C++: Add support for 'Element' content in dataflow.

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

@@ -1325,7 +1325,7 @@ import IsUnreachableInCall
  * Holds if access paths with `c` at their head always should be tracked at high
  * precision. This disables adaptive access path precision for such access paths.
  */
-predicate forceHighPrecision(Content c) { none() }
+predicate forceHighPrecision(Content c) { c instanceof ElementContent }
 
 /** Holds if `n` should be hidden from path explanations. */
 predicate nodeIsHidden(Node n) {
@@ -1396,7 +1396,8 @@ private predicate unionHasApproxName(Cpp::Union u, string s) { s = u.getName().c
 cached
 private newtype TContentApprox =
   TFieldApproxContent(string s) { fieldHasApproxName(_, s) } or
-  TUnionApproxContent(string s) { unionHasApproxName(_, s) }
+  TUnionApproxContent(string s) { unionHasApproxName(_, s) } or
+  TElementApproxContent()
 
 /** An approximated `Content`. */
 class ContentApprox extends TContentApprox {
@@ -1427,6 +1428,10 @@ private class UnionApproxContent extends ContentApprox, TUnionApproxContent {
   final override string toString() { result = s }
 }
 
+private class ElementApproxContent extends ContentApprox, TElementApproxContent {
+  final override string toString() { result = "ElementApprox" }
+}
+
 /** Gets an approximated value for content `c`. */
 pragma[inline]
 ContentApprox getContentApprox(Content c) {
@@ -1441,6 +1446,9 @@ ContentApprox getContentApprox(Content c) {
     u = c.(UnionContent).getUnion() and
     unionHasApproxName(u, prefix)
   )
+  or
+  c instanceof ElementContent and
+  result instanceof ElementApproxContent
 }
 
 /**
@@ -1700,6 +1708,14 @@ class DataFlowSecondLevelScope extends TDataFlowSecondLevelScope {
 /** Gets the second-level scope containing the node `n`, if any. */
 DataFlowSecondLevelScope getSecondLevelScope(Node n) { result.getANode() = n }
 
+/**
+ * Gets the maximum number of indirections to use for `ElementContent`.
+ *
+ * This should be equal to the largest number of stars (i.e., `*`s) in any
+ * `Element` content across all of our MaD summaries, sources, and sinks.
+ */
+int getMaxElementContentIndirectionIndex() { result = 5 }
+
 /**
  * Module that defines flow through iterators.
  * For example,

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

@@ -2083,6 +2083,9 @@ private newtype TContent =
       indirectionIndex =
         [1 .. max(Ssa::getMaxIndirectionsForType(getAFieldWithSize(u, bytes).getUnspecifiedType()))]
     )
+  } or
+  TElementContent(int indirectionIndex) {
+    indirectionIndex = [1 .. getMaxElementContentIndirectionIndex()]
   }
 
 /**
@@ -2193,6 +2196,25 @@ class UnionContent extends Content, TUnionContent {
   }
 }
 
+/**
+ * A `Content` that represents one of the elements of a
+ * container (e.g., `std::vector`).
+ */
+class ElementContent extends Content, TElementContent {
+  int indirectionIndex;
+
+  ElementContent() { this = TElementContent(indirectionIndex) }
+
+  pragma[inline]
+  override int getIndirectionIndex() {
+    pragma[only_bind_into](result) = pragma[only_bind_out](indirectionIndex)
+  }
+
+  override predicate impliesClearOf(Content c) { none() }
+
+  override string toString() { result = contentStars(this) + "element" }
+}
+
 /**
  * An entity that represents a set of `Content`s.
  *

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

@@ -147,7 +147,10 @@ predicate defaultAdditionalTaintStep(DataFlow::Node src, DataFlow::Node sink, st
  * of `c` at sinks and inputs to additional taint steps.
  */
 bindingset[node]
-predicate defaultImplicitTaintRead(DataFlow::Node node, DataFlow::ContentSet c) { none() }
+predicate defaultImplicitTaintRead(DataFlow::Node node, DataFlow::ContentSet c) {
+  node instanceof ArgumentNode and
+  c.isSingleton(any(ElementContent ec))
+}
 
 /**
  * Holds if `node` should be a sanitizer in all global taint flow configurations