Re: [PATCH] Support SHF_GNU_RETAIN ELF section flag

[Jozef Lawrynowicz <jozef.l@mittosystems.com>]

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.

However, it would certainly be a useful enhancement for the linker, and
is something I would be interested in working on in the future, so I
appreciate the offer.

Thanks,
Jozef

> 
> > As an aside.. we would run into limitations in the ELF format if trying
> > to mark the symbol itself as "retained". You cannot define "retain" as a
> > new symbol type or binding, because there is not a one-to-one mapping of
> > "retain" to the existing symbol types and bindings i.e. a retained
> > symbol might be STT_OBJECT or STT_FUNC, or STB_LOCAL or STB_GLOBAL.
> > st_other could be another place to store the requirement to "retain" a
> > symbol, but all those available bits are used up (albeit some
> > unofficially).
> >
> > I'm not saying there's no way it could be done, but these are the
> > hurdles I discovered when considering that path to implementing the
> > "retain" behavior.
> >
> > > About the .retain syntactic sugar: I also think it's not necessary, the
> > > .section directive with R flag merging is good enough IMHO.
> >
> > I'm OK with removing the .retain directive, since there is some
> > consensus it is not necessary. Also, I thought it made life easier for
> > GCC to mark sections as retained by omitting the section name from the
> > directive, but I've discovered a way around that now.
> >
> > Thanks,
> > Jozef
> 
> 
> 
> -- 
> H.J.