Bug 1468807

We should consider fixing this in Red Hat Enterprise Linux as well because it is visible with an interposed malloc (which could use vector instructions; the main malloc code in glibc does not when compiled with the system compiler).

+++ This bug was initially created as a clone of Bug #1440287 +++

Description of problem:
Every 64bit game that uses Unity (the game engine) won't start on Fedora 26 (deadlocks on a black screen). 32bit games work fine.

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--- Additional comment from Nicholas Miell on 2017-06-17 20:42:52 CEST ---

I'm seeing similar Unity hangs, except the sequence of events is:

1. Thread attempts to lazy init a TLS variable.
2. There is a SIGSEGV in _int_malloc() on a movaps %xmm0,(%rsp) instruction because RSP is misaligned due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58066
3. The Mono SIGSEGV handler gets called.
4. The SIGSEGV handler attempts to lazy init a TLS variable.
5. malloc deadlocks.

LD_PRELOADing the following simple stub fixes the affected Unity games:

#include <dlfcn.h>

__attribute__((force_align_arg_pointer)) void *__tls_get_addr (void *ti)
{
	void *(*tga)(void*) = dlsym(RTLD_NEXT, "__tls_get_addr");

	return tga(ti);
}

Why doesn't glibc's __tls_get_addr() have this attribute to deal with this gcc bug?

--- Additional comment from Nicholas Miell on 2017-06-17 20:46:21 CEST ---

Actually, the stack traces I'm seeing are literally identical to comment #0, except the signal is definitely SIGSEGV (from a MOVAPS instruction), not SIGPWR (from Mono's garbage collector).

--- Additional comment from Nicholas Miell on 2017-06-17 21:05 CEST ---

unzip align-tls-get-addr.zip
cd align-tls-get-addr
meson BUILD
ninja -C BUILD
LD_PRELOAD=$(pwd)/BUILD/tls_get_addr.so /path/to/bug1440287/repro.x86_64

For Steam, Set Launch Options to LD_PRELOAD=/full/path/to/align-tls-get-addr/BUILD/tls_get_addr.so %command%

--- Additional comment from Nicholas Miell on 2017-06-18 06:44 CEST ---

A proposed patch for glibc.

--- Additional comment from Florian Weimer on 2017-06-18 11:14:28 CEST ---

Thanks for tracking this down.  We will likely use a different approach upstream, involving a compatibility symbol for future glibc versions, and use a sysdeps override for backports.

--- Additional comment from Florian Weimer on 2017-06-29 15:55:49 CEST ---

Upstream patch posted for review: https://sourceware.org/ml/libc-alpha/2017-06/msg00922.html

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