From: Fangrui Song <i@maskray.me>
To: jozef.l@mittosystems.com
Cc: Michael Matz <matz@suse.de>, Binutils <binutils@sourceware.org>,
"H.J. Lu" <hjl.tools@gmail.com>,
ccoutant@gmail.com
Subject: Re: [PATCH] Support SHF_GNU_RETAIN ELF section flag
Date: Wed, 23 Sep 2020 16:29:43 -0700 [thread overview]
Message-ID: <20200923232943.kasbrmqtpone4yi7@gmail.com> (raw)
In-Reply-To: <CAMe9rOoahsvDNv_DxEH2hihzR9dVk7q9aF=nc732cCmV5niSkA@mail.gmail.com>
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.
next prev parent reply other threads:[~2020-09-23 23:29 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-22 20:29 Jozef Lawrynowicz
2020-09-22 23:24 ` Hans-Peter Nilsson
2020-09-22 23:58 ` H.J. Lu
2020-09-23 1:09 ` Fangrui Song
2020-09-23 9:58 ` Jozef Lawrynowicz
2020-09-23 13:39 ` H.J. Lu
2020-09-23 13:51 ` Michael Matz
2020-09-23 16:52 ` Jozef Lawrynowicz
2020-09-23 17:13 ` H.J. Lu
2020-09-23 18:47 ` Jozef Lawrynowicz
2020-09-23 19:03 ` H.J. Lu
2020-09-23 20:04 ` Jozef Lawrynowicz
2020-09-23 20:17 ` H.J. Lu
2020-09-23 23:29 ` Fangrui Song [this message]
2020-09-24 11:39 ` Jozef Lawrynowicz
2020-09-24 19:06 ` Fangrui Song
2020-09-24 13:27 ` Michael Matz
2020-09-24 13:18 ` H.J. Lu
2020-09-24 13:49 ` Jozef Lawrynowicz
2020-09-24 13:59 ` H.J. Lu
2020-09-24 16:56 ` Jozef Lawrynowicz
2020-09-24 17:04 ` H.J. Lu
2020-09-24 17:18 ` Jozef Lawrynowicz
2020-09-24 17:37 ` H.J. Lu
2020-09-23 12:13 ` Jozef Lawrynowicz
2020-09-23 13:59 ` Alan Modra
2020-09-23 16:54 ` Jozef Lawrynowicz
2020-09-28 11:35 ` Pedro Alves
2020-09-28 12:28 ` Jozef Lawrynowicz
2020-09-28 14:46 ` Pedro Alves
2020-09-29 13:18 ` Michael Matz
2020-09-29 13:22 ` Jozef Lawrynowicz
2020-09-29 13:48 ` Pedro Alves
2020-09-29 13:55 ` Michael Matz
2020-09-29 14:04 ` Pedro Alves
2020-09-29 14:10 ` Michael Matz
2020-09-29 14:11 ` Pedro Alves
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