[PowerPC] Fix sanitizer frame unwind on 32-bit ABIs

[seurer]

Summary

This fixes many sanitizer problems with -m32. It is really intended
for gcc but patches to the sanitizers make their way through llvm
first.

ref: https://gcc.gnu.org/ml/gcc-patches/2017-02/msg00855.html

(Note: patch being submitted by me on behalf of Segher Boessenkool
who works on gcc. See ref: above.)

[kcc]

This adds more ifdefs. Please don't. Instead please eliminate all ifdefs from inside the function body.
Or, better, remove all ifdefs completely
like this:

uhwptr GetPC1(uhwptr *frame) {

if (SANITIZER_PPC) return WHATEVER;
if (SANITIZER_S390) return WHATEVER;
return frame[1];

}

Please note that the current code was approved w/o me (https://reviews.llvm.org/D18895): I would never ever allow ifdefs like this.

[kcc]

Also, please make sure to CC llvm-commits to all code reviews in sanitizer run-time.

[segher]

In D31149#706988, @kcc wrote:

This adds more ifdefs. Please don't. Instead please eliminate all ifdefs from inside the function body.
Or, better, remove all ifdefs completely
like this:

uhwptr GetPC1(uhwptr *frame) {

if (SANITIZER_PPC) return WHATEVER;
if (SANITIZER_S390) return WHATEVER;
return frame[1];

}

Please note that the current code was approved w/o me (https://reviews.llvm.org/D18895): I would never ever allow ifdefs like this.

Hi Kostya,

The #ifdef this adds is for _CALL_SYSV, which is a preprocessor symbol that is defined if and only if
you are compiling for the SVR4 ABI (on PowerPC anyway). I cannot change the ABI. Do you want this
hidden in some other macro somewhere? If so, where?

[kcc]

+llvm-commits (please don't forget it)
The only place where the C preprocessor is welcome is lib/sanitizer_common/sanitizer_platform.h

[segher]

Ah, okay, that'll work fine. I'll make a new patch then (tomorrow).

Someone who can test it will have to handle the s390 part, sorry.

lib/sanitizer_common/sanitizer_stacktrace.cc

void BufferedStackTrace::FastUnwindStack(uptr pc, uptr bp, uptr stack_top,
   // Lowest possible address that makes sense as the next frame pointer.
   // Goes up as we walk the stack.
   uptr bottom = stack_bottom;
   // Avoid infinite loop when frame == frame[0] by using frame > prev_frame.
   while (IsValidFrame((uptr)frame, stack_top, bottom) &&
          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
     uhwptr pc1 = frame[1];
 #endif
     // Let's assume that any pointer in the 0th page (i.e. <0x1000 on i386 and
     // x86_64) is invalid and stop unwinding here.  If we're adding support for
     // a platform where this isn't true, we need to reconsider this check.