[PATCH] libdwfl: Handle unwind frame when the return address register isn't set.

[Mark Wielaard]

When we have unwound the frame and then cannot set the return address
we wouldn't set any error. That meant that a dwfl_thread_getframes ()
call could end in an error, but without any dwfl_errno set, producing
the "no error" error message.

If we cannot set the return address at the end of unwinding the frame
that means that either the return address register is bogus (error),
or that the return address is undefined (end of the call stack).

This fixes the run-backtrace-native-biarch.sh testcase for me on an
i386 on x86_64 setup with gcc 7.2.1 and glibc 2.17.

Signed-off-by: Mark Wielaard <mark@klomp.org>
---
 libdwfl/ChangeLog      |  5 +++++
 libdwfl/frame_unwind.c | 46 ++++++++++++++++++++++++++++++----------------
 2 files changed, 35 insertions(+), 16 deletions(-)

diff --git a/libdwfl/ChangeLog b/libdwfl/ChangeLog
index 9776f1c..b6262c2 100644
--- a/libdwfl/ChangeLog
+++ b/libdwfl/ChangeLog
@@ -1,3 +1,8 @@
+2018-04-10  Mark Wielaard  <mark@klomp.org>
+
+	* frame_unwind.c (unwind): If __libdwfl_frame_reg_get fails for
+	the return address either set an error or mark the pc undefined.
+
 2018-03-17  Mark Wielaard  <mark@klomp.org>
 
 	* libdwflP.h (struct __libdwfl_remote_mem_cache): New.
diff --git a/libdwfl/frame_unwind.c b/libdwfl/frame_unwind.c
index eaea495..8da691e 100644
--- a/libdwfl/frame_unwind.c
+++ b/libdwfl/frame_unwind.c
@@ -632,24 +632,38 @@ handle_cfi (Dwfl_Frame *state, Dwarf_Addr pc, Dwarf_CFI *cfi, Dwarf_Addr bias)
 	    ra_set = true;
 	}
     }
-  if (unwound->pc_state == DWFL_FRAME_STATE_ERROR
-      && __libdwfl_frame_reg_get (unwound,
-				  frame->fde->cie->return_address_register,
-				  &unwound->pc))
+  if (unwound->pc_state == DWFL_FRAME_STATE_ERROR)
     {
-      /* PPC32 __libc_start_main properly CFI-unwinds PC as zero.  Currently
-	 none of the archs supported for unwinding have zero as a valid PC.  */
-      if (unwound->pc == 0)
-	unwound->pc_state = DWFL_FRAME_STATE_PC_UNDEFINED;
+      if (__libdwfl_frame_reg_get (unwound,
+				   frame->fde->cie->return_address_register,
+				   &unwound->pc))
+	{
+	  /* PPC32 __libc_start_main properly CFI-unwinds PC as zero.
+	     Currently none of the archs supported for unwinding have
+	     zero as a valid PC.  */
+	  if (unwound->pc == 0)
+	    unwound->pc_state = DWFL_FRAME_STATE_PC_UNDEFINED;
+	  else
+	    {
+	      unwound->pc_state = DWFL_FRAME_STATE_PC_SET;
+	      /* In SPARC the return address register actually contains
+		 the address of the call instruction instead of the return
+		 address.  Therefore we add here an offset defined by the
+		 backend.  Most likely 0.  */
+	      unwound->pc += ebl_ra_offset (ebl);
+	    }
+	}
       else
-        {
-          unwound->pc_state = DWFL_FRAME_STATE_PC_SET;
-          /* In SPARC the return address register actually contains
-             the address of the call instruction instead of the return
-             address.  Therefore we add here an offset defined by the
-             backend.  Most likely 0.  */
-          unwound->pc += ebl_ra_offset (ebl);
-        }
+	{
+	  /* We couldn't set the return register, either it was bogus,
+	     or the return pc is undefined, maybe end of call stack.  */
+	  unsigned pcreg = frame->fde->cie->return_address_register;
+	  if (! ebl_dwarf_to_regno (ebl, &pcreg)
+	      || pcreg >= ebl_frame_nregs (ebl))
+	    __libdwfl_seterrno (DWFL_E_INVALID_REGISTER);
+	  else
+	    unwound->pc_state = DWFL_FRAME_STATE_PC_UNDEFINED;
+	}
     }
   free (frame);
 }
-- 
1.8.3.1