[PATCH] Support SHF_GNU_RETAIN ELF section flag

[Jozef Lawrynowicz]

On Thu, Sep 24, 2020 at 06:18:05AM -0700, H.J. Lu via Binutils wrote:
> On Wed, Sep 23, 2020 at 1:17 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Wed, Sep 23, 2020 at 1:04 PM Jozef Lawrynowicz
> > <jozef.l@mittosystems.com> wrote:
> > >
> > > On Wed, Sep 23, 2020 at 12:03:28PM -0700, H.J. Lu via Binutils wrote:
> > > > On Wed, Sep 23, 2020 at 11:47 AM Jozef Lawrynowicz
> > > > <jozef.l@mittosystems.com> wrote:
> > > > >
> > > > > On Wed, Sep 23, 2020 at 10:13:37AM -0700, H.J. Lu via Binutils wrote:
> > > > > > On Wed, Sep 23, 2020 at 9:52 AM Jozef Lawrynowicz
> > > > > > <jozef.l@mittosystems.com> wrote:
> > > > > > >
> > > > > > > On Wed, Sep 23, 2020 at 01:51:56PM +0000, Michael Matz wrote:
> > > > > > > > Hello,
> > > > > > > >
> > > > > > > > On Wed, 23 Sep 2020, H.J. Lu via Binutils wrote:
> > > > > > > >
> > > > > > > > > > I think that:
> > > > > > > > > >
> > > > > > > > > > >  .section .text,"ax"
> > > > > > > > > > >    ...
> > > > > > > > > > >  foo:
> > > > > > > > > > >    ...
> > > > > > > > > > >  .retain
> > > > > > > > > > >  retained_fn:
> > > > > > > > > > >    ...
> > > > > > > > > >
> > > > > > > > > > is some nice syntactic sugar compared to:
> > > > > > > > > >
> > > > > > > > > > >  .section .text,"ax"
> > > > > > > > > > >    ...
> > > > > > > > > > >  foo:
> > > > > > > > > > >    ...
> > > > > > > > > > >  .section .text,"axR"
> > > > > > > > > > >  retained_fn:
> > > > > > > > > > >    ...
> > > > > > > > > >
> > > > > > > > > > It's also partly for convenience; we have other directives which are
> > > > > > > > > > synonyms or short-hand for each other.
> > > > > > > > > >
> > > > > > > > >
> > > > > > > > > You don't need to keep the whole section when only one symbol should
> > > > > > > > > be kept.  Please drop the .retain directive.  GCC, as and ld should do the
> > > > > > > > > right thing with
> > > > > > > > >
> > > > > > > > > .section .text,"ax"
> > > > > > > > >    ...
> > > > > > > > > foo:
> > > > > > > > >   ...
> > > > > > > > >  .section .text,"axR"
> > > > > > > > >
> > > > > > > > >  retained_fn:
> > > > > > > > >
> > > > > > > > > where foo can be dropped and retained_fn will be kept.
> > > > > > > >
> > > > > > > > This is not what we discussed at the ABI list, the flag is per section, so
> > > > > > > > either the whole section is retained or not.  What you describe is
> > > > > > > > something else that would work on a per symbol basis, which would have to
> > > > > > > > be specified in a different way and might or might not be a good idea.
> > > > > > > > But let's not conflate these two.
> > > > > > >
> > > > > > > Also, the linker cannot currently dissect a section and remove a
> > > > > > > particular unused symbol anyway. Since garbage collection only operates
> > > > > > > on the section level, marking the section itself as "retained" seems
> > > > > > > most appropriate.
> > > > > >
> > > > > > It can be done.  If you put your branch on
> > > > > >
> > > > > > https://gitlab.com/x86-binutils/binutils-gdb
> > > > > >
> > > > > > I can help you implement it.
> > > > >
> > > > > It's not something I have time to look into at the moment, for now the
> > > > > aim is just to prevent garbage collection of sections.
> > > >
> > > > Linker and assembler already support it.   You just need to add SHF_GNU_RETAIN
> > > > to the framework.  Check how SHF_GNU_MBIND works.
> > >
> > > Sorry, I don't understand.
> > >
> > > Are you saying that LD already supports the garbage collection of
> > > individual unused symbol definitions from input sections? Whilst
> > > retaining other symbol definitions which are required by the program?
> > > I cannot find any reference to this.
> > >
> > > How does that relate to SHF_GNU_MBIND? I looked at all the references
> > > to "mbind" in Binutils and nothing seemed related garbage collection of
> > > sections, since SHF_GNU_MBIND is just used to indicate a particular
> > > section should be placed in a special memory area.
> >
> > For
> >
> > section .text,"ax"
> >    ...
> > foo:
> >   ...
> >  .section .text,"axR"
> > retained_fn:
> >
> > you need to create a new .text section with SHF_GNU_RETAIN for
> > retained_fn.   See get_section in obj-elf.c.  If you want to avoid
> 
> See users/hjl/elf/master branch at:
> 
> https://gitlab.com/x86-binutils/binutils-gdb/-/commits/users/hjl/elf/master
> 
> I removed the .retain directive.

Thanks, the formalization of section flag merging in the assembler is
nice.

My only comment is that I used the "STT_*" syntax in the .type directive
instead of % because some targets didn't like the % syntax and emmited
an error. I don't remember which, maybe it was mmix-elf, in which case
we don't care :)

When I re-test I'll see if the errors pop up again.

I still need to add the GNU OSABI handling, but with that and your
patches we are probably ready to apply after a final official sign-off.

How would that work logistically, I apply my patch and you apply yours
after? Or do we align off-master and apply as one patch?

Thanks,
Jozef

> 
> > merging .text section with SHF_GNU_RETAIN with other .text
> > sections by ld -r, linker needs to distinguish sections of the
> > same name with and without SHF_GNU_RETAIN.
> >
> > --
> > H.J.
> 
> 
> 
> -- 
> H.J.