>>> On 09.08.16 at 15:03, <jslaby@xxxxxxx> wrote:
> On 08/09/2016, 12:16 PM, Jan Beulich wrote:
>> While commit 55f1ea15216 ("efi: Fix for_each_efi_memory_desc_in_map()
>> for empty memmaps") made an attempt to deal with empty memory maps, it
>> didn't address the case where the desc_size field never gets set, as is
>> apparently the case when running under Xen.
>>
>> Reported-by: <lists@xxxxxxxxxxxx>
>> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>> Cc: Jiri Slaby <jslaby@xxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> Tested-by: <lists@xxxxxxxxxxxx>
>> ---
>> include/linux/efi.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> --- 4.8-rc1/include/linux/efi.h
>> +++ 4.8-rc1-EFI-memdesc-iterator-Xen/include/linux/efi.h
>> @@ -946,7 +946,7 @@ extern int efi_memattr_apply_permissions
>> /* Iterate through an efi_memory_map */
>> #define for_each_efi_memory_desc_in_map(m, md) \
>> for ((md) = (m)->map; \
>> - ((void *)(md) + (m)->desc_size) <= (m)->map_end; \
>> + ((void *)(md) + (m)->desc_size - 1) < (m)->map_end; \
>
> Is there any specific reason you change both the size and the comparator?
>
> IMO, either (readable)
> ((void *)(md) + (m)->desc_size) < (m)->map_end;
> or (mindfuck version)
> ((void *)(md) + (m)->desc_size - 1) <= (m)->map_end;
> is correct, not their mix.
We're not talking about an off-by-one getting fixed here: map_end
points past the valid range. The adjustment leverages overflow (or
underflow, to be precise) to produce correct behavior when
(m)->desc_size is zero.
Jan
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