Re: [PATCH] Support SHF_GNU_RETAIN ELF section flag

[Michael Matz <matz@suse.de>]

Hello,

On Wed, 23 Sep 2020, Fangrui Song wrote:

> Since SHF_GNU_RETAIN is a new feature, and we already have facility for 
> making arbitrary sections alive with R_*_NONE, can you highlight the 
> selling point of a new flag?

I wouldn't call this a facility.  I call it a hack that accidentally works 
for this purpose.

> Copying me previous reply here
> > We already have a way to create an artificial reference:
> > 
> >   .reloc ., R_X86_64_NONE, target_symbol
> > 
> > If we allow a relocation number for the second operand
> > 
> >   .reloc ., 0, target_symbol
> > 
> > this will be generic. You can insert the directives in a GC root (e.g.
> > _start or a symbol referenced by -u or maybe an .init_array)
> 
> If you do not want to touch the section containing the -e (--entry) symbol,
> you
> can use:
> 
>   .section .init_array.1,"a",@init_array
>   .reloc ., R_X86_64_NONE, retained_section

And what makes .init_array be retained?  To which data item does the 
relocation apply, what if .init_array is empty?  I think using this to 
force sections be retained is a terrible idea (I was split minds about 
calling this idea cute vs. terrible ;-) ).

> For a new section flag, there are a bunch of things needing thoughts

I think the answer to each of these things is obvious:

> * assembler
> 
> The .retain directive seems to be discouraged... For section flags:
> 
> .section .foo,"a"
> .section .foo,"aR"        # is this a new section
> .pushsection .foo,"aR"    # is this a new section
> 
> Does the compiler need to remember that a section has the flag?

Why should it need to remember?  (E.g. GCC does, but why would it matter?)  
The assembler merges the flags, no need to remember things.

> * linker
> - What does -r do on two sections of the same, one with the flag and the other
> without? (as HJ mentioned)

It merges the flag with logical or.

> - Does the output section have the flag?

Yes.

> - Does the flag retain other sections in the same section group?

Yes.


Ciao,
Michael.

> 
> 
> On 2020-09-23, H.J. Lu via Binutils 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
> > 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.
>