Re: [PATCH 1/4] efi/libstub: Allocate headspace in efi_get_memory_map()

[Jeffrey Hugo <jhugo-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

On 7/18/2016 9:14 AM, Ard Biesheuvel wrote:
> On 18 July 2016 at 17:11, Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org> wrote:
>> On Mon, Jul 18, 2016 at 09:03:31AM -0600, Jeffrey Hugo wrote:
>>> On 7/18/2016 5:00 AM, Mark Rutland wrote:
>>>> Hi,
>>>>
>>>> On Sun, Jul 17, 2016 at 02:45:59PM -0600, Jeffrey Hugo wrote:
>>>>> +   if (status == EFI_BUFFER_TOO_SMALL ||
>>>>> +                           (*buff_size - *map_size) / sizeof(*m) < 8) {
>>>>>            efi_call_early(free_pool, m);
>>>>> +           /*
>>>>> +            * Make sure there is 8 entries worth of headroom so that the
>>>>> +            * buffer can be reused for a new map after allocations are
>>>>> +            * no longer permitted.  Its unlikely that the map will grow to
>>>>> +            * exceed this headroom once we are ready to trigger
>>>>> +            * ExitBootServices()
>>>>> +            */
>>>>> +           *map_size += sizeof(*m) * 8;
>>
>>>> As a general note, it's a shame that we don't know desc_size first time
>>>> through. If descs grow, 8 * sizeof(*m) might be too small. That's an
>>>> existing problem though, so I guess it makes sense to be consistent and
>>>> use sizeof(*m) here.
>>>
>>> Yep.  Unless I've missed something, it seems like the two options
>>> are either make a decent attempt to guess at desc_size the first
>>> time (current approach), or intentionally fail the first time just
>>> to get the size.  The current approach doesn't seem perfect, but it
>>> does have a chance at passing with just one attempt.  I couldn't see
>>> a better solution, but if someone else has an idea, I'd like to hear
>>> it.
>>
>> Sure, that was more of an observation than a critique. In the absence of
>> any suggestions better, sticking with sizeof(*m) makes sense.
>>
>> If it does become a problem, we're likely to have to fix up other sites
>> regardless.
>>
>
> It is worth noting that the EDK2 implementation deliberately uses a
> desc_size that exceeds the size of the type. So '8 * sizeof(*m)' is
> almost never going to be 8 slots' worth of headroom.
>

Good point, desc_size does exceed sizeof(*m) when I test an EDK2 
implementation.  I think I can rework things to use desc_size instead, 
so that we do get the expected 8 slots of headroom.  I'll incorporate 
that in V2.

-- 
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.