fix: use integer division in get_printable_len for UTF-16 LE operands

`get_printable_len` returned a float for UTF-16 LE operands due to `/`
instead of `//`, violating the `-> int` annotation and silently
propagating a float into `_bb_has_stackstring`'s accumulator. Aligns
with the IDA extractor equivalent.

Closes SURF-58

CHANGELOG.md

@@ -48,8 +48,7 @@
 - fix: remove unreachable backports.functools_lru_cache fallback and dead dependency @williballenthin
 - fix: Scopes.from_dict uses cls instead of self so subclasses return the correct type @williballenthin
 - fix: correct wrong dict key in VMRay _compute_monitor_threads assertion (used thread_id instead of process_id) @williballenthin
-fix: replace assert with isinstance guard in get_callee for invalid MethodSpec tokens @williballenthin
-- fix: replace assert with isinstance guard in get_callee for invalid MethodSpec tokens @williballenthin
+- fix: use integer division in get_printable_len for UTF-16 LE operands @williballenthin (SURF-58)
 - fix: break thunk chain loop after resolving import to avoid duplicate API features @williballenthin (SURF-57)
 - fix: pass insn instead of oper to getOperValue/getOperAddr in viv insn extractor @williballenthin (SURF-56)
 - fix: implement extract_function_loop in dnfile extractor to detect backward branches as loops @williballenthin (SURF-55)

tests/test_helpers.py

@@ -14,6 +14,7 @@
 
 
 import codecs
+import dataclasses
 from pathlib import Path
 
 import pytest
@@ -40,6 +41,11 @@ from capa.features.common import (
     FORMAT_BINEXPORT2,
 )
 from capa.features.extractors import helpers
+from capa.features.extractors.viv.basicblock import (
+    get_printable_len,
+    is_printable_ascii,
+    is_printable_utf16le,
+)
 
 CD = Path(__file__).resolve().parent
 DRAKVUF_LOG_GZ = (
@@ -180,3 +186,37 @@ def test_get_file_taste_reads_first_bytes(tmp_path):
 def test_get_file_taste_missing_file_raises():
     with pytest.raises(IOError):
         get_file_taste(Path("/nonexistent/path/sample.exe"))
+
+
+def test_is_printable_ascii():
+    assert is_printable_ascii(b"AB") is True
+    assert is_printable_ascii(b"A\x00") is False
+    assert is_printable_ascii(b"\x80\x81") is False
+
+
+def test_is_printable_utf16le():
+    assert is_printable_utf16le(b"A\x00B\x00") is True
+    assert is_printable_utf16le(b"AB") is False
+    assert is_printable_utf16le(b"\x80\x00\x81\x00") is False
+
+
+def test_get_printable_len_returns_int():
+    @dataclasses.dataclass
+    class FakeOper:
+        tsize: int
+        imm: int
+
+    ascii_oper = FakeOper(tsize=4, imm=int.from_bytes(b"ABCD", "little"))
+    result = get_printable_len(ascii_oper)
+    assert isinstance(result, int)
+    assert result == 4
+
+    utf16_oper = FakeOper(tsize=4, imm=int.from_bytes(b"A\x00B\x00", "little"))
+    result = get_printable_len(utf16_oper)
+    assert isinstance(result, int)
+    assert result == 2
+
+    utf16_oper_8 = FakeOper(tsize=8, imm=int.from_bytes(b"A\x00B\x00C\x00D\x00", "little"))
+    result = get_printable_len(utf16_oper_8)
+    assert isinstance(result, int)
+    assert result == 4