netfilter-devel@vger.kernel.org Subject: Re: [PATCH nft 3/3,v2] netlink_linearize: skip set element expression in map statement key Message-ID: References: <20230926160216.152549-1-pablo@netfilter.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: netfilter-devel@vger.kernel.org On Wed, Sep 27, 2023 at 04:41:18PM +0200, Phil Sutter wrote: > On Wed, Sep 27, 2023 at 03:09:53PM +0200, Phil Sutter wrote: > > On Wed, Sep 27, 2023 at 01:19:31PM +0200, Pablo Neira Ayuso wrote: > > > Hi Phil, > > > > > > On Wed, Sep 27, 2023 at 01:10:09PM +0200, Phil Sutter wrote: [...] > > > > I actually considered forking the project. Or we just ship a copy of the > > > > lib with nftables sources? > > > > > > I would try to get back to them to refresh and retry. > > > > Oh well. I'll try an approach which eliminates the pointer if not > > enabled. The terse feedback and pessimistic replies right from the start > > convinced me though they just don't want it. > > OK, so I had a close look at the code and played a bit with pahole. My > approach to avoiding the extra pointer is to add another set of types > which json_t embed. So taking json_array_t as an example: > > | typedef struct { > | json_t json; > | size_t size; > | size_t entries; > | json_t **table; > | } json_array_t; > > I could introduce json_location_array_t: > > | typedef struct { > | json_array_t array; > | json_location_t *location; > | } json_location_array_t; > > The above structs are opaque to users, they only know about json_t. OK, so this new object type is hiding behind the json_t opaque type. > So I introduced a getter for the location data: > > | int json_get_location(json_t *json, int *line, int *column, > | int *position, int *length); > > In there, I have to map from json_t to the type in question. The problem > is to know whether I have a json_location_array_t or just a > json_array_t. The json_t may have been allocated by the input parser > with either JSON_STORE_LOCATION set or not or by json_array(). > > In order to make the decision, I need at least a bit in well-known > memory. Pahole tells there's a 4byte hole in json_t, but it may be > gone in 32bit builds (and enlarging json_t is a no-go, they consider > it ABI). The json_*_t structures don't show any holes, and extending > them means adding a mandatory word due to buffering, so I may just > as well store the location pointer itself in them. > > The only feasible alternative is to store location data separate from > the objects themselves, ideally in a hash table. This reduces the > overhead if not used by a failing hash table lookup in json_delete(). If I understood correctly, then this means you are ditching the json_location_array_t approach that you are detailing above. The hashtable approach might be sensible to follow, and such approach does not require any update to libjansson?