linux-kernel - Re: [PATCH 05/10] efi/libstub: distinguish between native/mixed not 32/64 bit

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Message-ID: <20191214230204.GB314531@rani.riverdale.lan>
Date:   Sat, 14 Dec 2019 18:02:05 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Ard Biesheuvel <ardb@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 05/10] efi/libstub: distinguish between native/mixed not
 32/64 bit

On Sat, Dec 14, 2019 at 10:14:38PM +0000, Ard Biesheuvel wrote:
> On Sat, 14 Dec 2019 at 22:30, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> >
> > On Sat, 14 Dec 2019 at 22:17, Arvind Sankar <nivedita@...m.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@...m.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@...m.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,
> 
> I think the answer is to have efi_table_ptr() for pointers and
> efi_table_attr() for other types.

Using __builtin_choose_expr avoids the warning:
	__ret = (__typeof__(__ret))
		__builtin_choose_expr(sizeof(at) < sizeof(ret),
				      (uintptr_t)at, at);

But having different efi_table_ macros sounds cleaner.