Re: [Xen-devel] [PATCH 3/4] x86/linker: add a reloc section to ELF binary

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.06.19 at 10:10, <roger.pau@xxxxxxxxxx> wrote:
> On Mon, Jun 24, 2019 at 01:24:02PM +0200, Daniel Kiper wrote:
>> On Fri, Jun 21, 2019 at 12:34:13AM -0600, Jan Beulich wrote:
>> > >>> On 19.06.19 at 17:06, <roger.pau@xxxxxxxxxx> wrote:
>> > > On Wed, Jun 19, 2019 at 06:57:05AM -0600, Jan Beulich wrote:
>> > >> >>> On 19.06.19 at 13:02, <roger.pau@xxxxxxxxxx> wrote:
>> > >> > If the hypervisor has been built with EFI support (ie: multiboot2).
>> > >> > This allows to position the .reloc section correctly in the output
>> > >> > binary, or else the linker might place .reloc before the .text
>> > >> > section.
>> > >> >
>> > >> > Note that the .reloc section is moved before .bss for two reasons: in
>> > >> > order for the resulting binary to not contain any section with data
>> > >> > after .bss, so that the file size can be smaller than the loaded
>> > >> > memory size, and because the data it contains is read-only, so it
>> > >> > belongs with the other sections containing read-only data.
>> > >>
>> > >> While this may be fine for ELF, I'm afraid it would be calling for
>> > >> subtle issues with xen.efi (i.e. the PE binary): There a .reloc
>> > >> section is generally expected to come after "normal" data
>> > >> sections.
>> > >
>> > > OK, would you like me to leave the .reloc section at the previous
>> > > position for EFI builds then?
>> >
>> > Well, this part is a requirement, not a question of me liking you
>> > to do so.
>> >
>> > > Or do we prefer to leave .reloc orphaned in the ELF build?
>> >
>> > Daniel might have an opinion here with his plans to actually
>> > add relocations there in the non-linker-generated-PE build. I
>> > don't have a strong opinion either way, as long as the
>> > current method of building gets left as is (or even simplified).
>> 
>> I would not drop .reloc section from xen-syms because it can be useful
>> for "manual" EFI image relocs generation. However, I am not strongly
>> tied to it. If you wish to drop it go ahead. I can readd it latter if
>> I get back to my new PE build work.
> 
> Do you mean that the dummy .reloc section added to non-PE builds can
> be dropped? (ie: remove xen/arch/x86/efi/relocs-dummy.S from the build)

Given my earlier reply it's not clear to me what you mean by "remove"
here. As a result ...

> I'm slightly lost, .reloc begin a section that's explicitly added to
> non-PE builds by relocs-dummy.S I assumed it was needed for some
> reason.

... it's also not clear what exactly you mean here, and hence whether
there's any reason needed beyond the reference to the two bounding
symbols by efi_arch_relocate_image().

Jan



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