From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.8 required=5.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=unavailable autolearn_force=no version=3.4.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 2B1C57D089 for ; Thu, 8 Nov 2018 22:29:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728277AbeKIHid (ORCPT ); Fri, 9 Nov 2018 02:38:33 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:34484 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728491AbeKIHic (ORCPT ); Fri, 9 Nov 2018 02:38:32 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=hk1JID0w158EIaGcr+HCkYTvTEFZ9mw1TMrwyhEXE3Y=; b=BtgqaeN5RxYXE0oy0PLqwjh7r t8QVPwzmJ0q7SL0wFaHgO1ObCjn5rI/5mmcFvxdaR3RRH37tE/4PzZ23IcZnFBTYK6H6UJ25H0fa7 3XTGbeV6la9bTYPN8n/ifIKXsxKmC3ge+wj0hyU378Bsj2KxB3xB5nk96VG/33vndqyPOusmlYvn4 TP0ZF2O7+Wwft6xZ+/YulihwWxhQWA2ncN7h1HlRLQwtfdthqVNTAr5bxYjTePk5DiyzyN5nD4z3P Qu0UtelLBNrCgq+Cei4bWbG235I8j5qbMUQB840PFn9LNax5URVU4AzilITACbFN06pIAc/QUVtxg dDiWwfikg==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1gKsMA-0005k8-PU; Thu, 08 Nov 2018 22:00:54 +0000 Date: Thu, 8 Nov 2018 14:00:54 -0800 From: Matthew Wilcox To: Dave Hansen Cc: Andy Lutomirski , Yu-cheng Yu , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , LKML , "open list:DOCUMENTATION" , Linux-MM , linux-arch , Linux API , Arnd Bergmann , Balbir Singh , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H. J. Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , "Ravi V. Shankar" , "Shanbhogue, Vedvyas" Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack Message-ID: <20181108220054.GP3074@bombadil.infradead.org> References: <20181011151523.27101-1-yu-cheng.yu@intel.com> <20181011151523.27101-5-yu-cheng.yu@intel.com> <4295b8f786c10c469870a6d9725749ce75dcdaa2.camel@intel.com> <043a17ef-dc9f-56d2-5fba-1a58b7b0fd4d@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <043a17ef-dc9f-56d2-5fba-1a58b7b0fd4d@intel.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Thu, Nov 08, 2018 at 01:48:54PM -0800, Dave Hansen wrote: > On 11/8/18 1:22 PM, Andy Lutomirski wrote: > >> +struct cet_kernel_state { > >> + u64 kernel_ssp; /* kernel shadow stack */ > >> + u64 pl1_ssp; /* ring-1 shadow stack */ > >> + u64 pl2_ssp; /* ring-2 shadow stack */ > >> +} __packed; > >> + > > Why are these __packed? It seems like it'll generate bad code for no > > obvious purpose. > > It's a hardware-defined in-memory structure. Granted, we'd need a > really wonky compiler to make that anything *other* than a nicely-packed > 24-byte structure, but the __packed makes it explicit. > > It is probably a really useful long-term thing to stop using __packed > and start using "__hw_defined" or something that #defines down to __packed. packed doesn't mean "don't leave gaps". It means: 'packed' The 'packed' attribute specifies that a variable or structure field should have the smallest possible alignment--one byte for a variable, and one bit for a field, unless you specify a larger value with the 'aligned' attribute. So Andy's right. It tells the compiler, "this struct will not be naturally aligned, it will be aligned to a 1-byte boundary". Which is silly. If we have struct b { unsigned long x; } __packed; struct a { char c; struct b b; }; we want struct b to start at offset 8, but with __packed, it will start at offset 1. Delete __packed. It doesn't do what you think it does.