tree 03b5f3c52034b98c5549b05e7c57b7c679a95f54
parent d2cc36dd8ce17a44d93832fedbfe3c2bc3306624
author Tom Stellard <tstellar@redhat.com> 1559952433 +0000
committer Tom Stellard <tstellar@redhat.com> 1559952433 +0000

Merging r361090:

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r361090 | maskray | 2019-05-17 20:16:00 -0700 (Fri, 17 May 2019) | 34 lines

[ARM][AArch64] Revert Android Bionic PT_TLS overaligning hack

This reverts D53906.

D53906 increased p_align of PT_TLS on ARM/AArch64 to 32/64 to make the
static TLS layout compatible with Android Bionic's ELF TLS. However,
this may cause glibc ARM/AArch64 programs to crash (see PR41527).

The faulty PT_TLS in the executable satisfies p_vaddr%p_align != 0. The
remainder is normally 0 but may be non-zero with the hack in place. The
problem is that we increase PT_TLS's p_align after OutputSections'
addresses are fixed (assignAddress()). It is possible that
p_vaddr%old_p_align = 0 while p_vaddr%new_p_align != 0.

For a thread local variable defined in the executable, lld computed TLS
offset (local exec) is different from glibc computed TLS offset from
another module (initial exec/generic dynamic). Note: PR41527 said the
bug affects initial exec but actually generic dynamic is affected as
well.

(glibc is correct in that it compute offsets that satisfy
`offset%p_align == p_vaddr%p_align`, which is a basic ELF requirement.
This hack appears to work on FreeBSD rtld, musl<=1.1.22, and Bionic, but
that is just because they (and lld) incorrectly compute offsets that
satisfy `offset%p_align = 0` instead.)

Android developers are fine to revert this patch, carry this patch in
their tree before figuring out a long-term solution (e.g. a dummy .tdata
with sh_addralign=64 sh_size={0,1} in crtbegin*.o files. The overhead is
now insignificant after D62059).

Reviewed By: rprichard, srhines

Differential Revision: https://reviews.llvm.org/D62055
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git-svn-id: https://llvm.org/svn/llvm-project/lld/branches/release_80@362858 91177308-0d34-0410-b5e6-96231b3b80d8