selftests/x86: Add a test for ptrace syscall restart and arg modification

selftests/x86: Add a test for ptrace syscall restart and arg modification

This tests assumptions about how fast syscall works wrt pt_regs
and, in particular, what happens if IP is decremented by 2
during a syscall.

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>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1c44dbfe59000ba135bbf35ccc5d2433a0b31618.1444091584.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>