RFC: Add GNU_PROPERTY_1_GLIBC_2_NEEDED

[Florian Weimer]

* H. J. Lu:

> I am not sure if I am following your concerns.   We have an ELF feature,
> like DT_RELR, which is tied to a glibc version.  The binary with DT_RELR
> will crash with the older glibcs.  And you DON'T want such a binary with
> a dependency on the required glibc version.  Can you tell me why?

Historically, such features have not been tied to a glibc version.  CET,
DT_AUDIT, AArch64 variant PCS support, nearly arbitrary calling
convention support on x86-64 all are not really version-specific (they
have been backported to varying degrees), and those involve dynamic
linker features.

In contrast, if DT_RELR support is indicated by a GLIBC_2.35 version
dependency, it is necessary to backport all of the GLIBC_2.35 symbol set
as part of the DT_RELR backport.  This means such backports are usually
not feasible.

>> >> 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.
>>
>
> Which ABI version? .note.ABI-tag or EI_ABIVERSION?  A binary linked
> against glibc 2.40 without DT_RELR can run with glibc 2.34.  But a binary
> linked against glibc 2.30 with DT_RELR won't run with glibc 2.34 at all.
> Increasing the ABI version doesn't solve the DT_RELR issue.

The way EI_ABIVERSION works is that the link editor produces the minimum
version needed by the features in the binary.

So if the link editor DT_RELR, it would produce a DT_CRITICAL_DT tag for
DT_RELR and set EI_ABIVERSION for critical DT tag support.  Similar for
other critical DT Tags.  If no critical DT tags are used, an earlier
EI_ABIVERSION can be used.

Thanks,
Florian