Model instance-attribute type flow

Use a field level step like JS and Ruby.

python/ql/lib/semmle/python/dataflow/new/internal/TypeTrackingImpl.qll

@@ -172,6 +172,8 @@ module TypeTrackingInput implements Shared::TypeTrackingInput<Location> {
   /** Holds if there is a level step from `nodeFrom` to `nodeTo`, which does not depend on the call graph. */
   predicate levelStepNoCall(Node nodeFrom, LocalSourceNode nodeTo) {
     TypeTrackerSummaryFlow::levelStepNoCall(nodeFrom, nodeTo)
+    or
+    localFieldStep(nodeFrom, nodeTo)
   }
 
   /**
@@ -317,6 +319,51 @@ module TypeTrackingInput implements Shared::TypeTrackingInput<Location> {
     )
   }
 
+  /**
+   * Holds if `ref` accesses attribute `attr` of `self`, where `self` is the first
+   * parameter of an instance method of `cls` (i.e. an access of the form `self.attr`).
+   *
+   * Static methods and class methods are excluded, since their first parameter is not a
+   * `self` instance reference.
+   */
+  private predicate selfAttrRef(Class cls, string attr, DataFlowPublic::AttrRef ref) {
+    exists(Function method, Name selfUse |
+      method = cls.getAMethod() and
+      not DataFlowDispatch::isStaticmethod(method) and
+      not DataFlowDispatch::isClassmethod(method) and
+      selfUse.getVariable() = method.getArg(0).(Name).getVariable() and
+      ref.getObject().asCfgNode().getNode() = selfUse and
+      ref.mayHaveAttributeName(attr)
+    )
+  }
+
+  /**
+   * Holds if `nodeFrom` is written to attribute `self.attr` in some instance method of a
+   * class, and `nodeTo` reads attribute `self.attr` in some (possibly different) instance
+   * method of the same class.
+   *
+   * This models flow through instance attributes (`self.foo`): a value stored into
+   * `self.foo` in one method can be read from `self.foo` in another method. Type-tracking
+   * handles the store and read steps via `AttrWrite`/`AttrRead`, but on its own it cannot
+   * relate the `self` of the writing method to the `self` of the reading method. Following
+   * the approach used for Ruby and JavaScript, we model this directly as a level step from
+   * the written value to the read reference, for any pair of methods on the class (not
+   * just from `__init__`).
+   *
+   * This is an over-approximation: it is instance-insensitive (it does not distinguish
+   * between different instances of the same class) and order-insensitive (it does not
+   * require the write to happen before the read), matching the precision of
+   * instance-attribute handling for Ruby and JavaScript.
+   */
+  private predicate localFieldStep(Node nodeFrom, LocalSourceNode nodeTo) {
+    exists(Class cls, string attr, DataFlowPublic::AttrWrite write, DataFlowPublic::AttrRead read |
+      selfAttrRef(cls, attr, write) and
+      nodeFrom = write.getValue() and
+      selfAttrRef(cls, attr, read) and
+      nodeTo = read
+    )
+  }
+
   /**
    * Holds if data can flow from `node1` to `node2` in a way that discards call contexts.
    */

python/ql/test/library-tests/dataflow/typetracking/attribute_tests.py

@@ -151,10 +151,10 @@ class MyClass2(object):
         self.foo = tracked # $ tracked=foo tracked
 
     def print_foo(self): # $ MISSING: tracked=foo
-        print(self.foo) # $ MISSING: tracked=foo tracked
+        print(self.foo) # $ tracked MISSING: tracked=foo
 
     def possibly_uncalled_method(self): # $ MISSING: tracked=foo
-        print(self.foo) # $ MISSING: tracked=foo tracked
+        print(self.foo) # $ tracked MISSING: tracked=foo
 
 instance = MyClass2()
 print(instance.foo) # $ MISSING: tracked=foo tracked

python/ql/test/library-tests/frameworks/hdbcli/pep249.py

@@ -11,11 +11,10 @@ cursor.close()
 
 # Connection stored in a class attribute (`self._conn`) and used in another method.
 #
-# This is currently NOT detected: the `Connection::instance()`/`execute()` predicates in
-# PEP249.qll are based on type tracking, which cannot follow a value that is stored into a
-# `self` attribute in one method and read from a `self` attribute in another method (see the
-# `MISSING` markers below). Regular (global) data flow handles this case correctly, so the
-# limitation is specific to the type-tracking-based modeling.
+# This is detected because type tracking includes a level step modelling flow through
+# instance attributes: a value written to `self._conn` in one method (here `__init__`) can
+# be read back from `self._conn` (directly or via a getter) in any other method on the same
+# class. This follows the same approach used for instance fields in Ruby and JavaScript.
 class Database:
     def __init__(self):
         self._conn = dbapi.connect(address="hostname", port=300, user="username")
@@ -26,10 +25,10 @@ class Database:
     def run_via_getter(self):
         conn = self.get_connection()
         cursor = conn.cursor()
-        cursor.execute("getter sql")  # $ MISSING: getSql="getter sql"
+        cursor.execute("getter sql")  # $ getSql="getter sql"
 
     def run_direct(self):
-        self._conn.execute("direct sql")  # $ MISSING: getSql="direct sql"
+        self._conn.execute("direct sql")  # $ getSql="direct sql"
 
 
 db = Database()