[PATCH v2] Support for SHF_GNU_RETAIN ELF Section Flag

[Fangrui Song]

On 2020-09-30, Jozef Lawrynowicz wrote:
>On Tue, Sep 29, 2020 at 05:10:09PM -0700, Roland McGrath wrote:
>> Since group semantics mean that if any section in a group is retained the
>> whole group must be retained, it seems to me that making the feature a
>> group flag rather than a section flag makes sense. I don't think it really
>> has any downside. It's a little bit more structure to generate a group but
>> everything already knows how to do it so that should be no problem.  While
>> it is true that multiple sections with the same name and different
>> attributes should already be fine, the place where multiple different
>> sections of the same name are usually seen today is when each different
>> section reusing a name is in a different group from the other sections with
>> the same name.
>>
>
>Let's assume:
>  GRP_RETAIN
>    Sections in a group with this flag set should always be included in
>    the linked object, even if they appear unused.
>
>One issue I have with the group method is conceptual; groups are supposed
>to be used to group related sections. They are related because of their
>meaning from the application perspective, not because of their metadata
>(i.e. sections to be retained are related by their metadata because they
>all should be retained).
>
>Just because groups have a side effect that can be leveraged to
>get a section to be retained in the final link, why does that mean we
>need to leverage that property when it it's not actually the simplest or
>most obvious way to implement the new behavior?
>
>SHF_GNU_RETAIN describes in the simplest possible way that the section
>should be retained in the final link, without relying on other
>constructs.
>
>Just because groups are discarded or retained as a group, doesn't mean
>we need to leverage groups to try and implement retention or discarding
>functionality.
>
>Why wasn't SHF_EXCLUDE implemented as a group flag? After all, groups
>are included or excluded from the link together.
>  GRP_EXCLUDE
>    Sections in a group with GRP_EXCLUDE set should be discarded from
>    the link.
>
>I mean, you could kind of use groups for anything when you decide
>grouping sections by metadata is OK.
>Why define SHT_NOBITS when you can create:
>  GRP_NOBITS
>    Sections in this group occupy no space in the file. They must have
>    type SHT_PROGBITS.
>
>Retention of a section is a property of the section. We are misusing ELF
>constructs by using groups to indicate an arbitrary section needs to be
>retained.
>
>Another issue (if more is needed) is about how to name the groups.
>* If it is mandated that GRP_RETAIN groups have the same name e.g.
>  "grp_retain", that means you can't put a section you want to
>  retain in a different logical group that makes sense from the
>  application perspective. So the other sections you would want to put
>  in a group with the retained section need to all be put in a bundle
>  with all the other GRP_RETAIN sections.
>* If GRP_RETAIN groups can have any name, so you can have multiple
>  GRP_RETAIN groups, how does the compiler decide how to name the
>  groups? It seems like it would be a mess. "grp_retain0", "grp_retain2"
>  ... "grp_retain10" from one object file, "grp_retain0"...
>  "grp_retain5" from another. Extra processing in the
>  linker to clean up these group names and keep them unique would be
>  required when performing a relocatable link.
>
>As a general point, what if I decide that there's enough pressure from
>the anti-SHF_GNU_RETAIN side that I change the implementation to use
>groups. But then those who already backed the flag prefer that method
>over using groups think the implementation should not have been changed.
>
>I feel like we already got enough backing for SHF_GNU_RETAIN between the
>GNU gABI and Binutils discussions. I understand, and welcome, more
>feedback, but I haven't been convinced any other method is obviously
>better than SHF_GNU_RETAIN, so why change it?
>
>I'm not saying it's possible to quantify which mechanism for "retain" is
>best, but SHF_GNU_RETAIN is the simplest, most obvious, and easiest to
>understand. Surely that gives it top marks?
>
>I'm also yet to hear one convincing reason why SHF_GNU_RETAIN is bad.
>As far as I can tell, the main argument against it stems from the fact
>that you can have two input sections with the same name but different
>SHF_GNU_RETAIN flag states. Not only is this a non-issue, groups have
>exactly the same problem!
>
>It would be great if a global maintainer to chime in on whether the
>attached patch is acceptable, because otherwise we are going to go back
>and forth forever.
>
>Thanks,
>Jozef

Hi Jozef,

I have checked with a few folks on the LLVM side. James Henderson and James Y
Knight seem to prefer a section flag to other mechanism. I prefer a
section flag, too (I was on the fence and wanted the alternatives to be considered).

About the section flag 'R' in assembly:

I'd still prefer an error to not add another exception to https://sourceware.org/pipermail/binutils/2020-February/109945.html
(a consensus GNU as and LLVM's integrated assembler have reached)

   .section retain,"a",@progbits
   .section retain,"aR",@progbits  # error for section flags change

If a separate section is desired, I'd prefer an explicit ,unique,0 or ,unique,1 or ...

   .section retain,"a",@progbits
   .section retain,"aR",@progbits,unique,0   # 'unique' is a binutils 2.35 feature (available in LLVM for a while)


About the use case in GCC (and probably Clang in the future):

   // a.h
   __attribute__((section("sec"))) inline void bar() { ... }

   // a.c
   #include "a.h"
   __attribute__((section("sec"), retain))   // retain may be the pre-existing used.
   void foo() { ... }

Perhaps there needs a function attribute to lower to ,unique,1 in assembly.