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ifunc resolving

Alan Modra amodra@gmail.com
Tue Jan 19 22:31:53 GMT 2021 
On Tue, Jan 19, 2021 at 03:59:54PM +0100, Florian Weimer via Binutils wrote:
> We have two positions that still need to be reconciled:
> 
> * IFUNC resolvers must not themselves have relocation dependencies
>   because they can be called at any time during relocation.  This
>   restricts the functionality available to an IFUNC resolver.

On many architectures this cannot be achieved without hand-crafted
assembly.  The reason is obvious.  Ifunc resolvers return an address.
Compilers load addresses from the GOT, particularly for PIC.  GOT
entries need relocation.

> * IFUNC resolvers may have relocation dependencies, but they may only be
>   called after the object that contains them has been relocated.  This
>   restricts how IFUNC symbols can be used (interposition is limited,
>   correct dependency ordering via DT_NEEDED is required).

No interposition in practice, because the object doing the interposing
is almost always relocated later.

> I do not think we have ever achieved consensus which position is the
> correct one.

There is a third possibility.  If ld.so defers all irelative and other
relocations using ifunc symbols until all non-ifunc relocations have
been performed, globally, then ifunc resolvers would only have the
restriction that they not call other ifuncs.

That idea was floated a very long time ago.  For some reason it is
too hard or too slow to do in ld.so.

> We have removed several IFUNC resolvers with IFUNC dependencies from
> glibc itself because we could not follow the interposition/dependency
> rules for the second approach.

Right, I'm inclined to say ifunc has a fatal design flaw without
deferred relocation processing.  If it's too hard for glibc itself,
it's too hard for anyone to use anywhere.

> Due to the x86-specific checks we have, those architectures currently
> land in the second category.  I do not know if other architecture
> maintainers agree.
> 
> Thanks,
> Florian
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Alan Modra
Australia Development Lab, IBM
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