From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack Date: Thu, 8 Nov 2018 16:45:26 -0800 Message-ID: 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> <20181108220054.GP3074@bombadil.infradead.org> <20181109003225.GQ3074@bombadil.infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: In-Reply-To: <20181109003225.GQ3074@bombadil.infradead.org> Sender: linux-kernel-owner@vger.kernel.org To: Matthew Wilcox Cc: Dave Hansen , 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 List-Id: linux-arch.vger.kernel.org On Thu, Nov 8, 2018 at 4:32 PM Matthew Wilcox wrote: > > On Thu, Nov 08, 2018 at 03:35:02PM -0800, Dave Hansen wrote: > > On 11/8/18 2:00 PM, Matthew Wilcox wrote: > > > 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. > > > > You're talking about how we want the struct laid out in memory if we > > have control over the layout. I'm talking about what happens if > > something *else* tells us the layout, like a hardware specification > > which is what is in play with the XSAVE instruction dictated layout > > that's in question here. > > > > What I'm concerned about is a structure like this: > > > > struct foo { > > u32 i1; > > u64 i2; > > }; > > > > If we leave that to natural alignment, we end up with a 16-byte > > structure laid out like this: > > > > 0-3 i1 > > 3-8 alignment gap > > 8-15 i2 > > I know you actually meant: > > 0-3 i1 > 4-7 pad > 8-15 i2 > > > Which isn't what we want. We want a 12-byte structure, laid out like this: > > > > 0-3 i1 > > 4-11 i2 > > > > Which we get with: > > > > struct foo { > > u32 i1; > > u64 i2; > > } __packed; > > But we _also_ get pessimised accesses to i1 and i2. Because gcc can't > rely on struct foo being aligned to a 4 or even 8 byte boundary (it > might be embedded in "struct a" from above). > In the event we end up with a hardware structure that has not-really-aligned elements, I suspect we can ask gcc for a new extension to help. Or maybe some hack like: struct foo { u32 i1; struct { u64 i2; } __attribute__((packed)); }; would do the trick. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr1-f68.google.com ([209.85.221.68]:45127 "EHLO mail-wr1-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727537AbeKIKXs (ORCPT ); Fri, 9 Nov 2018 05:23:48 -0500 Received: by mail-wr1-f68.google.com with SMTP id k15-v6so116615wre.12 for ; Thu, 08 Nov 2018 16:45:39 -0800 (PST) MIME-Version: 1.0 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> <20181108220054.GP3074@bombadil.infradead.org> <20181109003225.GQ3074@bombadil.infradead.org> In-Reply-To: <20181109003225.GQ3074@bombadil.infradead.org> From: Andy Lutomirski Date: Thu, 8 Nov 2018 16:45:26 -0800 Message-ID: Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack Content-Type: text/plain; charset="UTF-8" Sender: linux-arch-owner@vger.kernel.org List-ID: To: Matthew Wilcox Cc: Dave Hansen , 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" Message-ID: <20181109004526.qmllsJj4bJrSserl1yzUu0PnAqfdKB7htXapyRN56fU@z> On Thu, Nov 8, 2018 at 4:32 PM Matthew Wilcox wrote: > > On Thu, Nov 08, 2018 at 03:35:02PM -0800, Dave Hansen wrote: > > On 11/8/18 2:00 PM, Matthew Wilcox wrote: > > > 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. > > > > You're talking about how we want the struct laid out in memory if we > > have control over the layout. I'm talking about what happens if > > something *else* tells us the layout, like a hardware specification > > which is what is in play with the XSAVE instruction dictated layout > > that's in question here. > > > > What I'm concerned about is a structure like this: > > > > struct foo { > > u32 i1; > > u64 i2; > > }; > > > > If we leave that to natural alignment, we end up with a 16-byte > > structure laid out like this: > > > > 0-3 i1 > > 3-8 alignment gap > > 8-15 i2 > > I know you actually meant: > > 0-3 i1 > 4-7 pad > 8-15 i2 > > > Which isn't what we want. We want a 12-byte structure, laid out like this: > > > > 0-3 i1 > > 4-11 i2 > > > > Which we get with: > > > > struct foo { > > u32 i1; > > u64 i2; > > } __packed; > > But we _also_ get pessimised accesses to i1 and i2. Because gcc can't > rely on struct foo being aligned to a 4 or even 8 byte boundary (it > might be embedded in "struct a" from above). > In the event we end up with a hardware structure that has not-really-aligned elements, I suspect we can ask gcc for a new extension to help. Or maybe some hack like: struct foo { u32 i1; struct { u64 i2; } __attribute__((packed)); }; would do the trick.