Re: [PATCH] efi: stub: call get_memory_map() to obtain map and desc sizes

[Leif Lindholm <leif.lindholm-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On Fri, Jan 09, 2015 at 10:19:50AM +0000, Mark Rutland wrote:
> On Thu, Jan 08, 2015 at 07:09:22PM +0000, Ard Biesheuvel wrote:
> > On 8 January 2015 at 19:04, Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org> wrote:
> > > Hi Ard,
> > >
> > > On Thu, Jan 08, 2015 at 05:51:47PM +0000, Ard Biesheuvel wrote:
> > >> This fixes two minor issues in the implementation of get_memory_map():
> > >> - Currently, it assumes that sizeof(efi_memory_desc_t) == desc_size,
> > >>   which is usually true, but not mandated by the spec. (This was added
> > >>   intentionally to allow future additions to the definition of
> > >>   efi_memory_desc_t). The way the loop is implemented currently, the
> > >>   added slack space may be insufficient if desc_size is larger, which in
> > >>   some corner cases could result in the loop never terminating.
> > >> - It allocates 32 efi_memory_desc_t entries first (again, using the size
> > >>   of the struct instead of desc_size), and frees and reallocates if it
> > >>   turns out to be insufficient. Few implementations of UEFI have such small
> > >>   memory maps, which results in a unnecessary allocate/free pair on each
> > >>   invocation.
> > >>
> > >> Fix this by calling the get_memory_map() boot service first with a '0'
> > >> input value for map size to retrieve the map size and desc size from the
> > >> firmware and only then perform the allocation, using desc_size rather
> > >> than sizeof(efi_memory_desc_t).
> > >
> > > Is the desc_size guaranteed to be set up correctly if the size is too
> > > small?
> > >
> > > As far as I can see, for that case the spec only mandates that
> > > MemoryMapSize is updated nd EFI_BUFFER_TOO_SMALL is returned.
> > >
> > > It's not clear to me whether DescriptorSize or DescriptorVersion are
> > > initialised in cases other than success.
> > >
> > 
> > The way I read it, descriptor size and descriptor version are always
> > returned, e.g., as opposed to MapKey, which is only returned on
> > success, and the spec mentions that specifically.
> 
> I agree that that would be the sensible reading of the spec. I just fear
> that there's room for an implementor to read it slightly differently,
> and cause pain for us.

I disagree.
The intent is clear, and you can not follow the current text and
provide a version that does not do the right thing. Only the MapKey
return is conditional on success.
 
> > We could ask for clarification just to be sure.
> 
> I think we should.
> 
> Given it could take a while for any conclusion to be reached and
> published, I'm happy to go with the patch below in the meantime, so long
> as the issue gets raised.

We could ask for clarification of the spec, but I see no need to ask
for guidance on the current text.

/
    Leif