POWER PC-relative addressing and new text relocations

[Alan Modra]

On Mon, Sep 23, 2019 at 09:42:52AM +0200, Florian Weimer wrote:
> At Cauldron, the question came up whether the dynamic loader needs to
> be taught about the new relocations for PC-relative addressing.
> 
> I think they would only matter if we supported PC-relative addressing
> *and* text relocations.  Is that really necessary?
> 
> These text relocations would not work reliably anyway because the
> maximum displacement is not large enough.  For example, with the
> current process layout, it's impossible to reach shared objects from
> the main program and vice versa.  And some systems might want to add
> additional randomization, so that shared objects are not mapped closed
> together anymore.

We've been discussing this inside IBM too.  The conclusion is that
only one of the new relocs makes any possible sense as a dynamic
reloc, R_PPC64_TPREL34, and that one only if you allow
-ftls-model=local-exec when building shared libraries and accept that
DF_STATIC_TLS shared libraries that can't be dlopen'd are OK.

See https://sourceware.org/ml/binutils/2019-09/msg00164.html, which
doesn't allow even R_PPC64_TPREL34.  I haven't put this patch on the
binutils 2.33 branch.

-- 
Alan Modra
Australia Development Lab, IBM