[PATCH] elf/x86-64: Subtract __ImageBase for R_AMD64_IMAGEBASE

[H.J. Lu]

On Tue, Feb 23, 2021 at 6:10 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 23.02.2021 14:31, H.J. Lu wrote:
> > On Tue, Feb 23, 2021 at 5:15 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 23.02.2021 13:15, H.J. Lu via Binutils wrote:
> >>> When linking PE/x86-64 inputs to generate ELF executable, we need to
> >>> subtract __ImageBase, aka __executable_start, for R_AMD64_IMAGEBASE
> >>> relocation.
> >>
> >> Do you really mean PE/x86-64, not the respective COFF objects? In the
> >
> > What is the difference?  How should I call object files generated by
> > Microsoft Visual Studio?
>
> The call them COFF themselves, I believe.

I have an impression that Windows relocations are different from regular
COFF relocations.  coff-x86_64.c has COFF_WITH_PE in many places.
My patch is only for Windows/PE:

&& strcmp (abfd->xvec->name, "pe-x86-64") == 0

> >> description this may not matter much, but since there's at least one
> >> code comment also saying so, which may confuse readers, I'd like to
> >> understand this.
> >>
> >> Also, in how far is this x86-64-specific?
> >
> > I don't know.
> >
> >>> --- a/ld/testsuite/ld-x86-64/pe-x86-64-1.od
> >>> +++ b/ld/testsuite/ld-x86-64/pe-x86-64-1.od
> >>> @@ -2,6 +2,7 @@
> >>>  .*: +file format .*
> >>>
> >>>  SYMBOL TABLE:
> >>> +0+400000 g       .text\$mn   0000000000000000 __executable_start
> >>>  0+401000 g       .text\$mn   0000000000000000 getaddr1
> >>>  0+401020 g       .text\$mn   0000000000000000 begin
> >>>  0+403014 g       .bss        0000000000000000 __bss_start
> >>
> >> I don't think this artifical symbol should appear in the symbol
> >
> > This symbol is generated by linker.  It should be harmless.
>
> According to my experience, stray symbols can be confusing or worse.

__executable_start is well defined by linker.

> >> table. I also wonder what the consequences are if a set of linked
> >> object files specifies a symbol of this name.
> >
> > __executable_start is generated by linker.  If __ImageBase is
> > defined to another value, that value will be used as image base.
>
> So if I have an object defining __executable_start, which of the
> two will relocations be resolved against?
>

Your __executable_start will be used.

-- 
H.J.