From: Matthew Wilcox <willy@infradead.org>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
Yu-cheng Yu <yu-cheng.yu@intel.com>, X86 ML <x86@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
LKML <linux-kernel@vger.kernel.org>,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
linux-arch <linux-arch@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>,
Arnd Bergmann <arnd@arndb.de>,
Balbir Singh <bsingharora@gmail.com>,
Cyrill Gorcunov <gorcunov@gmail.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Eugene Syromiatnikov <esyr@redhat.com>,
Florian Weimer <fweimer@redhat.com>,
"H. J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
Jonathan Corbet <corbet@lwn.net>,
Kees Cook <keescook@chromium.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Nadav Amit <nadav.amit@gmail.com>,
Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
Peter Zijlstra <peterz@infradead.org>,
Randy Dunlap <rdunlap@infradead.org>,
"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
"Shanbhogue, Vedvyas" <vedvyas.shanbhogue@intel.com>
Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack
Date: Thu, 8 Nov 2018 16:32:25 -0800 [thread overview]
Message-ID: <20181109003225.GQ3074@bombadil.infradead.org> (raw)
In-Reply-To: <ead230ab-a904-50d6-c4cf-46d5804f6151@intel.com>
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).
> Now, looking at Yu-cheng's specific example, it doesn't matter. We've
> got 64-bit types and natural 64-bit alignment. Without __packed, we
> need to look out for natural alignment screwing us up. With __packed,
> it just does what it *looks* like it does.
The question is whether Yu-cheng's struct is ever embedded in another
struct. And if so, what does the hardware do?
next prev parent reply other threads:[~2018-11-09 0:32 UTC|newest]
Thread overview: 80+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-11 15:14 [PATCH v5 00/27] Control Flow Enforcement: Shadow Stack Yu-cheng Yu
2018-10-11 15:14 ` [PATCH v5 01/27] x86/cpufeatures: Add CPUIDs for Control Flow Enforcement Technology (CET) Yu-cheng Yu
2018-10-11 16:43 ` Borislav Petkov
2018-10-11 16:45 ` Yu-cheng Yu
2018-10-11 15:14 ` [PATCH v5 02/27] x86/fpu/xstate: Change names to separate XSAVES system and user states Yu-cheng Yu
2018-10-15 17:03 ` Borislav Petkov
2018-10-11 15:14 ` [PATCH v5 03/27] x86/fpu/xstate: Introduce XSAVES system states Yu-cheng Yu
2018-10-17 10:41 ` Borislav Petkov
2018-10-17 22:39 ` Randy Dunlap
2018-10-17 22:58 ` Borislav Petkov
2018-10-17 23:17 ` Randy Dunlap
2018-10-18 9:26 ` Borislav Petkov
2018-10-18 9:31 ` Pavel Machek
2018-10-18 12:10 ` Borislav Petkov
2018-10-18 18:33 ` Randy Dunlap
2018-10-18 9:24 ` Pavel Machek
2018-10-11 15:15 ` [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack Yu-cheng Yu
2018-11-08 18:40 ` Borislav Petkov
2018-11-08 20:40 ` Yu-cheng Yu
2018-11-08 23:52 ` Borislav Petkov
2018-11-11 11:31 ` Pavel Machek
2018-11-11 11:31 ` Pavel Machek
2018-11-11 14:59 ` Andy Lutomirski
2018-11-11 19:02 ` Pavel Machek
2018-11-08 20:46 ` Andy Lutomirski
2018-11-08 21:01 ` Yu-cheng Yu
2018-11-08 21:22 ` Andy Lutomirski
2018-11-08 21:31 ` Cyrill Gorcunov
2018-11-08 22:01 ` Andy Lutomirski
2018-11-08 22:18 ` Cyrill Gorcunov
2018-11-08 21:48 ` Dave Hansen
2018-11-08 22:00 ` Matthew Wilcox
2018-11-08 23:35 ` Dave Hansen
2018-11-09 0:32 ` Matthew Wilcox [this message]
2018-11-09 0:45 ` Andy Lutomirski
2018-11-09 17:13 ` Dave Hansen
2018-11-09 17:17 ` Matthew Wilcox
2018-11-09 17:20 ` Dave Hansen
2018-11-09 17:28 ` Dave Hansen
2018-11-11 11:31 ` Pavel Machek
2018-10-11 15:15 ` [PATCH v5 05/27] Documentation/x86: Add CET description Yu-cheng Yu
2018-11-13 18:43 ` Borislav Petkov
2018-11-13 21:02 ` Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 06/27] x86/cet: Control protection exception handler Yu-cheng Yu
2018-11-14 18:44 ` Borislav Petkov
2018-11-14 20:19 ` Yu-cheng Yu
2018-11-14 20:28 ` Borislav Petkov
2018-10-11 15:15 ` [PATCH v5 07/27] mm/mmap: Create a guard area between VMAs Yu-cheng Yu
2018-10-11 20:39 ` Jann Horn
2018-10-11 20:49 ` Yu-cheng Yu
2018-10-11 20:55 ` Andy Lutomirski
2018-10-12 21:49 ` Yu-cheng Yu
2018-10-12 13:17 ` Matthew Wilcox
2018-10-11 20:49 ` Dave Hansen
2018-10-12 10:24 ` Florian Weimer
2018-10-11 15:15 ` [PATCH v5 08/27] x86/cet/shstk: Add Kconfig option for user-mode shadow stack Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 09/27] mm: Introduce VM_SHSTK for shadow stack memory Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 10/27] mm/mmap: Prevent Shadow Stack VMA merges Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 11/27] x86/mm: Change _PAGE_DIRTY to _PAGE_DIRTY_HW Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 12/27] x86/mm: Introduce _PAGE_DIRTY_SW Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 13/27] drm/i915/gvt: Update _PAGE_DIRTY to _PAGE_DIRTY_BITS Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 14/27] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 15/27] x86/mm: Shadow stack page fault error checking Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 16/27] mm: Handle shadow stack page fault Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 17/27] mm: Handle THP/HugeTLB " Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 18/27] mm: Update can_follow_write_pte/pmd for shadow stack Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 19/27] mm: Introduce do_mmap_locked() Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 20/27] x86/cet/shstk: User-mode shadow stack support Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 21/27] x86/cet/shstk: Introduce WRUSS instruction Yu-cheng Yu
2018-11-06 18:43 ` Dave Hansen
2018-11-06 18:55 ` Andy Lutomirski
2018-11-06 20:21 ` Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 22/27] x86/cet/shstk: Signal handling for shadow stack Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 23/27] x86/cet/shstk: ELF header parsing of Shadow Stack Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 24/27] x86/cet/shstk: Handle thread shadow stack Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 25/27] mm/mmap: Add Shadow stack pages to memory accounting Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 26/27] x86/cet/shstk: Add arch_prctl functions for Shadow Stack Yu-cheng Yu
2018-10-11 15:15 ` [PATCH v5 27/27] x86/cet/shstk: Add Shadow Stack instructions to opcode map Yu-cheng Yu
2018-10-11 19:21 ` [PATCH v5 00/27] Control Flow Enforcement: Shadow Stack Dave Hansen
2018-10-11 19:29 ` Yu-cheng Yu
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