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=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED 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 DFF76C43441 for ; Mon, 26 Nov 2018 15:51:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9F85F20664 for ; Mon, 26 Nov 2018 15:51:13 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b="nFDPLzxD" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9F85F20664 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=efficios.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727136AbeK0Cpl (ORCPT ); Mon, 26 Nov 2018 21:45:41 -0500 Received: from mail.efficios.com ([167.114.142.138]:60236 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726255AbeK0Cpl (ORCPT ); Mon, 26 Nov 2018 21:45:41 -0500 Received: from localhost (ip6-localhost [IPv6:::1]) by mail.efficios.com (Postfix) with ESMTP id 374009B70F; Mon, 26 Nov 2018 10:51:10 -0500 (EST) Received: from mail.efficios.com ([IPv6:::1]) by localhost (mail02.efficios.com [IPv6:::1]) (amavisd-new, port 10032) with ESMTP id udde6FZuYZUr; Mon, 26 Nov 2018 10:51:09 -0500 (EST) Received: from localhost (ip6-localhost [IPv6:::1]) by mail.efficios.com (Postfix) with ESMTP id BD4979B706; Mon, 26 Nov 2018 10:51:09 -0500 (EST) DKIM-Filter: OpenDKIM Filter v2.10.3 mail.efficios.com BD4979B706 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=default; t=1543247469; bh=wOuNBBkexG3iXddtSPKLM3ni0N9RkO67OZVIGh4Jz/w=; h=Date:From:To:Message-ID:MIME-Version; b=nFDPLzxDX2pRPVLYEgnKqRri51HxaVTsE4s0rXLWMWXXs8WGd2k2zT5AJy5kTV9pi hRq7TJEklBibM+QmedW4O8Av6ixFbE6zPBNyQGl5iY49MtNBuWgbYOzyxsdeMczjwR afMBxvjJXHcNL9jDweowLuuzzKFYBGgeDBLIF7XB8WkZ7bYcXk0hgjYhs95MWfm8ZU 09F74IpriZTHBEq02RyOKzQr1+AKdGYX8ezp6tZim6P3iEjo+SV3hpmXgPfcCFoSm+ DfwUcXzv2xa7NjW1D3hpdk0XVNE2O+uYPMSxaA1eVYxQZN+CXCDkKUWQmXIuoIRRn0 B2uKO1w+QXxXQ== X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([IPv6:::1]) by localhost (mail02.efficios.com [IPv6:::1]) (amavisd-new, port 10026) with ESMTP id 18mJS-2HiVH5; Mon, 26 Nov 2018 10:51:09 -0500 (EST) Received: from mail02.efficios.com (mail02.efficios.com [167.114.142.138]) by mail.efficios.com (Postfix) with ESMTP id 9F2C89B6FF; Mon, 26 Nov 2018 10:51:09 -0500 (EST) Date: Mon, 26 Nov 2018 10:51:09 -0500 (EST) From: Mathieu Desnoyers To: Florian Weimer Cc: Rich Felker , carlos , Joseph Myers , Szabolcs Nagy , libc-alpha , Thomas Gleixner , Ben Maurer , Peter Zijlstra , "Paul E. McKenney" , Boqun Feng , Will Deacon , Dave Watson , Paul Turner , linux-kernel , linux-api Message-ID: <688718071.12798.1543247469553.JavaMail.zimbra@efficios.com> In-Reply-To: <87bm6cqm31.fsf@oldenburg.str.redhat.com> References: <20181121183936.8176-1-mathieu.desnoyers@efficios.com> <20181123142843.GJ23599@brightrain.aerifal.cx> <1150466925.11664.1542992720871.JavaMail.zimbra@efficios.com> <20181123173019.GK23599@brightrain.aerifal.cx> <865273158.11687.1542995541389.JavaMail.zimbra@efficios.com> <20181123183558.GM23599@brightrain.aerifal.cx> <1758017676.12041.1543007347347.JavaMail.zimbra@efficios.com> <87bm6cqm31.fsf@oldenburg.str.redhat.com> Subject: Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.142.138] X-Mailer: Zimbra 8.8.10_GA_3047 (ZimbraWebClient - FF52 (Linux)/8.8.10_GA_3041) Thread-Topic: glibc: Perform rseq(2) registration at nptl init and thread creation Thread-Index: PUX+I3v3zwDvk8MSf4HO3/oABovOyA== Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ----- On Nov 26, 2018, at 3:28 AM, Florian Weimer fweimer@redhat.com wrote: > * Mathieu Desnoyers: > >> Using a "weak" symbol in early adopter libraries is important, so they >> can be loaded together into the same process without causing loader >> errors due to many definitions of the same strong symbol. > > This is not how ELF dynamic linking works. If the symbol name is the > same, one definition interposes the others. > > You need to ensure that the symbol has the same size everywhere, though. > There are some tricky interactions with symbol versions, too. (The > interposing libraries must not use symbol versioning.) I was under the impression that loading the same strong symbol into an application multiple times would cause some kind of warning if non-weak. I did some testing to figure out which case I remembered would cause this. When compiling with "-fno-common", dynamic and static linking work fine, but trying to add multiple instances of a given symbol into a single object fails with: /tmp/ccSakXZV.o:(.bss+0x0): multiple definition of `a' /tmp/ccQBJBOo.o:(.bss+0x0): first defined here Even if the symbol has the same size. So considering that we don't care about compiling into a single object here, and only care about static and dynamic linking of libraries, indeed the "weak" symbol is not useful. So let's make __rseq_abi and __rseq_refcount strong symbols then ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com