Repository: angr/claripy

Issue:
Remove length information from string
### Description

Strings in claripy currently extend from Bits, and act on the surface as if their length is fixed. This isn't actually the case though, any op that goes through Z3 for instance loses this length information, as Z3 Strings do not have a length. Further, the primary use case for Strings, the Soot Engine in angr seemingly doesn't really care about the length of strings, it just hardcodes some high enough maximum values that it doesn't really matter. Let's remove it to be consistent with Z3 and other SMT solvers.

### Alternatives

_No response_

### Additional context

_No response_

Discussion:

Solution patch:
diff --git a/claripy/ast/bv.py b/claripy/ast/bv.py
index 6dff5de2..4de96dd5 100644
--- a/claripy/ast/bv.py
+++ b/claripy/ast/bv.py
@@ -134,7 +134,7 @@ class BV(Bits):
 
     @staticmethod
     def _from_int(like, value):
-        return BVV(value, like.length)
+        return BVV(value, like.length or 64)
 
     @staticmethod
     def _from_Bool(like, value):
diff --git a/claripy/ast/strings.py b/claripy/ast/strings.py
index 8631a6f1..8d701c7b 100644
--- a/claripy/ast/strings.py
+++ b/claripy/ast/strings.py
@@ -1,80 +1,23 @@
 from __future__ import annotations
 
 from claripy import operations
-from claripy.ast.base import _make_name
 
-from .bits import Bits
+from .base import Base, _make_name
 from .bool import Bool
-from .bv import BV, BVS, BVV
+from .bv import BV
 
 
-class String(Bits):
+class String(Base):
     """
-    Base class that represent the AST of a String object and implements all the operation useful to create and modify the AST.
+    Base class that represent the AST of a String object and implements all the
+    operation useful to create and modify the AST.
 
-    Do not instantiate this class directly, instead use StringS or StringV to construct a symbol or value, and then use
-    operations to construct more complicated expressions.
+    Do not instantiate this class directly, instead use StringS or StringV to
+    construct a symbol or value, and then use operations to construct more
+    complicated expressions.
     """
 
-    __slots__ = ("string_length",)
-
-    # Identifier used by composite solver in order to identify if a certain constraints contains
-    # variables of type string... In this case cvc4 would handle the solving part.
-    #
-    # TODO: Find a smarter way to do this!
-    STRING_TYPE_IDENTIFIER = "STRING_"
-    GENERATED_BVS_IDENTIFIER = "BVS_"
-    MAX_LENGTH = 10000
-
-    def __init__(self, *args, length: int, **kwargs):
-        """
-        :param length: The string bit length
-        """
-        super().__init__(*args, length=length, **kwargs)
-        self.string_length: int = self.length // 8
-
-    def __getitem__(self, rng):
-        """
-        This is a big endian indexer that takes its arguments as bits but returns bytes.
-        Returns the range of bits in self defined by: [rng.start, rng.end], indexed from the end
-        The bit range may not internally divide any bytes; i.e. low and (high+1) must both be divisible by 8
-        Examples:
-            self[7:0]  -- returns the last byte of self
-            self[15:0] -- returns the last two bytes of self
-            self[8:0]  -- Error! [8:0] is 9 bits, it asks for individual bits of the second to last byte!
-            self[8:1]  -- Error! [8:1] asks for 1 bit from the second to last byte and 7 from the last byte!
-        """
-        if type(rng) is slice:
-            bits_low = rng.start if rng.start is not None else 0
-            bits_high = rng.stop if rng.stop is not None else 8 * (self.string_length - 1)
-            if bits_high % 8 != 0 or (bits_low + 1) % 8 != 0:
-                raise ValueError("Bit indicies must not internally divide bytes!")
-            # high / low form a reverse-indexed byte index
-            high = bits_high // 8
-            low = bits_low // 8
-            if high < 0:
-                high = self.string_length + high
-            if low < 0:
-                low = self.string_length + low
-            # StrSubstr takes a front-indexed byte index as a starting point, and a length
-            start_idx = self.string_length - 1 - low
-            if high > low:
-                return StrSubstr(start_idx, 0, self)
-            return StrSubstr(start_idx, 1 + low - high, self)
-        raise ValueError("Only slices allowed for string extraction")
-
-    @staticmethod
-    # TODO: Figure out what to convert here
-    #
-    # The use case that i found useful here is during the concretization
-    # of a symbolic address in memory.
-    # When angr has to do this it creates a claripy.If constraints in this form
-    # If(condition, <value_read_if_condition_true>, <value_read_if_condition_false>).
-    # In some case <value_read_if_condition_false> can be MEM_UNINITIALIZED expressed as BVV
-    #
-    # What should we return in this case?
-    def _from_BV(like, value):  # pylint: disable=unused-argument
-        return value
+    __slots__ = ()
 
