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

[Ard Biesheuvel]

On 17 July 2014 16:09, Mark Salter <msalter at redhat.com> wrote:
> On Wed, 2014-07-16 at 23:13 +0200, Ard Biesheuvel wrote:
>> On 16 July 2014 23:03, Mark Salter <msalter at redhat.com> wrote:
>> > On Wed, 2014-07-16 at 22:38 +0200, Ard Biesheuvel wrote:
>> >> On 16 July 2014 21:45, Mark Salter <msalter at 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 think we need to come up with a loader which does load an image
>> > without kernel header so that we can test. Otherwise, we'll probably
>> > end up with buggy code anyway. The stub code assumes the the loaded
>> > image pointed to by the system table is the whole image. Seems like
>> > we'd need to add code to determine if it is whole kernel image or
>> > image without initial header. Stub would have to handle both cases.
>> > For instance, one case would want image placed at 2MiB+TEXT_OFFSET,
>> > other case would want 2MiB+TEXT_OFFSET+sizeof(kernel header).
>> >
>>
>> No, this has nothing to do with misaligned data.
>>
>> The PE/COFF .text section does not start at virtual offset #0 but at
>> virtual offset 'stext - efi_head'.
>> In other words, there is a hole in the virtual image where the header
>> is supposed to be.
>> So if there is no PE/COFF section describing what data should be put
>> at offset #0 by the loader, we can't assume the header is there, even
>> if ImageBase does start at #0
>
> I get that. You're supposing UEFI will always allocate memory for the
> full image, but only sometimes copy the PE/COFF headers. I can see your
> point from a PE/COFF perspective, but not so much from the UEFI spec
> perspective where the language leads me to think it treats the PE/COFF
> images as one unit wrt loading. In any case, it really isn't worth
> arguing about. I don't have any objection to the patch since it won't
> break anything from my perspective and it'll protect against breakage
> which could possibly occur with some firmware implementations.
>

I am reviving this old thread because it appears we may have seen an
issue involving shim and GRUB where data not covered by any loadable
PE/COFF section was not actually loaded to memory. In this case, it
was the ".reloc" section, not the header but the conclusion should be
the same.

@Peter: this is a second-hand account so perhaps you could fill in
with some details? Original thread is here:
http://marc.info/?l=linux-arm-kernel&m=140542202520933&w=2

-- 
Ard.