[PATCH v2] arm64/efi: efistub: jump to 'stext' directly, not through the header

Roy Franz
2014-07-16T21:03:56Z

On Wed, Jul 16, 2014 at 1:38 PM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> On 16 July 2014 21:45, Mark Salter <msalter@redhat.com> wrote:
>> On Wed, 2014-07-16 at 16:53 +0100, Mark Rutland wrote:
>>> On Wed, Jul 16, 2014 at 03:51:37PM +0100, Mark Salter wrote:
>>> > On Tue, 2014-07-15 at 12:58 +0200, Ard Biesheuvel wrote:
>>> > > After the EFI stub has done its business, it jumps into the kernel by branching
>>> > > to offset #0 of the loaded Image, which is where it expects to find the header
>>> > > containing a 'branch to stext' instruction.
>>> > >
>>> > > However, the header is not covered by any PE/COFF section, so the header may
>>> > > not actually be loaded at the expected offset. So instead, jump to 'stext'
>>> > > directly, which is at the base of the PE/COFF .text section, by supplying a
>>> > > symbol 'stext_offset' to efi-entry.o which contains the relative offset of
>>> > > stext into the Image. Also replace other open coded calculations of the same
>>> > > value with a reference to 'stext_offset'
>>> >
>>> > Have you actually seen a situation where the header isn't there?
>>> > Isn't the kernel header actually part of the pe/coff file and
>>> > firmware loads the whole file into RAM?
>>>
>>> From my understanding of Ard's earlier comments, this part isn't
>>> guaranteed per the UEFI spec.
>>>
>>> I would rather we weren't relying on implementation details.
>>>
>>
>> Could be. I didn't see anything about it in the UEFI spec, but I
>> probably wasn't exhaustive in my search. In any case, there's at
>> least one other place broken if the kernel header isn't included
>> in the loaded image.
>>
>
> I have not been able to find anything in the PE/COFF documents that
> tells you what to put in memory areas that are not covered by a
> section. Expecting the header to be there is indeed relying on an
> implementation detail, which seems risky.
> And indeed, if there are any other (non EFI related) uses of header
> fields in the kernel, it would be good to have a look at those well,

I looked through the UEFI spec, and didn't see anything really helpful
in this regard. However the "ImageBase" address that is returned
by the loadedImageProtocol is the address of the header, and this
address is aligned as specified in the PE/COFF header. It would
be strange to specify the alignment of something that is not expected
to be loaded.
It's probably worth asking on the edk2 list regarding what the UEFI
specification requires.

Roy