linux-kernel - Re: [PATCH] x86/fpu: move FPU state into separate cache

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Date:   Wed, 29 Mar 2017 14:35:57 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Borislav Petkov <bp@...e.de>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Brian Gerst <brgerst@...il.com>,
        He Chen <he.chen@...ux.intel.com>,
        Mathias Krause <minipli@...glemail.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Piotr Luc <piotr.luc@...el.com>, Kyle Huey <me@...ehuey.com>,
        Len Brown <len.brown@...el.com>, KVM <kvm@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH] x86/fpu: move FPU state into separate cache

On Wed, Mar 29, 2017 at 2:30 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> The trivial model might be to just declare the fpu part as an unsized
> array at the end:
>
>         /* Floating point and extended processor state */
>         struct fpu              fpu[];
>
> because there is no way in hell that any randomization code can move
> those kinds of unsized arrays around.

Side note: that approach would seem to have the added advantage that
because "fpu" now is an array, it syntactically acts like a pointer in
C, so now syntactically it's going to be equivalent to having a
"struct fpu *" pointer element, but from an allocation and code
generation standpoint it all is like allocating the fpu structure
together with the task struct.

                 Linus