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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 CFA30C4332D for ; Thu, 19 Mar 2020 18:17:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B422F20724 for ; Thu, 19 Mar 2020 18:17:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727023AbgCSSRr convert rfc822-to-8bit (ORCPT ); Thu, 19 Mar 2020 14:17:47 -0400 Received: from albireo.enyo.de ([37.24.231.21]:51982 "EHLO albireo.enyo.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726663AbgCSSRr (ORCPT ); Thu, 19 Mar 2020 14:17:47 -0400 Received: from [172.17.203.2] (helo=deneb.enyo.de) by albireo.enyo.de with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) id 1jEzjc-000123-KO; Thu, 19 Mar 2020 18:17:36 +0000 Received: from fw by deneb.enyo.de with local (Exim 4.92) (envelope-from ) id 1jEziG-0006VC-Ak; Thu, 19 Mar 2020 19:16:12 +0100 From: Florian Weimer To: Mathieu Desnoyers Cc: libc-alpha , carlos , Rich Felker , linux-api , Boqun Feng , Will Deacon , linux-kernel , Peter Zijlstra , Ben Maurer , Dave Watson , Thomas Gleixner , Paul , Paul Turner , Joseph Myers Subject: Re: [RFC PATCH glibc 4/8] glibc: Perform rseq(2) registration at C startup and thread creation (v15) References: <20200319144110.3733-1-mathieu.desnoyers@efficios.com> <20200319144110.3733-5-mathieu.desnoyers@efficios.com> <874kukpf9f.fsf@mid.deneb.enyo.de> <2147217200.3240.1584633395285.JavaMail.zimbra@efficios.com> <87r1xo5o2s.fsf@mid.deneb.enyo.de> <1302331358.3965.1584641354569.JavaMail.zimbra@efficios.com> Date: Thu, 19 Mar 2020 19:16:12 +0100 In-Reply-To: <1302331358.3965.1584641354569.JavaMail.zimbra@efficios.com> (Mathieu Desnoyers's message of "Thu, 19 Mar 2020 14:09:14 -0400 (EDT)") Message-ID: <87sgi4gqhf.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Mathieu Desnoyers: >> You also need to add an assert that the compiler supports >> __attribute__ ((aligned)) because ignoring it produces an >> ABI-incompatible header. > > Are you aware of some helper macro I should use to do this, or > is it done elsewhere in glibc ? I don't think we have any such GCC-only types yet. max_align_t is provided by GCC itself. >> The struct rseq/struct rseq_cs definitions >> are broken, they should not try to change the alignment. > > AFAIU, this means we should ideally not have used __attribute__((aligned)) > in the uapi headers in the first place. Why is it broken ? Compilers which are not sufficiently GCC-compatible define __attribute__(X) as the empty expansion, so you silently get a different ABI. There is really no need to specify 32-byte alignment here. Is not even the size of a standard cache line. It can result in crashes if these structs are heap-allocated using malloc, when optimizing for AVX2. For example, clang turns void clear (struct rseq *p) { memset (p, 0, sizeof (*p)); } into: vxorps %xmm0, %xmm0, %xmm0 vmovaps %ymm0, (%rdi) vzeroupper retq My understanding is that vmovaps will trap if the pointer is misaligned (“When the source or destination operand is a memory operand, the operand must be aligned on a 32-byte boundary or a general-protection exception (#GP) will be generated.”). > However, now that it is in the wild, it's a bit late to change that. I had forgotten about the alignment crashes. I think we should seriously consider changing the types. 8-(