C++: Use content approximations.

Author: MathiasVP

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowPrivate.qll

@@ -795,12 +795,63 @@ predicate additionalLambdaFlowStep(Node nodeFrom, Node nodeTo, boolean preserves
  */
 predicate allowParameterReturnInSelf(ParameterNode p) { none() }
 
+private predicate fieldHasApproxName(Field f, string s) {
+  s = f.getName().charAt(0) and
+  // Reads and writes of union fields are tracked using `UnionContent`.
+  not f.getDeclaringType() instanceof Cpp::Union
+}
+
+private predicate unionHasApproxName(Cpp::Union u, string s) { s = u.getName().charAt(0) }
+
+cached
+private newtype TContentApprox =
+  TFieldApproxContent(string s) { fieldHasApproxName(_, s) } or
+  TUnionApproxContent(string s) { unionHasApproxName(_, s) }
+
 /** An approximated `Content`. */
-class ContentApprox = Unit;
+class ContentApprox extends TContentApprox {
+  string toString() { none() } // overriden in subclasses
+}
+
+private class FieldApproxContent extends ContentApprox, TFieldApproxContent {
+  string s;
+
+  FieldApproxContent() { this = TFieldApproxContent(s) }
+
+  Field getAField() { fieldHasApproxName(result, s) }
+
+  string getPrefix() { result = s }
+
+  final override string toString() { result = s }
+}
+
+private class UnionApproxContent extends ContentApprox, TUnionApproxContent {
+  string s;
+
+  UnionApproxContent() { this = TUnionApproxContent(s) }
+
+  Cpp::Union getAUnion() { unionHasApproxName(result, s) }
+
+  string getPrefix() { result = s }
+
+  final override string toString() { result = s }
+}
 
 /** Gets an approximated value for content `c`. */
 pragma[inline]
-ContentApprox getContentApprox(Content c) { any() }
+ContentApprox getContentApprox(Content c) {
+  exists(string prefix, Field f |
+    prefix = result.(FieldApproxContent).getPrefix() and
+    f = c.(FieldContent).getField() and
+    fieldHasApproxName(f, prefix)
+  )
+  or
+  exists(string prefix, Cpp::Union u |
+    prefix = result.(UnionApproxContent).getPrefix() and
+    u = c.(UnionContent).getUnion() and
+    unionHasApproxName(u, prefix)
+  )
+}
 
 private class MyConsistencyConfiguration extends Consistency::ConsistencyConfiguration {
   override predicate argHasPostUpdateExclude(ArgumentNode n) {

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -1628,9 +1628,7 @@ private newtype TContent =
     // field can be read by any read of the union's fields.
     indirectionIndex =
       [1 .. max(Ssa::getMaxIndirectionsForType(u.getAField().getUnspecifiedType()))]
-  } or
-  TCollectionContent() or // Not used in C/C++
-  TArrayContent() // Not used in C/C++.
+  }
 
 /**
  * A description of the way data may be stored inside an object. Examples
@@ -1682,22 +1680,14 @@ class UnionContent extends Content, TUnionContent {
 
   Field getAField() { result = u.getAField() }
 
+  Union getUnion() { result = u }
+
   pragma[inline]
   int getIndirectionIndex() {
     pragma[only_bind_into](result) = pragma[only_bind_out](indirectionIndex)
   }
 }
 
-/** A reference through an array. */
-class ArrayContent extends Content, TArrayContent {
-  override string toString() { result = "[]" }
-}
-
-/** A reference through the contents of some collection-like container. */
-private class CollectionContent extends Content, TCollectionContent {
-  override string toString() { result = "<element>" }
-}
-
 /**
  * An entity that represents a set of `Content`s.
  *