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Date: Fri, 1 Jun 2018 22:59:11 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] Fix TLS layout of TLS variant I when there is a
 gap above TP

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.

Rich

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