[PATCH] Support SHF_GNU_RETAIN ELF section flag

[H.J. Lu]

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
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.