unwinder: fix unwind abort for uleb128 case

When unwind instruction is 0xb2,the subsequent instructions
are uleb128 bytes.
For now,it uses only the first uleb128 byte in code.

For vsp increments of 0x204~0x400,use one uleb128 byte like below:
0xc06a00e4 <unwind_test_work>: 0x80b27fac
  Compact model index: 0
  0xb2 0x7f vsp = vsp + 1024
  0xac      pop {r4, r5, r6, r7, r8, r14}

For vsp increments larger than 0x400,use two uleb128 bytes like below:
0xc06a00e4 <unwind_test_work>: @0xc0cc9e0c
  Compact model index: 1
  0xb2 0x81 0x01 vsp = vsp + 1032
  0xac      pop {r4, r5, r6, r7, r8, r14}
The unwind works well since the decoded uleb128 byte is also 0x81.

For vsp increments larger than 0x600,use two uleb128 bytes like below:
0xc06a00e4 <unwind_test_work>: @0xc0cc9e0c
  Compact model index: 1
  0xb2 0x81 0x02 vsp = vsp + 1544
  0xac      pop {r4, r5, r6, r7, r8, r14}
In this case,the decoded uleb128 result is 0x101(vsp=0x204+(0x101<<2)).
While the uleb128 used in code is 0x81(vsp=0x204+(0x81<<2)).
The unwind aborts at this frame since it gets incorrect vsp.

To fix this, add uleb128 decode to cover all the above case.

Signed-off-by: chao an <[email protected]>

arch/arm/src/common/arm_backtrace_unwind.c

@@ -338,6 +338,31 @@ static int unwind_exec_pop_subset_r0_to_r3(struct unwind_ctrl_s *ctrl,
   return 0;
 }
 
+static unsigned long unwind_decode_uleb128(struct unwind_ctrl_s *ctrl)
+{
+  unsigned long bytes = 0;
+  unsigned long insn;
+  unsigned long result = 0;
+
+  /* unwind_get_byte() will advance `ctrl` one instruction at a time, so
+   * loop until we get an instruction byte where bit 7 is not set.
+   *
+   * Note: This decodes a maximum of 4 bytes to output 28 bits data where
+   * max is 0xfffffff: that will cover a vsp increment of 1073742336, hence
+   * it is sufficient for unwinding the stack.
+   */
+
+  do
+    {
+      insn = unwind_get_byte(ctrl);
+      result |= (insn & 0x7f) << (bytes * 7);
+      bytes++;
+    }
+  while (!!(insn & 0x80) && (bytes != sizeof(result)));
+
+  return result;
+}
+
 /****************************************************************************
  * Name: unwind_pop_register
  *
@@ -403,7 +428,7 @@ static int unwind_exec_content(struct unwind_ctrl_s *ctrl)
     }
   else if (content == 0xb2)
     {
-      unsigned long uleb128 = unwind_get_byte(ctrl);
+      unsigned long uleb128 = unwind_decode_uleb128(ctrl);
 
       ctrl->vrs[SP] += 0x204 + (uleb128 << 2);
     }