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

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Message-ID: <CAKv+Gu_JQz=xd_UmqiuZ8TvA+ksT_rY4iXP_j7OdW4F5sfZt9g@mail.gmail.com>
Date:   Sat, 14 Dec 2019 19:54:25 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     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, 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;                                                          \
> > > +})
> >
> > The casting of `at' is appropriate if the attr is a pointer type which
> > needs to be zero-extended to 64-bit, but for other fields it is
> > unnecessary at best and possibly dangerous.  There are probably no
> > instances currently where it is called for a non-pointer field, but is
> > it possible to detect if the type is pointer and avoid the cast if not?
>
> To clarify, I mean the casting via `unsigned long' -- casting to type of
> __ret should be ok. We could also use uintptr_t for cleanliness when the
> cast is required?

Could you give an example of how it could break?