Re: [RFC] Symbol meta-information ELF extension

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Tue, Feb 18, 2020 at 10:54 AM Jozef Lawrynowicz
<jozef.l@mittosystems.com> wrote:
>
> On Tue, 18 Feb 2020 06:50:58 -0800
> "H.J. Lu" <hjl.tools@gmail.com> wrote:
>
> > On Tue, Feb 18, 2020 at 6:38 AM Jozef Lawrynowicz
> > <jozef.l@mittosystems.com> wrote:
> > >
> > > On Tue, 18 Feb 2020 05:58:34 -0800
> > > "H.J. Lu" <hjl.tools@gmail.com> wrote:
> > >
> > > > On Tue, Feb 18, 2020 at 5:47 AM Jozef Lawrynowicz
> > > > <jozef.l@mittosystems.com> wrote:
> > > > >
> > > > > On Tue, 18 Feb 2020 04:58:02 -0800
> > > > > "H.J. Lu" <hjl.tools@gmail.com> wrote:
> > > > >
> > > > > > On Tue, Feb 18, 2020 at 2:26 AM Jozef Lawrynowicz
> > > > > > <jozef.l@mittosystems.com> wrote:
> > > > > > >
> > > > > > > Hi,
> > > > > > >
> > > > > > > I've been working with Texas Instruments to develop an ELF extension which
> > > > > > > allows additional information about symbols ("symbol meta-information") to
> > > > > > > be stored in ELF files, in a section called .symtab_meta.
> > > > > > >
> > > > > > > The aim of symbol meta-information is to provide a extensible format for
> > > > > > > propagating additional information about symbols from the source code through to
> > > > > > > the link stage.
> > > > > > >
> > > > > > > I hope I can get some feedback on this proposal, and its current implementation,
> > > > > > > from the upstream community so I can make any adjustments required for the
> > > > > > > eventual upstreaming of this feature.
> > > > > >
> > > > > > Have you looked at Solaris syminfo section
> > > > > >
> > > > > > https://docs.oracle.com/cd/E26502_01/html/E26507/chapter7-17.html
> > > > > >
> > > > > > Can you extend it to do what you want?
> > > > > >
> > > > >
> > > > > It looks like there is only some minimal support for the Syminfo section in
> > > > > Binutils. The support is only there so that readelf can dump information about
> > > > > the Syminfo section from an object file.
> > > > >
> > > > > There's no implementation to enable the creation of a Syminfo section with
> > > > > user-specified flags for certain symbols, nor are there any hooks into any
> > > > > other Binutils programs, so I can't see any advantage to try and leverage that
> > > > > existing functionality.
> > > >
> > > > syminfo is a very useful ELF extension.  My question is if binutils supports
> > > > syminfo, can it be extended to meet your need?
> > > >
> > > >
> > >
> > > Well, a syminfo entry has a field to store flags which describes the extra
> > > information about the symbol, similar to the "kind" field of a symbol
> > > meta-information entry.
> > >
> > > typedef struct {
> > >         Elf64_Half      si_boundto;
> > >         Elf64_Half      si_flags;
> > > } Elf64_Syminfo;
> > >
> > > Meanwhile, syminfo augments the si_flags value by using si_boundto to point
> > > to an index in the .dynamic section. This is similar to how the "value" field of
> > > a symbol meta-information entry augments its "kind" field, but this
> > > functionality must work in static executables which do not have a .dynamic
> > > section.
> > >
> > > So one way to extend syminfo to meet the general need of this proposed symbol
> > > meta-information functionality then we would need a new field to store the
> > > value associated with the "si_flags".
> > >
> > > typedef struct {
> > >         Elf64_Half      si_boundto;
> > >         Elf64_Half      si_flags;
> > >         Elf64_Addr      si_value;
> > > } Elf64_Syminfo_new;
> >
> > You should start a discussion to extend ELF symbol info at
> >
> > https://groups.google.com/forum/#!forum/generic-abi
>
> Thanks for the suggestion. I'll work with TI to get something posted there soon.
>
> Can you clarify whether you still think this would work best as an extension to
> Solaris Syminfo?
>
> To be honest I have a problem with that given that Syminfo is a target specific
> extension to ELF (which just happens to be implemented in a generic way in
> Readelf), and we want to add symbol meta-information as a generic extension
> to ELF.
>
> There's no advantage to building upon Syminfo in Binutils given the
> implementation is limited to readelf dumping the Syminfo table.
>
> If Syminfo was already part of the gABI I would be on board with working from
> that to add the new functionality.
>
> I guess the other option is to take Syminfo, extend it with the new
> functionality we want, then propose it as generic ELF extension. I wonder if
> there would be legal issues doing all that without any input from Oracle.
>
> Do they "own" the spec? Would I be able to propose their spec with some
> extensions to ELF gABI? (These are somewhat rhetorical questions...)

https://groups.google.com/forum/#!forum/generic-abi

is the place to discuss ELF gABI.


-- 
H.J.