[Bug libdw/26321] Possible bug in dwarf_getlocation

[mark at klomp dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=26321

Mark Wielaard <mark at klomp dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mark at klomp dot org

--- Comment #1 from Mark Wielaard <mark at klomp dot org> ---
Thanks for reviewing the (existing) code. I had to read the code a couple of
times myself to be convinced it is actually correct. Do the following comments
convince you? Then I'll check them in so the next person doesn't get too
confused:

diff --git a/libdw/dwarf_getlocation.c b/libdw/dwarf_getlocation.c
index 6fb3ff3d..bca65112 100644
--- a/libdw/dwarf_getlocation.c
+++ b/libdw/dwarf_getlocation.c
@@ -162,7 +162,10 @@ dwarf_getlocation_implicit_value (Dwarf_Attribute *attr,
const Dwarf_Op *op,
 }

 /* DW_AT_data_member_location can be a constant as well as a loclistptr.
-   Only data[48] indicate a loclistptr.  */
+   Only data[48] indicate a loclistptr.  On success returns zero and fills
+   in llbuf (when not NULL) and sets listlen to 1.  Returns 1 when this
+   isn't an constant DW_AT_data_member_location offset.  Returns -1 and
+   sets dwarf_errno on failure.  */
 static int
 check_constant_offset (Dwarf_Attribute *attr,
                       Dwarf_Op **llbuf, size_t *listlen)
@@ -667,7 +670,7 @@ dwarf_getlocation (Dwarf_Attribute *attr, Dwarf_Op **llbuf,
size_t *listlen)

   int result = check_constant_offset (attr, llbuf, listlen);
   if (result != 1)
-    return result;
+    return result; /* Either success 0, or -1 to indicate error.  */

   /* If it has a block form, it's a single location expression.
      Except for DW_FORM_data16, which is a 128bit constant.  */
@@ -898,6 +901,7 @@ dwarf_getlocation_addr (Dwarf_Attribute *attr, Dwarf_Addr
address,
        }
     }

+  /* If check_constant_offset is successful, we are done with 1 result.  */
   int result = check_constant_offset (attr, llbufs, listlens);
   if (result != 1)
     return result ?: 1;
@@ -989,7 +993,7 @@ dwarf_getlocations (Dwarf_Attribute *attr, ptrdiff_t
offset, Dwarf_Addr *basep,
              *endp = -1;
              return 1;
            }
-         return result;
+         return result; /* Something bad, dwarf_errno has been set.  */
        }

       /* We must be looking at a true loclistptr, fetch the initial

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