From owner-freebsd-hackers@freebsd.org Fri Feb 21 13:19:48 2020 Return-Path: Delivered-To: freebsd-hackers@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id 25CED25D6DF for ; Fri, 21 Feb 2020 13:19:48 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) server-signature RSA-PSS (4096 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 48PBrg3hLbz3yl8 for ; Fri, 21 Feb 2020 13:19:47 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kib@localhost [127.0.0.1]) by kib.kiev.ua (8.15.2/8.15.2) with ESMTPS id 01LDJdNY020093 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO) for ; Fri, 21 Feb 2020 15:19:42 +0200 (EET) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua 01LDJdNY020093 Received: (from kostik@localhost) by tom.home (8.15.2/8.15.2/Submit) id 01LDJd7V020091 for freebsd-hackers@freebsd.org; Fri, 21 Feb 2020 15:19:39 +0200 (EET) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Fri, 21 Feb 2020 15:19:39 +0200 From: Konstantin Belousov To: freebsd-hackers@freebsd.org Subject: Re: How much libc++ ABI changes FreeBSD can consume? Message-ID: <20200221131939.GV29554@kib.kiev.ua> References: <20200220141655.GP29554@kib.kiev.ua> <20200221120325.GA86511@bec.de> <20200221123637.GT29554@kib.kiev.ua> <20200221125846.GA88921@bec.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200221125846.GA88921@bec.de> X-Spam-Status: No, score=-1.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FORGED_GMAIL_RCVD,FREEMAIL_FROM, NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on tom.home X-Rspamd-Queue-Id: 48PBrg3hLbz3yl8 X-Spamd-Bar: / Authentication-Results: mx1.freebsd.org; dkim=none; dmarc=fail reason="No valid SPF, No valid DKIM" header.from=gmail.com (policy=none); spf=softfail (mx1.freebsd.org: 2001:470:d5e7:1::1 is neither permitted nor denied by domain of kostikbel@gmail.com) smtp.mailfrom=kostikbel@gmail.com X-Spamd-Result: default: False [-1.00 / 15.00]; ARC_NA(0.00)[]; NEURAL_HAM_MEDIUM(-1.00)[-0.998,0]; RCVD_TLS_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; FREEMAIL_FROM(0.00)[gmail.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; MIME_GOOD(-0.10)[text/plain]; PREVIOUSLY_DELIVERED(0.00)[freebsd-hackers@freebsd.org]; TO_DN_NONE(0.00)[]; R_SPF_SOFTFAIL(0.00)[~all:c]; RCPT_COUNT_ONE(0.00)[1]; NEURAL_HAM_LONG(-1.00)[-1.000,0]; IP_SCORE_FREEMAIL(0.00)[]; HAS_XAW(0.00)[]; IP_SCORE(0.00)[ip: (-3.17), ipnet: 2001:470::/32(-4.65), asn: 6939(-3.58), country: US(-0.05)]; SUBJECT_ENDS_QUESTION(1.00)[]; FROM_EQ_ENVFROM(0.00)[]; R_DKIM_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; ASN(0.00)[asn:6939, ipnet:2001:470::/32, country:US]; RCVD_COUNT_TWO(0.00)[2]; FREEMAIL_ENVFROM(0.00)[gmail.com]; DMARC_POLICY_SOFTFAIL(0.10)[gmail.com : No valid SPF, No valid DKIM,none] X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 21 Feb 2020 13:19:48 -0000 On Fri, Feb 21, 2020 at 01:58:46PM +0100, Joerg Sonnenberger wrote: > On Fri, Feb 21, 2020 at 02:36:37PM +0200, Konstantin Belousov wrote: > > On Fri, Feb 21, 2020 at 01:03:25PM +0100, Joerg Sonnenberger wrote: > > > On Fri, Feb 21, 2020 at 03:24:32AM -0600, Zhihao Yuan wrote: > > > > On Thu, Feb 20, 2020 at 8:17 AM Konstantin Belousov wrote: > > > > > > > > > > 3. Is MFC required for libc++ updates? If so, how > > > > > > does that affect ABI changes? > > > > > It is highly desirable to get libc++ synced between head and all actively > > > > > supported stable versions. > > > > > > > > > > > 4. Is there any desire to make C++ ABI breakage > > > > > > smoother by ultilzing mechanisms such as > > > > > > Symbol.map? > > > > > Yes. More expanded answer below. > > > > > > > > > > Right now any libc++ ABI breakage requires dso version bump. We try hard > > > > > to avoid that because it trivially leads to a situation when multiple > > > > > libc++'s are loaded into same process, unless everything is recompiled > > > > > against same lib. In other words, bumping version for such fundamental > > > > > library is too troublesome. > > > > > > > > > > Symver provides a solution for gradual ABI changes, but by policy > > > > > we never provide symbol versioning for third-party libraries unless > > > > > upstream maintains the versioning. The reason is that we cannot enforce > > > > > upstream ABI policy, which would make versioning broken by updates and > > > > > then pointless. > > > > > > > > > > So for instance libstdc++.so from gcc is versioned, while ncurses are not. > > > > > > > > > > > > > To summarize what I heard, even if libc++ > > > > stabilizes V2 ABI, we do not want to do an > > > > "ABI break since release 1X" thing. If we > > > > really upgrade, we break all stable versions. > > > > And we hope/encourage libc++ to > > > > version symbols like what libstdc++ does, > > > > correct? > > > > > > Symbol versioning helps really little for this kind of ABI breaks. It > > > still ends up effectively being a flag day as libraries build before and > > > after don't interact that well with each other. > > It helps some, and from my undertanding, symver + properly used inline > > namespaces cover much, if not everything. > > Take a look at all the pain libstdc++ had with its new string > implementation. Symbol versioning is fine as long as the ABI doesn't > extend beyond the boundaries of the depending DSOs. But if you want to > change e.g. std::string, all libraries to be mixed need to version any > interface containing it and that is effectively as much trouble as just > doing the DSO bump. Right, as I understand, inline namespaces were the reaction to this and similar issues. With new namespace, std::string methods would get different mangled name. I am not claiming that either approach (or its combination) cover everything, but the situation should be not that dim. E.g., after ino64 changes, I am aware of only one serious breakage when running binaries built for older struct stat, on newer system, and this comes from Firefox doing dlsym("stat").