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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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).
> 
> Also a remark regarding the title - in my builds there already is
> a .reloc section in the ELF images, so "add" doesn't really seem
> correct to me. It sits right after .rodata, and I would it doesn't
> get folded into there because - for some reason - .rodata is
> actually marked writable.

AFAICT .rodata is marked writable because it contains .data.param and
.data.rel.ro. I'm unsure why we need .data.rel.ro, I would assume that
once the final binary has been linked .data.rel.ro would be empty,
since there's no run time linking or relocation as Xen is a standalone
binary.

Regarding .data.param it should be renamed to .rodata.param, and I
should take a look at why it's marked as 'WA' instead of 'A'.

Thanks, Roger.

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