[Linux-ia64] Question about .opd section

[Linux-ia64] Question about .opd section

[Wichmann, Mats D]

The gnu toolchain (well, binutils) emits a .opd section
for  architectures which use function descriptors -
it's not unique to Itanium.

But I can't find much information about it.  Google
has been unhelpful (to me, anyway) on this one. It's
described as holding function descriptors, but I'm not 
clear who uses this information - is this used at
runtime?

Is this section considered normal/required for
Linux/ia64?  I'm asking because it wasn't in the
Itanium psABI, and thus didn't make it into the
current version of the Itanium LSB spec; our
application checker tool is flagging this as an
unknown section.

Thanks,

Mats

Re: [Linux-ia64] Question about .opd section

[David Mosberger]

>>>>> On Mon, 3 Feb 2003 08:37:48 -0800, "Wichmann, Mats D" <mats.d.wichmann@intel.com> said:

  Mats> The gnu toolchain (well, binutils) emits a .opd section
  Mats> for  architectures which use function descriptors -
  Mats> it's not unique to Itanium.

As far as I know, this is an HPism (originally coming from HP-UX
PA-RISC).  "opd" stands for "official procedure descriptor".  the
"official" is in the sense that the address of each descriptor serves
as the canonical "address" of the function it's describing; i.e., this
is what makes function-pointer comparisons work.

  Mats> But I can't find much information about it.  Google has been
  Mats> unhelpful (to me, anyway) on this one. It's described as
  Mats> holding function descriptors, but I'm not clear who uses this
  Mats> information - is this used at runtime?

Nothing at run-time directly references the .opd section, but of
course whenever you call a function through a pointer, you'll end up
loading the function's global-pointer and it's entry point from a
descriptor.  The main program's descriptor are stored in .opd (for
shared objects, the function descriptors are created at runtime, by
the runtime loader).

  Mats> Is this section considered normal/required for
  Mats> Linux/ia64?  I'm asking because it wasn't in the
  Mats> Itanium psABI, and thus didn't make it into the
  Mats> current version of the Itanium LSB spec; our
  Mats> application checker tool is flagging this as an
  Mats> unknown section.

It might be good to document it in the LSB.  There might be tools out
there that assume that .opd contains nothing but function descriptors,
so if someone put something else in there, things might go wrong badly
(which reeminds me: the ptrace-support in libunwind accesses .opd as a
fallback-mechanism to determine the global-pointer of the main
program; so there is at least one library out there that would break
if .opd contained other stuff).

	--david

Re: [Linux-ia64] Question about .opd section

[Jim Wilson]

binutils@sources.redhat.com is a better place to ask questions like this.
This is where the binutils experts hang out.

I found a comment in the IA-64 bfd support that says .opd contains "official
procedure descriptors".  I don't know where the terminology came from though.

The function descriptors are required for executing IA-64 code, so this is
definitely a normal section when using the GNU tools.  The section is
required if present.  The section is created by the linker, if one of the
object files being linked uses a relocation type that requires that a
function descriptor be created, e.g. LTOFF_FPTR22.  The 2-word function
descriptor goes in the .opd section, the address of the descriptor goes in
the GOT, and the gcc generated code loads the address from the GOT.

I don't have easy access to any other IA-64 compiler, so I can't comment on
how other compilers handle this.

Jim