     @staticmethod
     def _from_str(like, value):  # pylint: disable=unused-argument
@@ -107,38 +50,22 @@ class String(Bits):
         """
         return StrIndexOf(self, pattern, start_idx, bitlength)
 
-    def raw_to_bv(self):
-        """
-        A counterpart to FP.raw_to_bv - does nothing and returns itself.
-        """
-        if self.symbolic:
-            return BVS(
-                next(iter(self.variables)).replace(self.STRING_TYPE_IDENTIFIER, self.GENERATED_BVS_IDENTIFIER),
-                self.length,
-            )
-        return BVV(ord(self.args[0]), self.length)
-
-    def raw_to_fp(self):
-        return self.raw_to_bv().raw_to_fp()
 
-
-def StringS(name, size, uninitialized=False, explicit_name=False, **kwargs):
+def StringS(name, uninitialized=False, explicit_name=False, **kwargs):
     """
     Create a new symbolic string (analogous to z3.String())
 
     :param name:                 The name of the symbolic string (i. e. the name of the variable)
-    :param size:                 The size in bytes of the string (i. e. the length of the string)
     :param uninitialized:        Whether this value should be counted as an "uninitialized" value in the course of an
                                  analysis.
     :param bool explicit_name:   If False, an identifier is appended to the name to ensure uniqueness.
 
     :returns:                    The String object representing the symbolic string
     """
-    n = _make_name(String.STRING_TYPE_IDENTIFIER + name, size, False if explicit_name is None else explicit_name)
+    n = _make_name(name, 0, False if explicit_name is None else explicit_name)
     return String(
         "StringS",
-        n,
-        length=8 * size,
+        (n,),
         symbolic=True,
         uninitialized=uninitialized,
         variables=frozenset((n,)),
@@ -146,41 +73,28 @@ def StringS(name, size, uninitialized=False, explicit_name=False, **kwargs):
     )
 
 
-def StringV(value, length: int | None = None, **kwargs):
+def StringV(value, **kwargs):
     """
     Create a new Concrete string (analogous to z3.StringVal())
 
     :param value:  The constant value of the concrete string
-    :param length: The byte length of the string
 
     :returns:      The String object representing the concrete string
     """
 
-    if length is None:
-        length = len(value)
-
-    if length < len(value):
-        raise ValueError("Can't make a concrete string value longer than the specified length!")
-
-    return String("StringV", (value, length), length=8 * length, **kwargs)
+    return String("StringV", (value,), **kwargs)
 
 
-StrConcat = operations.op("StrConcat", String, String, calc_length=operations.str_concat_length_calc)
-StrSubstr = operations.op("StrSubstr", (BV, BV, String), String, calc_length=operations.substr_length_calc)
+StrConcat = operations.op("StrConcat", String, String)
+StrSubstr = operations.op("StrSubstr", (BV, BV, String), String)
 StrLen = operations.op("StrLen", (String), BV, calc_length=lambda *_: 64)
-StrReplace = operations.op(
-    "StrReplace",
-    (String, String, String),
-    String,
-    extra_check=operations.str_replace_check,
-    calc_length=operations.str_replace_length_calc,
-)
+StrReplace = operations.op("StrReplace", (String, String, String), String)
 StrContains = operations.op("StrContains", (String, String), Bool)
 StrPrefixOf = operations.op("StrPrefixOf", (String, String), Bool)
 StrSuffixOf = operations.op("StrSuffixOf", (String, String), Bool)
 StrIndexOf = operations.op("StrIndexOf", (String, String, BV), BV, calc_length=lambda *_: 64)
 StrToInt = operations.op("StrToInt", (String), BV, calc_length=lambda *_: 64)
-IntToStr = operations.op("IntToStr", (BV,), String, calc_length=operations.int_to_str_length_calc)
+IntToStr = operations.op("IntToStr", (BV,), String)
 StrIsDigit = operations.op("StrIsDigit", (String,), Bool)
 
