-mno-tls-direct-seg-refs support in glibc for i386 PV Xen

[Florian Weimer]

* Andrew Cooper:

> Oh, so the i386 TLS model relies on the calculation wrapping (modulo 4G)
> when the segment limit is 4G, instead of taking a fault?

That's about it.

> Intel states this is behaviour is implementation specific (SDM Vol3
> 5.3.1) and may fault, while AMD doesn't discuss it at all as far as I
> can tell (APM Vol2 4.12 is the right section, but I can't see this
> discussed).
>
> While I can believe it probably works on every processor these days, it
> does seem like dodgy ground to base an ABI on.

Sure, but it has been this way since the beginnings of NPTL, for close
to twenty years now.  The TCB is at positive offsets, and the user TLS
data at negative offsets.

> It also means that Xen isn't necessarily the only affected party.  I'm
> pretty sure GRSecurity use reduced segment limits as well.

Mostly for CS and DS, I believe, for the fake NX handling.  I think that
was never upstream, but some vendor kernels had variants of it.

> I also bet it doesn't work reliably under emulation.

It has to, given that it's so pervasively used under Linux. 8-/

Thanks,
Florian