Date: Sat, 2 Jun 2018 21:11:21 -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 09:15:23AM +0200, Szabolcs Nagy wrote: > * 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. I just setup libc-test to run under qemu-system-sh4, and confirmed failure before the patch, success with the patch applied. I don't have any other qemu-system environments setup, but success here looks like a good sign! Rich
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