 # Equality / inequality check
diff --git a/claripy/backends/backend_concrete/backend_concrete.py b/claripy/backends/backend_concrete/backend_concrete.py
index 2e677e6b..117b03cc 100644
--- a/claripy/backends/backend_concrete/backend_concrete.py
+++ b/claripy/backends/backend_concrete/backend_concrete.py
@@ -61,7 +61,7 @@ class BackendConcrete(Backend):
         return bv.BVV(value, size)
 
     @staticmethod
-    def StringV(value, size):  # pylint: disable=unused-argument
+    def StringV(value):
         return strings.StringV(value)
 
     @staticmethod
diff --git a/claripy/backends/backend_z3.py b/claripy/backends/backend_z3.py
index 309a22c5..9e9c772d 100644
--- a/claripy/backends/backend_z3.py
+++ b/claripy/backends/backend_z3.py
@@ -377,12 +377,10 @@ class BackendZ3(Backend):
 
     @condom
     def StringV(self, ast):
-        return z3.StringVal(ast.args[0].ljust(ast.args[1], "\0"), ctx=self._context)
+        return z3.StringVal(ast.args[0], ctx=self._context)
 
     @condom
     def StringS(self, ast):
-        # Maybe this should be an error? Warning for now to support reliant code
-        log.warning("Converting claripy StringS' to z3 looses length information.")
         return z3.String(ast.args[0], ctx=self._context)
 
     #
diff --git a/claripy/frontend/frontend.py b/claripy/frontend/frontend.py
index 15baeee4..0aab4405 100644
--- a/claripy/frontend/frontend.py
+++ b/claripy/frontend/frontend.py
@@ -61,8 +61,7 @@ class Frontend:
         if isinstance(e, BV):
             return [BVV(v, e.size()) for v in values]
         if isinstance(e, String):
-            return [StringV(v, length=e.string_length) for v in values]
-
+            return [StringV(v) for v in values]
         # TODO: Implement support for other types
         raise NotImplementedError
 
diff --git a/claripy/operations.py b/claripy/operations.py
index 8f6fdccc..cfee1356 100644
--- a/claripy/operations.py
+++ b/claripy/operations.py
@@ -169,48 +169,10 @@ def extract_length_calc(high, low, _):
     return high + 1 - low
 
 
-def str_basic_length_calc(str_1):
-    return str_1.length
-
-
-def int_to_str_length_calc(int_val):  # pylint: disable=unused-argument
-    return 8 * claripy.ast.String.MAX_LENGTH
-
-
-def str_replace_check(*args):
-    str_1, str_2, _ = args
-    if str_1.length < str_2.length:
-        return False, "The pattern that has to be replaced is longer than the string itself"
-    return True, ""
-
-
-def substr_length_calc(start_idx, count, strval):  # pylint: disable=unused-argument
-    # FIXME: How can I get the value of a concrete object without a solver
-    return strval.length if not count.concrete else 8 * count.args[0]
-
-
 def ext_length_calc(ext, orig):
     return orig.length + ext
 
 
-def str_concat_length_calc(*args):
-    return sum(arg.length for arg in args)
-
-
-def str_replace_length_calc(*args):
-    str_1, str_2, str_3 = args
-    # Return the maximum length that the string can assume after the replace
-    # operation
-    #
-    # If the part that has to be replaced if greater than
-    # the replacement than the we have the maximum length possible
-    # when the part that has to be replaced is not found inside the string
-    if str_2.length >= str_3.length:
-        return str_1.length
-    # Otherwise We have the maximum length when teh replacement happens
-    return str_1.length - str_2.length + str_3.length
-
-
 #
 # Operation lists
 #


