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Re: POWER PC-relative addressing and new text relocations
- From: Alan Modra <amodra at gmail dot com>
- To: Florian Weimer <fw at deneb dot enyo dot de>
- Cc: binutils at sourceware dot org, libc-alpha at sourceware dot org, gcc at gcc dot gnu dot org
- Date: Mon, 23 Sep 2019 18:01:22 +0930
- Subject: Re: POWER PC-relative addressing and new text relocations
- References: <87o8zbcvb7.fsf@mid.deneb.enyo.de>
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