RFC: Add GNU_PROPERTY_1_GLIBC_2_NEEDED

[Florian Weimer]

* H. J. Lu:

>> This proposal may conflict in spirit with the glibc proposal to support
>> preloadable symbol version (so you can add _dl_find_eh_frame@GLIBC_2.35
>> to a glibc 2.28 installation, for example).  So far, symbol versions
>
> Why will adding a glibc version dependency change the preload
> behavior?

Previously, we thought we could relax the version coverage check to
enable adding completely new symbol versions by preloading an
implementation.  With BIND_NOW, this is completely safe because missing
symbols are still detected.  But this turns unreliable once glibc
versions are tied to ELF implementation features.  Preloading an
implementation of _dl_find_eh_frame@GLIBC_2.35 (for example) will not
add dynamic linker features first implemented in glibc 2.35.

>> The problem that linkers and loaders ignore unknown types should be
>> tackled in a different way, e.g. by flagging critical types in some way.
>> See:
>>
>>   Critical program headers and dynamic tags
>>   <https://groups.google.com/g/generic-abi/c/vdG_G4l3N-Y/m/SB3DurdbBAAJ>
>>
>
> This won't help the existing ld.so binaries which this proposal
> is addressing.

We need to increase the ABI version once, to signal the requirement for
critical tags checking.

Thanks,
Florian