Test patch:
diff --git a/tests/test_solver.py b/tests/test_solver.py
index 2b0d3448..97926b4b 100644
--- a/tests/test_solver.py
+++ b/tests/test_solver.py
@@ -455,7 +455,7 @@ class StandardTests(TestCase):
         x = claripy.BVS("x", 32)
         y = claripy.BVS("y", 32)
         z = claripy.BVS("z", 32)
-        str_1 = claripy.StringS("sym_str_1", 1024)
+        str_1 = claripy.StringS("sym_str_1")
         c = claripy.And(x == 1, y == 2, z == 3, str_1 == claripy.StringV("cavallo"))
         s.add(c)
         assert len(s._solver_list) == 4
diff --git a/tests/test_strings.py b/tests/test_strings.py
index 8e4939e1..a8435b35 100644
--- a/tests/test_strings.py
+++ b/tests/test_strings.py
@@ -14,7 +14,7 @@ class TestStrings(unittest.TestCase):
 
     def test_concat(self):
         str_concrete = claripy.StringV("conc")
-        str_symbol = claripy.StringS("symb_concat", 4, explicit_name=True)
+        str_symbol = claripy.StringS("symb_concat", explicit_name=True)
         solver = self.get_solver()
         res = str_concrete + str_symbol
         solver.add(res == claripy.StringV("concrete"))
@@ -38,7 +38,7 @@ class TestStrings(unittest.TestCase):
         self.assertEqual(["conc"], list(result))
 
     def test_substr(self):
-        str_symbol = claripy.StringS("symb_subst", 4, explicit_name=True)
+        str_symbol = claripy.StringS("symb_subst", explicit_name=True)
         solver = self.get_solver()
         solver.add(claripy.StrSubstr(1, 2, str_symbol) == claripy.StringV("o"))
         self.assertTrue(solver.satisfiable())
@@ -57,7 +57,7 @@ class TestStrings(unittest.TestCase):
         self.assertEqual(list(result), ["concrete"])
 
     def test_replace(self):
-        str_to_replace_symb = claripy.StringS("symb_repl", 4, explicit_name=True)
+        str_to_replace_symb = claripy.StringS("symb_repl", explicit_name=True)
         sub_str_to_repl = claripy.StringV("a")
         replacement = claripy.StringV("b")
         solver = self.get_solver()
@@ -88,7 +88,7 @@ class TestStrings(unittest.TestCase):
         self.assertEqual(["cane"], list(result))
 
     def test_ne(self):
-        str_symb = claripy.StringS("symb_ne", 12, explicit_name=True)
+        str_symb = claripy.StringS("symb_ne", explicit_name=True)
         solver = self.get_solver()
         solver.add(str_symb != claripy.StringV("concrete"))
         self.assertTrue(solver.satisfiable())
@@ -97,7 +97,7 @@ class TestStrings(unittest.TestCase):
         self.assertTrue("concrete" not in result)
 
     def test_length(self):
-        str_symb = claripy.StringS("symb_length", 12, explicit_name=True)
+        str_symb = claripy.StringS("symb_length", explicit_name=True)
         solver = self.get_solver()
         # TODO: How do we want to deal with the size of a symbolic string?
         solver.add(claripy.StrLen(str_symb) == 14)
@@ -119,7 +119,7 @@ class TestStrings(unittest.TestCase):
             self.assertTrue(len(r) == 8)
 
     def test_or(self):
-        str_symb = claripy.StringS("Symb_or", 4, explicit_name=True)
+        str_symb = claripy.StringS("Symb_or", explicit_name=True)
         solver = self.get_solver()
         res = claripy.Or((str_symb == claripy.StringV("abc")), (str_symb == claripy.StringV("ciao")))
         solver.add(res)
@@ -129,7 +129,7 @@ class TestStrings(unittest.TestCase):
         self.assertEqual({"ciao", "abc"}, set(result))
 
