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 Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id D4133C433EF for ; Mon, 18 Jul 2022 07:52:26 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4LmZ1j0pdxz3cdr for ; Mon, 18 Jul 2022 17:52:25 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=aculab.com (client-ip=185.58.85.151; helo=eu-smtp-delivery-151.mimecast.com; envelope-from=david.laight@aculab.com; receiver=) Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.85.151]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4LmZ194Czjz3bl6 for ; Mon, 18 Jul 2022 17:51:54 +1000 (AEST) Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-252-gbDn5GdWMXeh8y7fCCM7XA-1; Mon, 18 Jul 2022 08:51:48 +0100 X-MC-Unique: gbDn5GdWMXeh8y7fCCM7XA-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) by AcuMS.aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) with Microsoft SMTP Server (TLS) id 15.0.1497.36; Mon, 18 Jul 2022 08:51:45 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.036; Mon, 18 Jul 2022 08:51:45 +0100 From: David Laight To: 'Michael Ellerman' , Sudip Mukherjee , Benjamin Herrenschmidt , Paul Mackerras , Kees Cook Subject: RE: mainline build failure of powerpc allmodconfig for prom_init_check Thread-Topic: mainline build failure of powerpc allmodconfig for prom_init_check Thread-Index: AQHYmmCqgiI6vbQbKkWuMJXdAKKy2K2Dv4mw Date: Mon, 18 Jul 2022 07:51:45 +0000 Message-ID: <72a95f0949d149d1b07bd161f3d59cc1@AcuMS.aculab.com> References: <87cze3docs.fsf@mpe.ellerman.id.au> In-Reply-To: <87cze3docs.fsf@mpe.ellerman.id.au> 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: quoted-printable X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Linus Torvalds , "linuxppc-dev@lists.ozlabs.org" , linux-kernel , "linux-hardening@vger.kernel.org" Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" From: Michael Ellerman > Sent: 18 July 2022 05:41 ... > So we're memsetting all of args to 254, not zero. >=20 > That's happening because allmodconfig with gcc 12 enables > CONFIG_INIT_STACK_ALL_PATTERN, whereas gcc 11 doesn't. I can't help feeling it would be better if that generated a call to a memset64() function. Saving loads of tests at the top of the function, and (most of?) the constant expansion to 64bit. Although and explicit 'stack clear' function would be better for the kernel - since it would give the option of patching it away at startup. I really can't help feeling that initialising on-stack arrays will kill performance. While kernel stack frames have to be relatively small, in userspace very large on-stack arrays can be allocated (and correctly bound checked) knowing that the cost is minimal (maybe a TLB miss). =09David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)