Re: Relocation fixup during AARCH64 SEC PE-COFF generation?

[Achin Gupta <achin.gupta@arm.com>]

On Tue, Sep 13, 2016 at 04:25:32PM +0100, Ard Biesheuvel wrote:
> On 13 September 2016 at 16:16, Achin Gupta <achin.gupta@arm.com> wrote:
> > On Tue, Sep 13, 2016 at 03:43:41PM +0100, Ard Biesheuvel wrote:
> >> On 13 September 2016 at 15:12, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> >> > On 13 September 2016 at 15:03, Achin Gupta <achin.gupta@arm.com> wrote:
> >> >> Hi All,
> >> >>
> >> >> Upon entry into UEFI, the ArmPlatformPkg/PrePi/PeiUniCore.inf SEC module
> >> >> executes directly from within the firmware volume. The FV would typically be
> >> >> loaded in DRAM by ARM Trusted Firmware. The rule in ArmJuno.fdf for SEC file
> >> >> types converts a PE-COFF module into a stripped Terse executable as follows:
> >> >>
> >> >> [Rule.AARCH64.SEC]
> >> >>   FILE SEC = $(NAMED_GUID) RELOCS_STRIPPED FIXED {
> >> >>     TE  TE    Align = Auto              $(INF_OUTPUT)/$(MODULE_NAME).efi
> >> >>   }
> >> >>
> >> >> The GCC_ARM_AARCH64_DLINK_COMMON variable includes the "--emit-relocs" option
> >> >> when the ELF image is generated. Since the SEC module is able to "XIP" from the
> >> >> within FV, this means that the relocations have to be fixed up at link time
> >> >> before the TE image is created.
> >> >>
> >> >
> >> > Correct. GenFv relocates the image based on the runtime address of the
> >> > firmware volume.
> >> >
> >> >> It seems to me that in GenFw.c, at least the static relocations are fixed up
> >> >> when an ELF image is converted to PE-COFF. Can someone confirm if I am correct
> >> >> in understanding that dynamic relocations will be left as is in the PE-COFF
> >> >> image while static relocations will be fixed up?
> >> >>
> >> >
> >> > We usually don't emit dynamic relocations, since we are not linking
> >> > shared objects.
> >>
> >> ... or more specifically, all absolute static relocations are
> >> converted into PE/COFF base relocations, all relative static
> >> relocations are validated against the PE/COFF layout (which we keep
> >> 1:1 identical with the ELF section layout), and all dynamic
> >> relocations are ignored.
> >
> > Thanks Ard. I expect the relative static relocations to be fixed by the PE-COFF
> > loader at runtime.
> >
>
> No. The PE/COFF loader only processes base relocations, which are
> essentially pointers into the image where absolute addresses are
> stored, and which need to have an offset applied based on the
> difference between the link time base address and the load time base
> address. Like in ELF, no relative dynamic relocations exist.
>
> Only because of the ELF to PE/COFF conversion, we have to ensure that
> any relative references are still valid after the conversion.

Obviously I am not well versed with the correct terminology :(

I guess we are both talking about the base relocations done in
PeCoffLoaderRelocateImage() (MdePkg/Library/BasePeCoffLib/BasePeCoff.c) i.e. the
EFI_IMAGE_REL_BASED_* types defined in EfiImage.h?

>
> >> Note that this implies that proxy generating
> >> static relocations are rejected, since --emit-relocs does not produce
> >> any output for the proxies.
> >
> > You lost me here. What do you mean by "proxy generating static relocations"?
> >
>
> Basically, GOT based relocations that result in a GOT entry to be
> emitted when linking a shared object. Since those relocations are
> essentially instructions to the linker to emit such a GOT entry, the
> absolute relocation itself which causes the GOT entry to hold the
> correct value at runtime is not present in the object file, will hence
> not be emitted by --emit-relocs, and so will not have a corresponding
> PE/COFF base relocation emitted for it. This is the reason why we
> currently cannot support such GOT based relocations.

makes sense! Thanks for the clarification.

Thanks,
Achin


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