linux-kernel - Re: [PATCH 03/10] efi/libstub: use a helper to iterate over a EFI handle array

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Date:   Sat, 14 Dec 2019 16:07:57 -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 03/10] efi/libstub: use a helper to iterate over a EFI
 handle array

On Sat, Dec 14, 2019 at 08:40:57PM +0000, Ard Biesheuvel wrote:
> On Sat, 14 Dec 2019 at 21:33, Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >
> > On Sat, Dec 14, 2019 at 06:57:28PM +0100, Ard Biesheuvel wrote:
> > > Iterating over a EFI handle array is a bit finicky, since we have
> > > to take mixed mode into account, where handles are only 32-bit
> > > while the native efi_handle_t type is 64-bit.
> > >
> > > So introduce a helper, and replace the various occurrences of
> > > this pattern.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > > ---
> > >
> > > +#define for_each_efi_handle(handle, array, size, i)                  \
> > > +     for (i = 1, handle = efi_is_64bit()                             \
> > > +             ? (efi_handle_t)(unsigned long)((u64 *)(array))[0]      \
> > > +             : (efi_handle_t)(unsigned long)((u32 *)(array))[0];     \
> > > +         i++ <= (size) / (efi_is_64bit() ? sizeof(efi_handle_t)      \
> > > +                                          : sizeof(u32));            \
> > > +         handle = efi_is_64bit()                                     \
> > > +             ? (efi_handle_t)(unsigned long)((u64 *)(array))[i]      \
> > > +             : (efi_handle_t)(unsigned long)((u32 *)(array))[i])
> > > +
> > >  /*
> > >   * The UEFI spec and EDK2 reference implementation both define EFI_GUID as
> > >   * struct { u32 a; u16; b; u16 c; u8 d[8]; }; and so the implied alignment
> > > --
> > > 2.17.1
> > >
> >
> > This would access one past the array, no? Eg if the array has one
> > handle, i is incremented to 2 the first time the condition is checked,
> > then the loop increment will access array[2] before the condition is
> > checked again. There seem to be at least a couple of other for_each
> > macros that might have similar issues.
> >
> 
> Indeed.
> 
> > How about the below instead?
> >
> > #define for_each_efi_handle(handle, array, size, i)                     \
> >         for (i = 0;                                                     \
> >             (i < (size) / (efi_is_64bit() ? sizeof(efi_handle_t)        \
> >                                           : sizeof(u32))) &&            \
> >             ((handle = efi_is_64bit()                                   \
> >                 ? ((efi_handle_t *)(array))[i]                          \
> >                 : (efi_handle_t)(unsigned long)((u32 *)(array))[i]), 1);\
> >             i++)
> >
> 
> Yeah, that looks correct to me, but perhaps we can come up with
> something slightly more readable? :-)
> (Not saying my code was better in that respect)

:) The idiom of the && with , operator is copied from for_each_bvec in bvec.h.

Perhaps more readable with helper macros:

#define __efi_handle_size (efi_is_64bit() ? sizeof(efi_handle_t)	\
					  : sizeof(u32))
#define __efi_handle_elem(array, i)					\
	(efi_is_64bit() ? ((efi_handle_t *)(array))[i]			\
			: (efi_handle_t)(uintptr_t)((u32 *)(array))[i])

#define for_each_efi_handle(handle, array, size, i)			\
	for (i = 0; i < (size) / __efi_handle_size &&			\
		    (handle = __efi_handle_elem(array, i), 1); i++)