Re: xen/link: Move .data.rel.ro sections into .rodata for final link

[David Woodhouse <dwmw2@infradead.org>]

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On Sun, 2017-07-30 at 00:16 -0600, Jan Beulich wrote:
> > 
> > > 
> > > > 
> > > > David Woodhouse <dwmw2@infradead.org> 07/20/17 5:22 PM >>>
> > This includes stuff lke the hypercall tables which we really want
> > to be read-only. And they were going into .data.read-mostly.
> Yes, we'd like them to be read-only, but what if EFI properly assigned r/o
> permissions to the .rodata section when loading xen.efi? We'd then be
> unable to apply relocations when switching from 1:1 to virtual mappings
> (see efi_arch_relocate_image()).


FWIW it does look like TianoCore has gained the ability to mark
sections as read-only, in January of this year:
https://github.com/tianocore/edk2/commit/d0e92aad46

It doesn't actually seem to be complete — even with subsequent fixes
since that commit, it doesn't look like it catches the case of data
sections without EFI_IMAGE_SCN_MEM_WRITE, such as .rodata. 

And even if/when that gets fixed you'll note that the protection is
deliberately torn down in ExitBootServices(), specifically for the case
you're concerned about below — because you'll need to do the
relocations.

So I don't think there should be a problem here.

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