fix: guard get_operand_expressions against empty expression tree

`_build_expression_tree` already returns `[]` for the Ghidra bug where
an operand has no expressions (see
https://github.com/NationalSecurityAgency/ghidra/issues/6817), but
`get_operand_expressions` then called the recursive walker
unconditionally with `tree_index=0`, which indexed into the empty list
and raised `IndexError`. Add an early-return guard so callers receive
`[]` instead.

CHANGELOG.md

@@ -32,6 +32,7 @@
 - fix: loader.py reads entire file for magic byte check @williballenthin #3029
 - fix: freeze/__init__.py: logically impossible condition @williballenthin #3030
 - fix: EXTENSIONS_ELF never referenced @williballenthin #3031
+- fix: guard get_operand_expressions against empty expression tree so Ghidra-exported BinExport2 files with empty operands no longer raise IndexError @williballenthin
 - fix: add return after zero-offset yield in extract_insn_offset_features so Offset(0) is not emitted twice @williballenthin
 - fix: use f-string in binexport2 extractor so unexpected global feature value appears in ValueError message @williballenthin
 - fix: correct scale/displacement expressions in get_operand_phrase_info 5-expression branch (used expression3 operator instead of expression4 value) @williballenthin

capa/features/extractors/binexport2/helpers.py

@@ -333,6 +333,8 @@ def _fill_operand_expression_list(
 
 def get_operand_expressions(be2: BinExport2, op: BinExport2.Operand) -> list[BinExport2.Expression]:
     tree = _build_expression_tree(be2, op)
+    if not tree:
+        return []
 
     expressions: list[BinExport2.Expression] = []
     _fill_operand_expression_list(be2, op, tree, 0, expressions)

tests/test_binexport_accessors.py

@@ -340,6 +340,26 @@ BE2 = ParseDict(
 )
 
 
+def test_get_operand_expressions_empty_operand():
+    be2 = ParseDict(
+        {
+            "expression": [
+                {"type": BinExport2.Expression.REGISTER, "symbol": "x0"},
+            ],
+            "operand": [
+                {"expression_index": [0]},
+                {},
+            ],
+        },
+        BinExport2(),
+    )
+    normal_op = be2.operand[0]
+    empty_op = be2.operand[1]
+
+    assert len(get_operand_expressions(be2, normal_op)) == 1
+    assert get_operand_expressions(be2, empty_op) == []
+
+
 def test_is_stack_register_expression():
     mov = ParseDict(BE2_DICT["instruction"][0], BinExport2.Instruction())
     add = ParseDict(BE2_DICT["instruction"][2], BinExport2.Instruction())