selftests/x86/mpx: Fix incorrect bounds with old _sigfault

For distributions with old userspace header files, the _sigfault
structure is different. mpx-mini-test fails with the following
error:

  [root@Purley]# mpx-mini-test_64 tabletest
  XSAVE is supported by HW & OS
  XSAVE processor supported state mask: 0x2ff
  XSAVE OS supported state mask: 0x2ff
   BNDREGS: size: 64 user: 1 supervisor: 0 aligned: 0
    BNDCSR: size: 64 user: 1 supervisor: 0 aligned: 0
  starting mpx bounds table test
  ERROR: siginfo bounds do not match shadow bounds for register 0

Fix it by using the correct offset of _lower/_upper in _sigfault.
RHEL needs this patch to work.
Signed-off-by: Rui Wang <rui.y.wang@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dave.hansen@linux.intel.com
Fixes: e754aedc ("x86/mpx, selftests: Add MPX self test")
Link: http://lkml.kernel.org/r/1513586050-1641-1-git-send-email-rui.y.wang@intel.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/testing/selftests/x86/mpx-mini-test.c

@@ -315,11 +315,39 @@ static inline void *__si_bounds_upper(siginfo_t *si)
 	return si->si_upper;
 }
 #else
+
+/*
+ * This deals with old version of _sigfault in some distros:
+ *
+
+old _sigfault:
+        struct {
+            void *si_addr;
+	} _sigfault;
+
+new _sigfault:
+	struct {
+		void __user *_addr;
+		int _trapno;
+		short _addr_lsb;
+		union {
+			struct {
+				void __user *_lower;
+				void __user *_upper;
+			} _addr_bnd;
+			__u32 _pkey;
+		};
+	} _sigfault;
+ *
+ */
+
 static inline void **__si_bounds_hack(siginfo_t *si)
 {
 	void *sigfault = &si->_sifields._sigfault;
 	void *end_sigfault = sigfault + sizeof(si->_sifields._sigfault);
-	void **__si_lower = end_sigfault;
+	int *trapno = (int*)end_sigfault;
+	/* skip _trapno and _addr_lsb */
+	void **__si_lower = (void**)(trapno + 2);
 
 	return __si_lower;
 }
@@ -331,7 +359,7 @@ static inline void *__si_bounds_lower(siginfo_t *si)
 
 static inline void *__si_bounds_upper(siginfo_t *si)
 {
-	return (*__si_bounds_hack(si)) + sizeof(void *);
+	return *(__si_bounds_hack(si) + 1);
 }
 #endif