[PATCH v2] Support for SHF_GNU_RETAIN ELF Section Flag

[H.J. Lu]

On Wed, Sep 30, 2020 at 3:19 AM Jozef Lawrynowicz
<jozef.l@mittosystems.com> 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.
>

I like SHF_GNU_RETAIN which is complementary to SHF_EXCLUDE.

-- 
H.J.