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SubjectRe: [PATCH 03/10] efi/libstub: use a helper to iterate over a EFI handle array
On Sat, Dec 14, 2019 at 09:04:10PM +0000, Ard Biesheuvel wrote:
> On Sat, 14 Dec 2019 at 21:40, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> >
> > On Sat, 14 Dec 2019 at 21:33, Arvind Sankar <nivedita@alum.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@kernel.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)
>
> How about
>
> #define efi_get_handle_at(array, idx) \
> (efi_is_64bit() ? (efi_handle_t)(unsigned long)((u64 *)(array))[idx] \
> : (efi_handle_t)(unsigned long)((u32 *)(array))[i])
>
>
> #define efi_get_handle_num(size) \
> ((size) / (efi_is_64bit() ? sizeof(u64) : sizeof(u32)))
>
> #define for_each_efi_handle(handle, array, size, i) \
> for (i = 0; \
> i < efi_get_handle_num(size) && \
> ((handle = efi_get_handle_at((array), i)) || true); \
> i++)

Heh, I came up with almost the same thing, but yours is slightly better,
You have a typo in efi_get_handle_at (i instead of idx in the second
line).

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