Fix arm64 frame-pointer stackwalker

src/processor/stackwalker_arm64.cc

Index: src/processor/stackwalker_arm64.cc
diff --git a/src/processor/stackwalker_arm64.cc b/src/processor/stackwalker_arm64.cc
index 31119a97e1d087aa22453ab5aabfc4053d5f1fb4..e8587325c5f1e3286732525661eafa0cb8852c44 100644
--- a/src/processor/stackwalker_arm64.cc
+++ b/src/processor/stackwalker_arm64.cc
@@ -195,13 +195,22 @@ StackFrameARM64* StackwalkerARM64::GetCallerByFramePointer(
     return NULL;
   }
 
-  uint64_t caller_lr = 0;
-  if (last_fp && !memory_->GetMemoryAtAddress(last_fp + 8, &caller_lr)) {
-    BPLOG(ERROR) << "Unable to read caller_lr from last_fp + 8: 0x"
+  // The memory at last_fp + 8 is the last frame's LR (callee's lr), which is
+  // the PC of the caller.
+  uint64_t caller_pc = 0;
+  if (last_fp && !memory_->GetMemoryAtAddress(last_fp + 8, &caller_pc)) {
+    BPLOG(ERROR) << "Unable to read caller_pc from last_fp + 8: 0x"
                  << std::hex << (last_fp + 8);
     return NULL;
   }
 
+  uint64_t caller_lr = 0;
+  if (last_fp && !memory_->GetMemoryAtAddress(caller_fp + 8, &caller_lr)) {
+    BPLOG(ERROR) << "Unable to read caller_lr from caller_fp + 8: 0x"
+                 << std::hex << (caller_fp + 8);
+    return NULL;
+  }
+
   uint64_t caller_sp = last_fp ? last_fp + 16 :
       last_frame->context.iregs[MD_CONTEXT_ARM64_REG_SP];
 
@@ -213,8 +222,7 @@ StackFrameARM64* StackwalkerARM64::GetCallerByFramePointer(
   frame->context = last_frame->context;
   frame->context.iregs[MD_CONTEXT_ARM64_REG_FP] = caller_fp;
   frame->context.iregs[MD_CONTEXT_ARM64_REG_SP] = caller_sp;
-  frame->context.iregs[MD_CONTEXT_ARM64_REG_PC] =
-      last_frame->context.iregs[MD_CONTEXT_ARM64_REG_LR];
+  frame->context.iregs[MD_CONTEXT_ARM64_REG_PC] = caller_pc;
   frame->context.iregs[MD_CONTEXT_ARM64_REG_LR] = caller_lr;
   frame->context_validity = StackFrameARM64::CONTEXT_VALID_PC |
                             StackFrameARM64::CONTEXT_VALID_LR |