libdw: Handle empty location expression for (indirect) DIE locations.

When dwarf_getlocation_implicit_pointer and dwarf_getlocation_attr
refer to a DIE that doesn't contain a DW_AT_location then don't generate
an error, but return an empty location expression to signal the actual
value pointed to is not available. This isn't invalid DWARF. Also make
sure that __libdw_intern_expression handles empty location expressions.

Signed-off-by: Mark Wielaard <mjw@redhat.com>

5 files changed, 42 insertions, 8 deletions

diff --git a/libdw/ChangeLog b/libdw/ChangeLog
index 91e10833..79bb4b57 100644
--- a/libdw/ChangeLog
+++ b/libdw/ChangeLog
@@ -1,3 +1,14 @@
+2013-12-09  Mark Wielaard  <mjw@redhat.com>
+
+	* dwarf_getlocation.c (__libdw_intern_expression): Handle empty
+	location expressions.
+	* dwarf_getlocation_attr.c (dwarf_getlocation_attr): When no
+	location found, return empty location expression.
+	* dwarf_getlocation_implicit_pointer.c
+	(dwarf_getlocation_implicit_pointer): Likewise.
+	(__libdw_empty_loc_attr): New internal function.
+	* libdwP.h (__libdw_empty_loc_attr): Define.
+
 2013-11-27  Mark Wielaard  <mjw@redhat.com>
 
 	* libdw.map (ELFUTILS_0.158): Add dwfl_module_addrsym_elf and
diff --git a/libdw/dwarf_getlocation.c b/libdw/dwarf_getlocation.c
index ff25fc7c..4124ae36 100644
--- a/libdw/dwarf_getlocation.c
+++ b/libdw/dwarf_getlocation.c
@@ -202,6 +202,13 @@ __libdw_intern_expression (Dwarf *dbg, bool other_byte_order,
 			   bool cfap, bool valuep,
 			   Dwarf_Op **llbuf, size_t *listlen, int sec_index)
 {
+  /* Empty location expressions don't have any ops to intern.  */
+  if (block->length == 0)
+    {
+      *listlen = 0;
+      return 0;
+    }
+
   /* Check whether we already looked at this list.  */
   struct loc_s fake = { .addr = block->data };
   struct loc_s **found = tfind (&fake, cache, loc_compare);
@@ -465,8 +472,8 @@ __libdw_intern_expression (Dwarf *dbg, bool other_byte_order,
   if (unlikely (n == 0))
     {
       /* This is not allowed.
-
-	 XXX Is it?  */
+	 It would mean an empty location expression, which we handled
+	 already as a special case above.  */
       goto invalid;
     }
 
diff --git a/libdw/dwarf_getlocation_attr.c b/libdw/dwarf_getlocation_attr.c
index bf155840..cb290456 100644
--- a/libdw/dwarf_getlocation_attr.c
+++ b/libdw/dwarf_getlocation_attr.c
@@ -74,8 +74,8 @@ dwarf_getlocation_attr (attr, op, result)
 	    return -1;
 	  if (INTUSE(dwarf_attr) (&die, DW_AT_location, result) == NULL)
 	    {
-	      __libdw_seterrno (DWARF_E_INVALID_DWARF);
-	      return -1;
+	      __libdw_empty_loc_attr (result, attr->cu);
+	      return 0;
 	    }
 	}
 	break;
@@ -88,8 +88,8 @@ dwarf_getlocation_attr (attr, op, result)
 	  if (INTUSE(dwarf_attr) (&die, DW_AT_location, result) == NULL
 	      && INTUSE(dwarf_attr) (&die, DW_AT_const_value, result) == NULL)
 	    {
-	      __libdw_seterrno (DWARF_E_INVALID_DWARF);
-	      return -1;
+	      __libdw_empty_loc_attr (result, attr->cu);
+	      return 0;
 	    }
 	}
 	break;
diff --git a/libdw/dwarf_getlocation_implicit_pointer.c b/libdw/dwarf_getlocation_implicit_pointer.c
index 322fdb8a..f93d17ec 100644
--- a/libdw/dwarf_getlocation_implicit_pointer.c
+++ b/libdw/dwarf_getlocation_implicit_pointer.c
@@ -34,6 +34,18 @@
 #include <dwarf.h>
 
 
+static unsigned char empty_exprloc = 0;
+
+void
+internal_function
+__libdw_empty_loc_attr (Dwarf_Attribute *attr, struct Dwarf_CU *cu )
+{
+  attr->code = DW_AT_location;
+  attr->form = DW_FORM_exprloc;
+  attr->valp = &empty_exprloc;
+  attr->cu = cu;
+}
+
 int
 dwarf_getlocation_implicit_pointer (attr, op, result)
      Dwarf_Attribute *attr;
@@ -57,8 +69,8 @@ dwarf_getlocation_implicit_pointer (attr, op, result)
   if (INTUSE(dwarf_attr) (&die, DW_AT_location, result) == NULL
       && INTUSE(dwarf_attr) (&die, DW_AT_const_value, result) == NULL)
     {
-      __libdw_seterrno (DWARF_E_INVALID_DWARF);
-      return -1;
+      __libdw_empty_loc_attr (result, attr->cu);
+      return 0;
     }
 
   return 0;
diff --git a/libdw/libdwP.h b/libdw/libdwP.h
index f02a5bf2..35ab6e79 100644
--- a/libdw/libdwP.h
+++ b/libdw/libdwP.h
@@ -632,6 +632,10 @@ int __check_build_id (Dwarf *dw, const uint8_t *build_id, const size_t id_len)
   internal_function;
 #endif /* ENABLE_DWZ */
 
+/* Fills in the given attribute to point at an empty location expression.  */
+void __libdw_empty_loc_attr (Dwarf_Attribute *attr, struct Dwarf_CU *cu)
+  internal_function;
+
 
 /* Aliases to avoid PLTs.  */
 INTDECL (dwarf_aggregate_size)