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The vdso-based sigreturn mechanism is fragile and isn't used by
modern glibc so, if we break it, we'll only notice when someone
tests an unusual libc.

Add an explicit selftest.

[ I wrote this while debugging a Bionic breakage -- my first guess
  was that I had somehow messed up sigreturn.  I've caused problems in
  that code before, and it's really easy to fail to notice it because
  there's nothing on a modern distro that needs vdso-based sigreturn. ]
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/32946d714156879cd8e5d8eab044cd07557ed558.1452628504...