IsComparisons.qll

/** INTERNAL - Helper predicates for queries that inspect the comparison of objects using `is`. */

import python

/** Holds if the comparison `comp` uses `is` or `is not` (represented as `op`) to compare its `left` and `right` arguments. */
predicate comparison_using_is(Compare comp, ControlFlowNode left, Cmpop op, ControlFlowNode right) {
  exists(CompareNode fcomp | fcomp = comp.getAFlowNode() |
    fcomp.operands(left, op, right) and
    (op instanceof Is or op instanceof IsNot)
  )
}

/** Holds if the class `c` overrides the default notion of equality or comparison. */
predicate overrides_eq_or_cmp(ClassValue c) {
  major_version() = 2 and c.hasAttribute("__eq__")
  or
  c.declaresAttribute("__eq__") and not c = Value::named("object")
  or
  exists(ClassValue sup | sup = c.getASuperType() and not sup = Value::named("object") |
    sup.declaresAttribute("__eq__")
  )
  or
  major_version() = 2 and c.hasAttribute("__cmp__")
}

/** Holds if the class `cls` is likely to only have a single instance throughout the program. */
predicate probablySingleton(ClassValue cls) {
  strictcount(Value inst | inst.getClass() = cls) = 1
  or
  cls = Value::named("None").getClass()
}

/** Holds if using `is` to compare instances of the class `c` is likely to cause unexpected behavior. */
predicate invalid_to_use_is_portably(ClassValue c) {
  overrides_eq_or_cmp(c) and
  // Exclude type/builtin-function/bool as it is legitimate to compare them using 'is' but they implement __eq__
  not c = Value::named("type") and
  not c = ClassValue::builtinFunction() and
  not c = Value::named("bool") and
  // OK to compare with 'is' if a singleton
  not probablySingleton(c)
}

/** Holds if the control flow node `f` points to either `True`, `False`, or `None`. */
predicate simple_constant(ControlFlowNode f) {
  exists(Value val | f.pointsTo(val) |
    val = Value::named("True") or val = Value::named("False") or val = Value::named("None")
  )
}

private predicate cpython_interned_value(Expr e) {
  exists(string text | text = e.(StrConst).getText() |
    text.length() = 0
    or
    text.length() = 1 and text.regexpMatch("[U+0000-U+00ff]")
  )
  or
  exists(int i | i = e.(IntegerLiteral).getN().toInt() | -5 <= i and i <= 256)
  or
  exists(Tuple t | t = e and not exists(t.getAnElt()))
}

/**
 * The set of values that can be expected to be interned across
 * the main implementations of Python. PyPy, Jython, etc tend to
 * follow CPython, but it varies, so this is a best guess.
 */
private predicate universally_interned_value(Expr e) {
  e.(IntegerLiteral).getN().toInt() = 0
  or
  exists(Tuple t | t = e and not exists(t.getAnElt()))
  or
  e.(StrConst).getText() = ""
}

/** Holds if the expression `e` points to an interned constant in CPython. */
predicate cpython_interned_constant(Expr e) {
  exists(Expr const | e.pointsTo(_, const) | cpython_interned_value(const))
}

/** Holds if the expression `e` points to a value that can be reasonably expected to be interned across all implementations of Python. */
predicate universally_interned_constant(Expr e) {
  exists(Expr const | e.pointsTo(_, const) | universally_interned_value(const))
}

private predicate comparison_both_types(Compare comp, Cmpop op, ClassValue cls1, ClassValue cls2) {
  exists(ControlFlowNode op1, ControlFlowNode op2 |
    comparison_using_is(comp, op1, op, op2) or comparison_using_is(comp, op2, op, op1)
  |
    op1.inferredValue().getClass() = cls1 and
    op2.inferredValue().getClass() = cls2
  )
}

private predicate comparison_one_type(Compare comp, Cmpop op, ClassValue cls) {
  not comparison_both_types(comp, _, _, _) and
  exists(ControlFlowNode operand |
    comparison_using_is(comp, operand, op, _) or comparison_using_is(comp, _, op, operand)
  |
    operand.inferredValue().getClass() = cls
  )
}

/**
 * Holds if using `is` or `is not` as the operator `op` in the comparison `comp` would be invalid when applied to the class `cls`.
 */
predicate invalid_portable_is_comparison(Compare comp, Cmpop op, ClassValue cls) {
  // OK to use 'is' when defining '__eq__'
  not exists(Function eq | eq.getName() = "__eq__" or eq.getName() = "__ne__" |
    eq = comp.getScope().getScope*()
  ) and
  (
    comparison_one_type(comp, op, cls) and invalid_to_use_is_portably(cls)
    or
    exists(ClassValue other | comparison_both_types(comp, op, cls, other) |
      invalid_to_use_is_portably(cls) and
      invalid_to_use_is_portably(other)
    )
  ) and
  // OK to use 'is' when comparing items from a known set of objects
  not exists(Expr left, Expr right, Value val |
    comp.compares(left, op, right) and
    exists(ImmutableLiteral il | il.getLiteralValue() = val)
  |
    left.pointsTo(val) and right.pointsTo(val)
    or
    // Simple constant in module, probably some sort of sentinel
    exists(AstNode origin |
      not left.pointsTo(_) and
      right.pointsTo(val, origin) and
      origin.getScope().getEnclosingModule() = comp.getScope().getEnclosingModule()
    )
  ) and
  // OK to use 'is' when comparing with a member of an enum
  not exists(Expr left, Expr right, AstNode origin |
    comp.compares(left, op, right) and
    enum_member(origin)
  |
    left.pointsTo(_, origin) or right.pointsTo(_, origin)
  )
}

private predicate enum_member(AstNode obj) {
  exists(ClassValue cls, AssignStmt asgn | cls.getASuperType().getName() = "Enum" |
    cls.getScope() = asgn.getScope() and
    asgn.getValue() = obj
  )
}