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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Sat, 14 Dec 2019 21:30:08 +0000
Message-ID: <CAKv+Gu85yLS6cYaGPTLc=hjHjvjjYYX-E0wCwKK+1W+T9dxAcQ@mail.gmail.com>
Subject: Re: [PATCH 05/10] efi/libstub: distinguish between native/mixed not
 32/64 bit
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Ard Biesheuvel <ardb@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-efi <linux-efi@vger.kernel.org>,
        Hans de Goede <hdegoede@redhat.com>,
        Matthew Garrett <matthewgarrett@google.com>,
        Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
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Sender: linux-kernel-owner@vger.kernel.org
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On Sat, 14 Dec 2019 at 22:17, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Sat, Dec 14, 2019 at 08:27:50PM +0000, Ard Biesheuvel wrote:
> > On Sat, 14 Dec 2019 at 21:13, Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > >
> > > On Sat, Dec 14, 2019 at 07:54:25PM +0000, Ard Biesheuvel wrote:
> > > > On Sat, 14 Dec 2019 at 20:49, Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > > > >
> > > > > On Sat, Dec 14, 2019 at 02:46:27PM -0500, Arvind Sankar wrote:
> > > > > > On Sat, Dec 14, 2019 at 06:57:30PM +0100, Ard Biesheuvel wrote:
> > > > > > > +
> > > > > > > +#define efi_table_attr(table, attr, instance) ({                   \
> > > > > > > +   __typeof__(((table##_t *)0)->attr) __ret;                       \
> > > > > > > +   if (efi_is_native()) {                                          \
> > > > > > > +           __ret = ((table##_t *)instance)->attr;                  \
> > > > > > > +   } else {                                                        \
> > > > > > > +           __typeof__(((table##_32_t *)0)->attr) at;               \
> > > > > > > +           at = (((table##_32_t *)(unsigned long)instance)->attr); \
> > > > > > > +           __ret = (__typeof__(__ret))(unsigned long)at;           \
> > > > > > > +   }                                                               \
> > > > > > > +   __ret;                                                          \
> > > > > > > +})
> > > > > >
> > Yes. I'm open to suggestions on how to improve this, but mixed mode is
> > somewhat of a maintenance burden, so if new future functionality needs
> > to leave mixed mode behind, I'm not too bothered.
> >
>
> Maybe just do
>         if (sizeof(at) < sizeof(__ret))
>                 __ret = (__typeof__(__ret))(uintptr_t)at;
>         else
>                 __ret = (__typeof__(__ret))at;
> That should cover most of the cases.

But the compiler will still be unhappy about the else clause if __ret
is a pointer type, since we'll be casting an u32 to a pointer,