dwarf-reader: Ignore zero length location expressions from DW_AT_location

Location expressions might occasionally be of length 0. E.g. a reason
for that are thread local variables that do not exactly have a
location to refer to. Compilers/Linkers may choose an empty location
description. E.g.  see the dwarfdump output for the added testcase
based on libandroid.so (from AOSP).

$ dwarfdump libandroid.so|egrep -B9 "gChoreographerE$"

LOCAL_SYMBOLS:
< 1><0x00000022> DW_TAG_namespace
                   DW_AT_name           android
< 2><0x00000027>   DW_TAG_variable
                     DW_AT_name           gChoreographer
                     DW_AT_type           <0x00000b65>
                     DW_AT_decl_file      0x00000003 .../choreographer.cpp
                     DW_AT_decl_line      0x00000059
                     DW_AT_location       len 0x0000: :
                     DW_AT_linkage_name   _ZN7androidL14gChoreographerE

The DW_AT_location is properly read by elfutils' dwarf_location(), but
is not useful for us to proceed with. Hence early exit on this.

	* src/abg-dwarf-reader.cc (die_location_expr): Ignore zero
	  length location expressions.
	* tests/data/Makefile.am: Add new test files.
	* tests/data/test-read-dwarf/test-libandroid.so: New test file.
	* tests/data/test-read-dwarf/test-libandroid.so.abi: Likewise.
	* tests/test-read-dwarf.cc: Add new test case.

Reported-by: Dan Albert <danalbert@google.com>
Reviewed-by: Giuliano Procida <gprocida@google.com>
Cc: Mark Wielaard <mark@klomp.org>
Signed-off-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>

src/abg-dwarf-reader.cc

@@ -9385,9 +9385,9 @@ compare_dies_cu_decl_file(const Dwarf_Die* l, const Dwarf_Die *r, bool &result)
 /// @param expr_len the length of the resulting dwarf expression.
 /// This is set iff the function returns true.
 ///
-/// @return true if the attribute exists and has a dwarf expression as
-/// value.  In that case the expr and expr_len arguments are set to
-/// the resulting dwarf exprssion.
+/// @return true if the attribute exists and has a non-empty dwarf expression
+/// as value.  In that case the expr and expr_len arguments are set to the
+/// resulting dwarf expression.
 static bool
 die_location_expr(const Dwarf_Die* die,
 		  unsigned attr_name,
@@ -9404,6 +9404,10 @@ die_location_expr(const Dwarf_Die* die,
   size_t len = 0;
   bool result = (dwarf_getlocation(&attr, expr, &len) == 0);
 
+  // Ignore location expressions where reading them succeeded but
+  // their length is 0.
+  result &= len > 0;
+
   if (result)
     *expr_len = len;
 

tests/data/Makefile.am

@@ -520,6 +520,8 @@ test-read-dwarf/test-PR26568-2.o     \
 test-read-dwarf/test-PR26568-1.o     \
 test-read-dwarf/test-PR26568-1.o.abi \
 test-read-dwarf/test-PR26568-2.o.abi \
+test-read-dwarf/test-libandroid.so \
+test-read-dwarf/test-libandroid.so.abi \
 \
 test-annotate/test0.abi			\
 test-annotate/test1.abi			\

tests/test-read-dwarf.cc

@@ -394,6 +394,13 @@ InOutSpec in_out_specs[] =
     "data/test-read-dwarf/test-PR26568-2.o.abi",
     "output/test-read-dwarf/test-PR26568-2.o.abi",
   },
+  {
+    "data/test-read-dwarf/test-libandroid.so",
+    "",
+    HASH_TYPE_ID_STYLE,
+    "data/test-read-dwarf/test-libandroid.so.abi",
+    "output/test-read-dwarf/test-libandroid.so.abi",
+  },
   // This should be the last entry.
   {NULL, NULL, SEQUENCE_TYPE_ID_STYLE, NULL, NULL}
 };