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=-3.8 required=3.0 tests=BAYES_00, 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 EA695C433DB for ; Mon, 8 Mar 2021 10:01:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B85AC64EB2 for ; Mon, 8 Mar 2021 10:01:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231544AbhCHKB2 convert rfc822-to-8bit (ORCPT ); Mon, 8 Mar 2021 05:01:28 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.85.151]:40339 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231522AbhCHKB0 (ORCPT ); Mon, 8 Mar 2021 05:01:26 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-126-UoBJENP0MXWJd9ek_G47xg-1; Mon, 08 Mar 2021 10:01:21 +0000 X-MC-Unique: UoBJENP0MXWJd9ek_G47xg-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Mon, 8 Mar 2021 10:01:20 +0000 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Mon, 8 Mar 2021 10:01:20 +0000 From: David Laight To: 'Peter Zijlstra' , Linus Torvalds CC: Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , "Namhyung Kim" , Linux Kernel Mailing List Subject: RE: "struct perf_sample_data" alignment Thread-Topic: "struct perf_sample_data" alignment Thread-Index: AQHXEZrQ+54heyJXCUKE4zwwZZ2YcKp5304Q Date: Mon, 8 Mar 2021 10:01:20 +0000 Message-ID: References: In-Reply-To: Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ... > What I remember is that since perf_sample_data is fairly large, > unconditionally initializing the whole thing is *slow* (and > -fauto-var-init=zero will hurt here). That will hurt everywhere. I can also imagine it hiding bugs and making people shrink on-stack arrays to the point where they either overrun or cause an unexpected error or string truncation. Initialising to zero is a bad choice if the aim is to avoid leaking stack. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)