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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Thu, 8 Nov 2018 16:45:26 -0800
Message-ID: <CALCETrWbYwxDtkp7jjf=L7xFubEOP3+DuCQFHdf7bVy0MqqvXQ@mail.gmail.com>
Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for
 shadow stack
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        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>
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On Thu, Nov 8, 2018 at 4:32 PM Matthew Wilcox <willy@infradead.org> 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.