From 224bef8ec492e8c2a88f17f05ab7982495b43d6a Mon Sep 17 00:00:00 2001
From: Christopher Ferris <cferris@google.com>
Date: Tue, 18 Aug 2015 15:41:31 -0700
Subject: [PATCH] Modify ip to get the instr being executed.

The ip values being reported are always pointing at the next
instruction to execute, not the currently executing instruction.
Change the ip address to point at the currently executing instruction.

This fixes a problem where the next instruction is not actually
part of the same function, so if an addr2line is used, it reports
the wrong value.

This does not modify the ip for mips.

Bug: 22565486
Change-Id: Ie0df4c9727f62ce06948b9a2b0e4b7956eb6e752
---
 libc/bionic/debug_stacktrace.cpp | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/libc/bionic/debug_stacktrace.cpp b/libc/bionic/debug_stacktrace.cpp
index 71e876b95..41009117b 100644
--- a/libc/bionic/debug_stacktrace.cpp
+++ b/libc/bionic/debug_stacktrace.cpp
@@ -85,14 +85,14 @@ static _Unwind_Reason_Code trace_function(__unwind_context* context, void* arg)
     return _URC_NO_REASON;
   }
 
-#if defined(__arm__)
-  /*
-   * The instruction pointer is pointing at the instruction after the bl(x), and
-   * the _Unwind_Backtrace routine already masks the Thumb mode indicator (LSB
-   * in PC). So we need to do a quick check here to find out if the previous
-   * instruction is a Thumb-mode BLX(2). If so subtract 2 otherwise 4 from PC.
-   */
+  // The instruction pointer is pointing at the instruction after the return
+  // call on all architectures.
+  // Modify the pc to point at the real function.
   if (ip != 0) {
+#if defined(__arm__)
+    // We need to do a quick check here to find out if the previous
+    // instruction is a Thumb-mode BLX(2). If so subtract 2 otherwise
+    // 4 from PC.
     short* ptr = reinterpret_cast<short*>(ip);
     // Thumb BLX(2)
     if ((*(ptr-1) & 0xff80) == 0x4780) {
@@ -100,8 +100,15 @@ static _Unwind_Reason_Code trace_function(__unwind_context* context, void* arg)
     } else {
       ip -= 4;
     }
-  }
+#elif defined(__aarch64__)
+    // All instructions are 4 bytes long, skip back one instruction.
+    ip -= 4;
+#elif defined(__i386__) || defined(__x86_64__)
+    // It's difficult to decode exactly where the previous instruction is,
+    // so subtract 1 to estimate where the instruction lives.
+    ip--;
 #endif
+  }
 
   state->frames[state->frame_count++] = ip;
   return (state->frame_count >= state->max_depth) ? _URC_END_OF_STACK : _URC_NO_REASON;