x86/entry/vsyscall: Add CONFIG to control default

Most modern systems can run with vsyscall=none. In an effort to
provide a way for build-time defaults to lack legacy settings,
this adds a new CONFIG to select the type of vsyscall mapping to
use, similar to the existing "vsyscall" command line parameter.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: 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: Josh Triplett <josh@joshtriplett.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150813005519.GA11696@www.outflux.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/vsyscall/vsyscall_64.c

@@ -38,7 +38,14 @@
 #define CREATE_TRACE_POINTS
 #include "vsyscall_trace.h"
 
-static enum { EMULATE, NATIVE, NONE } vsyscall_mode = EMULATE;
+static enum { EMULATE, NATIVE, NONE } vsyscall_mode =
+#ifdef CONFIG_LEGACY_VSYSCALL_NATIVE
+	NATIVE;
+#elif CONFIG_LEGACY_VSYSCALL_NONE
+	NONE;
+#else
+	EMULATE;
+#endif
 
 static int __init vsyscall_setup(char *str)
 {