From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-Status: No, score=-4.2 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,MAILING_LIST_MULTI, SPF_HELO_PASS,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from sourceware.org (server2.sourceware.org [IPv6:2620:52:3:1:0:246e:9693:128c]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by dcvr.yhbt.net (Postfix) with ESMTPS id 58E191F55B for ; Mon, 25 May 2020 15:20:35 +0000 (UTC) Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id D2B183890432; Mon, 25 May 2020 15:20:33 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org D2B183890432 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sourceware.org; s=default; t=1590420033; bh=HYjKBsVG8/aQRDDWxnpTEgM4JQmNET1Q9YOObHs51wo=; h=To:Subject:References:Date:In-Reply-To:List-Id:List-Unsubscribe: List-Archive:List-Post:List-Help:List-Subscribe:From:Reply-To:Cc: From; b=WBLf5jVjOVVvkqdm9HdwLhya9RcCHFhQ8Rxu1JHdMRCF0dcAhLR27iGilpi3eVbX9 MZpu/cdSZCcU0C+QlHsUT95LaKhRCmeS9nr++CTD5WVOld2MUQAqRRpsr4cMOewoPl MW1KbUqP2T74WAkN+WXdIY3A7wJEsfTpVhzLyqGQ= Received: from us-smtp-1.mimecast.com (us-smtp-1.mimecast.com [207.211.31.81]) by sourceware.org (Postfix) with ESMTP id 64FB4383E80F for ; Mon, 25 May 2020 15:20:31 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 64FB4383E80F Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-491-c78kMZeNNO2R3tqKxuLtLA-1; Mon, 25 May 2020 11:20:27 -0400 X-MC-Unique: c78kMZeNNO2R3tqKxuLtLA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 92B0B80183C; Mon, 25 May 2020 15:20:24 +0000 (UTC) Received: from oldenburg2.str.redhat.com (ovpn-112-121.ams2.redhat.com [10.36.112.121]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8A4AB5798D; Mon, 25 May 2020 15:20:21 +0000 (UTC) To: Mathieu Desnoyers Subject: Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19) References: <20200501021439.2456-1-mathieu.desnoyers@efficios.com> <20200501021439.2456-2-mathieu.desnoyers@efficios.com> <87v9kqbzse.fsf@oldenburg2.str.redhat.com> <941087675.33347.1590418305398.JavaMail.zimbra@efficios.com> Date: Mon, 25 May 2020 17:20:19 +0200 In-Reply-To: <941087675.33347.1590418305398.JavaMail.zimbra@efficios.com> (Mathieu Desnoyers's message of "Mon, 25 May 2020 10:51:45 -0400 (EDT)") Message-ID: <87367ovy6k.fsf@oldenburg2.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: Florian Weimer via Libc-alpha Reply-To: Florian Weimer Cc: Rich Felker , libc-alpha , Peter Zijlstra , linux-api , Boqun Feng , Will Deacon , linux-kernel , Ben Maurer , Dave Watson , Thomas Gleixner , Paul , Paul Turner , Joseph Myers Errors-To: libc-alpha-bounces@sourceware.org Sender: "Libc-alpha" * Mathieu Desnoyers: > The larger question here is: considering that we re-implement the entire > uapi header within glibc (which includes the uptr addition), do we still > care about using the header provided by the Linux kernel ? We don't care, but our users do. Eventually, they want to include and to get new constants that are not yet known to glibc. > Having different definitions depending on whether a kernel header is > installed or not when including a glibc header seems rather unexpected. Indeed. > *If* we want to use the uapi header, I think something is semantically > missing. Here is the scheme I envision. We could rely on the kernel header > version.h to figure out which of glibc or kernel uapi header is more > recent. Any new concept we try to integrate into glibc (e.g. uptr) > should go into the upstream Linux uapi header first. I think we should always prefer the uapi header. The Linux version check does not tell you anything about backports. > For the coming glibc e.g. 2.32, we use the kernel uapi header if > kernel version is >= 4.18.0. Within glibc, the fallback implements > exactly the API exposed by the kernel rseq.h header. Agreed. > As we eventually introduce the uptr change into the Linux kernel, and > say it gets merged for Linux 5.9.0, we mirror this change into glibc > (e.g. release 2.33), and bump the Linux kernel version cutoff to 5.9.0. > So starting from that version, we use the Linux kernel header only if > version >= 5.9.0, else we fallback on glibc's own implementation. Fortunately, we don't need to settle this today. 8-) Let's stick to the 4.18 definitions for the fallback for now, and discuss the incorporation of future changes later. >>> +/* Ensure the compiler supports __attribute__ ((aligned)). */ >>> +_Static_assert (__alignof__ (struct rseq_cs) >= 32, "alignment"); >>> +_Static_assert (__alignof__ (struct rseq) >= 32, "alignment"); >> >> This needs #ifndef __cplusplus or something like that. I'm surprised >> that this passes the installed header tests. > > Would the following be ok ? > > #ifdef __cplusplus > #define rseq_static_assert static_assert > #else > #define rseq_static_assert _Static_assert > #endif > > /* Ensure the compiler supports __attribute__ ((aligned)). */ > rseq_static_assert (__alignof__ (struct rseq_cs) >= 32, "alignment"); > rseq_static_assert (__alignof__ (struct rseq) >= 32, "alignment"); Seems reasonable, yes. __alignof__ is still a GCC extension. C++11 has alignof, C11 has _Alignof. So you could use something like this (perhaps without indentation for the kernel header version): #ifdef __cplusplus # if __cplusplus >= 201103L # define rseq_static_assert(x) static_assert x; # define rseq_alignof alignof # endif #elif __STDC_VERSION__ >= 201112L # define rseq_static_assert(x) _Static_assert x; # define rseq_alignof _Alignof #endif #ifndef rseq_static_assert # define rseq_static_assert /* nothing */ #endif rseq_static_assert ((rseq_alignof__ (struct rseq_cs) >= 32, "alignment")) rseq_static_assert ((rseq_alignof (struct rseq) >= 32, "alignment")) And something similar for _Alignas/attribute aligned, with an error for older standards and !__GNUC__ compilers (because neither the type nor __thread can be represented there). Thanks, Florian