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Date: Sat, 2 Jun 2018 09:15:23 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] Fix TLS layout of TLS variant I when there is a
 gap above TP

* Rich Felker <dalias@...c.org> [2018-06-01 22:59:11 -0400]:
> On Sat, Jun 02, 2018 at 01:52:01AM +0200, Szabolcs Nagy wrote:
> > In TLS variant I the TLS is above TP (or above a fixed offset from TP)
> > but on some targets there is a reserved gap above TP before TLS starts.
> > 
> > This matters for the local-exec tls access model when the offsets of
> > TLS variables from the TP are hard coded by the linker into the
> > executable, so the libc must compute these offsets the same way as the
> > linker.  The tls offset of the main module has to be
> > 
> > 	alignup(GAP_ABOVE_TP, main_tls_align).
> > 
> > If there is no TLS in the main module then the gap can be ignored
> > since musl does not use it and the tls access models of shared
> > libraries are not affected.
> > 
> > The previous setup only worked if (tls_align & -GAP_ABOVE_TP) == 0
> > (i.e. TLS did not require large alignment) because the gap was
> > treated as a fixed offset from TP.  Now the TP points at the end
> > of the pthread struct (which is aligned) and there is a gap above
> > it (which may also need alignment).
> > 
> > The fix required changing TP_ADJ and __pthread_self on affected
> > targets (aarch64, arm and sh) and in the tlsdesc asm the offset to
> > access the dtv changed too.
> > ---
> 
> On first glance it all looks right. I'll read in more detail soon.
> Thanks!
> 
> > passed my simple local-exec tests.
> 
> Did you test all archs or just some? I think we should at least run
> libc-test (if it sufficiently tests TLS) on the affected archs to make
> sure there are no regressions.
> 

ran libc-test on various targets via qemu-user, i didnt see any regressions.

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