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Re: [PATCH 2/8] x86/EFI: sections may not live at VA 0 in PE binaries
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Wed, 21 Apr 2021 14:57:01 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Wed, 21 Apr 2021 12:57:17 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Wed, Apr 21, 2021 at 12:32:42PM +0200, Jan Beulich wrote:
> On 21.04.2021 10:52, Roger Pau Monné wrote:
> > On Thu, Apr 01, 2021 at 11:44:45AM +0200, Jan Beulich wrote:
> >> PE binaries specify section addresses by (32-bit) RVA. GNU ld up to at
> >> least 2.36 would silently truncate the (negative) difference when a
> >> section is placed below the image base. Such sections would also be
> >> wrongly placed ahead of all "normal" ones. Since, for the time being,
> >> we build xen.efi with --strip-debug anyway, .stab* can't appear. And
> >> .comment has an entry in /DISCARD/ already anyway in the EFI case.
> >>
> >> Because of their unclear origin, keep the directives for the ELF case
> >> though.
> >
> > It's my understadng thonse sections are only there for debug purposes,
> > and never part of the final xen binary as they are stripped?
> >
> > Could we maybe remove the section load address of 0 and instead just
> > use the (NOLOAD) directive?
>
> (NOLOAD) is meaningless for PE.
>
> > Does it really matter to place them at address 0?
>
> That's the convention for ELF, and also what ld defaults to for debugging
> sections.
>
> > I also wonder, is this change fixing some existing bug, or it's just a
> > cleanup change?
>
> If there were sections at 0, the resulting PE binary would end up broken.
> Prior to binutils 2.37 this brokenness is silent, i.e. the linker doesn't
> issue any form of diagnostic. The change therefore is addressing a latent
> issue - if any such section started being non-empty, we'd be in trouble.
>
> > I also only see the .comment section in my binary output, so maybe
> > it's fine to just remove them from the script?
>
> Which binary are you referring to - ELF or PE? In the former case, yes,
> that's what the statement is for. In the latter case I can't see how this
> would be, with .comment being explicitly part of /DISCARD/ in that case.
So from a bit of searching I just did it seems like stab sections
where used during the 90s with ELF, but that this has long been
superseded by DWARF 2 becoming the default in the late 90s, hence I
think it would be fine to just remove those sections even in the ELF
case?
Thanks, Roger.
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