Re: EFI: Stash ROMs if they're not in the PCI BAR

[David Woodhouse <dwmw2-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

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On Mon, 2013-03-18 at 16:02 +0000, Matthew Garrett wrote:
> On Mon, 2013-03-18 at 11:36 +0000, Matt Fleming wrote:
> > On Fri, 2013-03-15 at 08:57 +0000, David Woodhouse wrote:
> > > True. It probably doesn't *matter* because the size is zero so the
> > > firmware is just going to ignore the pointer anyway. Although in that
> > > case I wonder why we couldn't have just passed NULL. Perhaps we expected
> > > that some firmware might do some validation on the pointer before
> > > getting to the size check?
> > 
> > I doubt that the firmware checks the validity of pci_handle when size is
> > zero, and I agree it's worth passing NULL to silence the warning (which
> > is also more explicit that just initialising pci_handle), unless Matthew
> > knows of a reason we shouldn't do that?
> 
> No reason I can think of, and any failure will be pretty immediately
> obvious.

I agree with Matthew that I can think of no reason why a sane
implementation of EFI would ever actually dereference the pointer in
that case¹.

But there's an important difference in the way I phrased it...

I can't see why a *sane* implementation of EFI would do a lot of things.
Many of which have actually been seen in the field, much to our
distress.

So I'm starting to to think "hm, maybe we should give it a valid pointer
anyway, just in case".

Remember: Any sufficiently advanced incompetence is indistinguishable
from malice.

Assuming malice on the part of firmware authors is a fairly good way to
model their potential behaviour. So I'm perfectly prepared that someone
would ship an implementation which does *no* input checking, ever.
Except for gratuitously checking that particular pointer for NULL and
bailing out with an 'invalid parameters' error, even if the length was
zero.

-- 
dwmw2

¹ Although I haven't actually checked the spec to see if the pointer is
  required to be valid even when length is zero.



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