Re: [Xen-devel] [PATCH 3/3] x86/EFI: don't accept 64-bit base relocations on page tables

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.08.16 at 18:35, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 19/08/16 13:57, Jan Beulich wrote:
>>>>> On 19.08.16 at 14:39, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 19/08/16 08:52, Jan Beulich wrote:
>>>> Page tables get pre-populated with physical addresses which, due to
>>>> living inside the Xen image, will never exceed 32 bits in width. That
>>>> in turn results in the tool generating the relocations to produce
>>>> 32-bit relocations for them instead of the 64-bit ones needed for
>>>> relocating virtual addresses. Hence instead of special casing page
>>>> tables in the processing of 64-bit relocations, let's be more rigid
>>>> and refuse them (as being indicative of something else having gone
>>>> wrong in the build process).
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>> Is it an ABI requirement to use the minimal available relocation?  It is
>>> certainly suboptimal to use a 64bit relocation when a 32bit one would
>>> do, but I wouldn't bet that it is unconditional avoided by all toolchains.
>> What ABI? The tool in question is one of our own making. And the
>> way relocations get generated it's hard to tell those that have to
>> be 32-bit (in early boot code and trampoline code) from those that
>> may as well be 64-bit ones (in page tables).
>>
>>> It is currently the case that Xen needs to live below 4GB physical, so
>>> from that point of view a 64bit relocation will not be required in the
>>> pagetables.
>> And even if Xen didn't itself have this requirement, the EFI loader
>> would always put us below 4Gb.
> 
> Why is this necessarily true?
> 
> xen.efi is built as a 64bit PE, not 32bit.

The file format doesn't matter here. And we'd have other problems
to solve if we got loaded above 4Gb.

Jan


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