rs6000: Fix sanitizer frame unwind on 32-bit ABIs

This fixes more than half of our testcase failures on BE.


libsanitizer/
	* sanitizer_common/sanitizer_stacktrace.cc
	(BufferedStackTrace::FastUnwindStack): Use the correct frame offset
	for PowerPC SYSV ABI.

From-SVN: r258525

libsanitizer/ChangeLog

+2018-03-14  Segher Boessenkool  <segher@kernel.crashing.org>
+
+	* sanitizer_common/sanitizer_stacktrace.cc
+	(BufferedStackTrace::FastUnwindStack): Use the correct frame offset
+	for PowerPC SYSV ABI.
+
 2018-02-14  Igor Tsimbalist  <igor.v.tsimbalist@intel.com>
 
 	PR target/84148

libsanitizer/sanitizer_common/sanitizer_stacktrace.cc

@@ -78,14 +78,21 @@ void BufferedStackTrace::FastUnwindStack(uptr pc, uptr bp, uptr stack_top,
          IsAligned((uptr)frame, sizeof(*frame)) &&
          size < max_depth) {
 #ifdef __powerpc__
-    // PowerPC ABIs specify that the return address is saved at offset
-    // 16 of the *caller's* stack frame.  Thus we must dereference the
-    // back chain to find the caller frame before extracting it.
+    // PowerPC ABIs specify that the return address is saved on the
+    // *caller's* stack frame.  Thus we must dereference the back chain
+    // to find the caller frame before extracting it.
     uhwptr *caller_frame = (uhwptr*)frame[0];
     if (!IsValidFrame((uptr)caller_frame, stack_top, bottom) ||
         !IsAligned((uptr)caller_frame, sizeof(uhwptr)))
       break;
+    // For most ABIs the offset where the return address is saved is two
+    // register sizes.  The exception is the SVR4 ABI, which uses an
+    // offset of only one register size.
+#ifdef _CALL_SYSV
+    uhwptr pc1 = caller_frame[1];
+#else
     uhwptr pc1 = caller_frame[2];
+#endif
 #elif defined(__s390__)
     uhwptr pc1 = frame[14];
 #else