     def test_lt_etc(self):
-        str_symb = claripy.StringS("Symb_2", 4)
+        str_symb = claripy.StringS("Symb_2")
         solver = self.get_solver()
         c1 = claripy.StrLen(str_symb) <= 4
         c2 = claripy.StrLen(str_symb) < 4
@@ -142,7 +142,7 @@ class TestStrings(unittest.TestCase):
         self.assertFalse(solver.satisfiable())
 
     def test_substr_BV_concrete_index(self):
-        str_symbol = claripy.StringS("symb_subst", 4, explicit_name=True)
+        str_symbol = claripy.StringS("symb_subst", explicit_name=True)
         solver = self.get_solver()
         bv1 = claripy.BVV(1, 32)
         bv2 = claripy.BVV(2, 32)
@@ -152,7 +152,7 @@ class TestStrings(unittest.TestCase):
         self.assertEqual("on", solver.eval(str_symbol, 1)[0][1:3])
 
     def test_substr_BV_symbolic_index(self):
-        str_symbol = claripy.StringS("symb_subst", 4, explicit_name=True)
+        str_symbol = claripy.StringS("symb_subst", explicit_name=True)
         solver = self.get_solver()
         start = claripy.BVS("start_idx", 32)
         count = claripy.BVS("count", 32)
@@ -170,7 +170,7 @@ class TestStrings(unittest.TestCase):
         self.assertEqual("on", solver.eval(str_symbol, 1, extra_constraints=(start == 1, count == 4))[0][1:])
 
     def test_substr_BV_mixed_index(self):
-        str_symbol = claripy.StringS("symb_subst", 4, explicit_name=True)
+        str_symbol = claripy.StringS("symb_subst", explicit_name=True)
         solver = self.get_solver()
         start = claripy.BVS("symb_subst_start_idx", 32, explicit_name=True)
         count = claripy.BVV(2, 32)
@@ -182,7 +182,7 @@ class TestStrings(unittest.TestCase):
         self.assertEqual("on", solver.eval(str_symbol, 1, extra_constraints=(start == 2,))[0][2:4])
 
     def test_contains(self):
-        str_symb = claripy.StringS("symb_contains", 4, explicit_name=True)
+        str_symb = claripy.StringS("symb_contains", explicit_name=True)
         res = claripy.StrContains(str_symb, claripy.StringV("an"))
         solver = self.get_solver()
         solver.add(res)
@@ -202,7 +202,7 @@ class TestStrings(unittest.TestCase):
         self.assertEqual((True,), solver.eval(res, 2))
 
     def test_prefix(self):
-        str_symb = claripy.StringS("symb_prefix", 4, explicit_name=True)
+        str_symb = claripy.StringS("symb_prefix", explicit_name=True)
         res = claripy.StrPrefixOf(claripy.StringV("an"), str_symb)
         solver = self.get_solver()
         solver.add(res)
@@ -213,7 +213,7 @@ class TestStrings(unittest.TestCase):
             self.assertTrue(sol.startswith("an"))
 
     def test_suffix(self):
-        str_symb = claripy.StringS("symb_suffix", 4, explicit_name=True)
+        str_symb = claripy.StringS("symb_suffix", explicit_name=True)
         res = claripy.StrSuffixOf(claripy.StringV("an"), str_symb)
         solver = self.get_solver()
         solver.add(res)
@@ -244,7 +244,7 @@ class TestStrings(unittest.TestCase):
         self.assertEqual((True,), solver.eval(res, 2))
 
     def test_index_of(self):
-        str_symb = claripy.StringS("symb_suffix", 4, explicit_name=True)
+        str_symb = claripy.StringS("symb_suffix", explicit_name=True)
         res = claripy.StrIndexOf(str_symb, claripy.StringV("an"), 0)
         solver = self.get_solver()
 
@@ -270,7 +270,7 @@ class TestStrings(unittest.TestCase):
 
     @unittest.skip("Usually hangs")
     def test_index_of_symbolic_start_idx(self):
-        str_symb = claripy.StringS("symb_index_of", 4, explicit_name=True)
+        str_symb = claripy.StringS("symb_index_of", explicit_name=True)
         start_idx = claripy.BVS("symb_start_idx", 32, explicit_name=True)
 
         solver = self.get_solver()