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[PATCH] Support SHF_GNU_RETAIN ELF section flag

Fangrui Song i@maskray.me
Wed Sep 23 23:29:43 GMT 2020 
Hi Jozef,

I saw your proposal https://sourceware.org/pipermail/gnu-gabi/2020q3/000429.html
I did not subscribe to gnu-gabi before yesterday so it is inconvenient for me to
reply there. 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?

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

(I find that gold has an internal error with such a relocation.)
But GNU ld should have been supported this for a very long time.

(I added these directives to llvm last year: https://reviews.llvm.org/D62014 )

---

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

* 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?
(Think how this works with __attribute__((section(...))); many asm streamers are
one-pass)

* linker
- What does -r do on two sections of the same, one with the flag and the other
without? (as HJ mentioned)
- Does the output section have the flag?
- Does the flag retain other sections in the same section group?


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.
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