RFC: Add GNU_PROPERTY_1_GLIBC_2_NEEDED

[H.J. Lu]

On Thu, Oct 28, 2021 at 7:08 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * 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.

So you would like to backport DT_RELR.

> >> >> 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.
>

There is no DT_CRITICAL_DT support in the older glibcs.  The only option
is EI_ABIVERSION and I don't think we need DT_CRITICAL_DT.   We update
EI_ABIVERSION whenever there is a new feature added.  I think it is one
missing piece in the original DT_RELR proposal.

-- 
H.J.