On 16 July 2014 23:03, Mark Salter <msalter@xxxxxxxxxx> wrote: > On Wed, 2014-07-16 at 22:38 +0200, Ard Biesheuvel wrote: >> On 16 July 2014 21:45, Mark Salter <msalter@xxxxxxxxxx> 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 > Am I just missing something here? Your "arm64/efi: efistub: get text > offset and image size from the Image header" patch makes no sense if we > can't rely on header being there. > No, you're right, I reversed my position after doing that work and digging into the details. I will drop those patches and proposed this one instead. -- Ard. -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
