Data flow: Sync files

ql/src/codeql_ruby/dataflow/internal/DataFlowImplCommon.qll

@@ -0,0 +1,841 @@
+private import DataFlowImplSpecific::Private
+private import DataFlowImplSpecific::Public
+import Cached
+
+/**
+ * The cost limits for the `AccessPathFront` to `AccessPathApprox` expansion.
+ *
+ * `apLimit` bounds the acceptable fan-out, and `tupleLimit` bounds the
+ * estimated per-`AccessPathFront` tuple cost. Access paths exceeding both of
+ * these limits are represented with lower precision during pruning.
+ */
+predicate accessPathApproxCostLimits(int apLimit, int tupleLimit) {
+  apLimit = 10 and
+  tupleLimit = 10000
+}
+
+/**
+ * The cost limits for the `AccessPathApprox` to `AccessPath` expansion.
+ *
+ * `apLimit` bounds the acceptable fan-out, and `tupleLimit` bounds the
+ * estimated per-`AccessPathApprox` tuple cost. Access paths exceeding both of
+ * these limits are represented with lower precision.
+ */
+predicate accessPathCostLimits(int apLimit, int tupleLimit) {
+  apLimit = 5 and
+  tupleLimit = 1000
+}
+
+cached
+private module Cached {
+  /**
+   * Holds if `p` is the `i`th parameter of a viable dispatch target of `call`.
+   * The instance parameter is considered to have index `-1`.
+   */
+  pragma[nomagic]
+  private predicate viableParam(DataFlowCall call, int i, ParameterNode p) {
+    p.isParameterOf(viableCallable(call), i)
+  }
+
+  /**
+   * Holds if `arg` is a possible argument to `p` in `call`, taking virtual
+   * dispatch into account.
+   */
+  cached
+  predicate viableParamArg(DataFlowCall call, ParameterNode p, ArgumentNode arg) {
+    exists(int i |
+      viableParam(call, i, p) and
+      arg.argumentOf(call, i) and
+      compatibleTypes(getNodeType(arg), getNodeType(p))
+    )
+  }
+
+  pragma[nomagic]
+  private ReturnPosition viableReturnPos(DataFlowCall call, ReturnKindExt kind) {
+    viableCallable(call) = result.getCallable() and
+    kind = result.getKind()
+  }
+
+  /**
+   * Holds if a value at return position `pos` can be returned to `out` via `call`,
+   * taking virtual dispatch into account.
+   */
+  cached
+  predicate viableReturnPosOut(DataFlowCall call, ReturnPosition pos, Node out) {
+    exists(ReturnKindExt kind |
+      pos = viableReturnPos(call, kind) and
+      out = kind.getAnOutNode(call)
+    )
+  }
+
+  /** Provides predicates for calculating flow-through summaries. */
+  private module FlowThrough {
+    /**
+     * The first flow-through approximation:
+     *
+     * - Input access paths are abstracted with a Boolean parameter
+     *   that indicates (non-)emptiness.
+     */
+    private module Cand {
+      /**
+       * Holds if `p` can flow to `node` in the same callable using only
+       * value-preserving steps.
+       *
+       * `read` indicates whether it is contents of `p` that can flow to `node`.
+       */
+      pragma[nomagic]
+      private predicate parameterValueFlowCand(ParameterNode p, Node node, boolean read) {
+        p = node and
+        read = false
+        or
+        // local flow
+        exists(Node mid |
+          parameterValueFlowCand(p, mid, read) and
+          simpleLocalFlowStep(mid, node)
+        )
+        or
+        // read
+        exists(Node mid |
+          parameterValueFlowCand(p, mid, false) and
+          readStep(mid, _, node) and
+          read = true
+        )
+        or
+        // flow through: no prior read
+        exists(ArgumentNode arg |
+          parameterValueFlowArgCand(p, arg, false) and
+          argumentValueFlowsThroughCand(arg, node, read)
+        )
+        or
+        // flow through: no read inside method
+        exists(ArgumentNode arg |
+          parameterValueFlowArgCand(p, arg, read) and
+          argumentValueFlowsThroughCand(arg, node, false)
+        )
+      }
+
+      pragma[nomagic]
+      private predicate parameterValueFlowArgCand(ParameterNode p, ArgumentNode arg, boolean read) {
+        parameterValueFlowCand(p, arg, read)
+      }
+
+      pragma[nomagic]
+      predicate parameterValueFlowsToPreUpdateCand(ParameterNode p, PostUpdateNode n) {
+        parameterValueFlowCand(p, n.getPreUpdateNode(), false)
+      }
+
+      /**
+       * Holds if `p` can flow to a return node of kind `kind` in the same
+       * callable using only value-preserving steps, not taking call contexts
+       * into account.
+       *
+       * `read` indicates whether it is contents of `p` that can flow to the return
+       * node.
+       */
+      predicate parameterValueFlowReturnCand(ParameterNode p, ReturnKind kind, boolean read) {
+        exists(ReturnNode ret |
+          parameterValueFlowCand(p, ret, read) and
+          kind = ret.getKind()
+        )
+      }
+
+      pragma[nomagic]
+      private predicate argumentValueFlowsThroughCand0(
+        DataFlowCall call, ArgumentNode arg, ReturnKind kind, boolean read
+      ) {
+        exists(ParameterNode param | viableParamArg(call, param, arg) |
+          parameterValueFlowReturnCand(param, kind, read)
+        )
+      }
+
+      /**
+       * Holds if `arg` flows to `out` through a call using only value-preserving steps,
+       * not taking call contexts into account.
+       *
+       * `read` indicates whether it is contents of `arg` that can flow to `out`.
+       */
+      predicate argumentValueFlowsThroughCand(ArgumentNode arg, Node out, boolean read) {
+        exists(DataFlowCall call, ReturnKind kind |
+          argumentValueFlowsThroughCand0(call, arg, kind, read) and
+          out = getAnOutNode(call, kind)
+        )
+      }
+
+      predicate cand(ParameterNode p, Node n) {
+        parameterValueFlowCand(p, n, _) and
+        (
+          parameterValueFlowReturnCand(p, _, _)
+          or
+          parameterValueFlowsToPreUpdateCand(p, _)
+        )
+      }
+    }
+
+    /**
+     * The final flow-through calculation:
+     *
+     * - Calculated flow is either value-preserving (`read = TReadStepTypesNone()`)
+     *   or summarized as a single read step with before and after types recorded
+     *   in the `ReadStepTypesOption` parameter.
+     * - Types are checked using the `compatibleTypes()` relation.
+     */
+    private module Final {
+      /**
+       * Holds if `p` can flow to `node` in the same callable using only
+       * value-preserving steps and possibly a single read step, not taking
+       * call contexts into account.
+       *
+       * If a read step was taken, then `read` captures the `Content`, the
+       * container type, and the content type.
+       */
+      predicate parameterValueFlow(ParameterNode p, Node node, ReadStepTypesOption read) {
+        parameterValueFlow0(p, node, read) and
+        if node instanceof CastingNode
+        then
+          // normal flow through
+          read = TReadStepTypesNone() and
+          compatibleTypes(getNodeType(p), getNodeType(node))
+          or
+          // getter
+          compatibleTypes(read.getContentType(), getNodeType(node))
+        else any()
+      }
+
+      pragma[nomagic]
+      private predicate parameterValueFlow0(ParameterNode p, Node node, ReadStepTypesOption read) {
+        p = node and
+        Cand::cand(p, _) and
+        read = TReadStepTypesNone()
+        or
+        // local flow
+        exists(Node mid |
+          parameterValueFlow(p, mid, read) and
+          simpleLocalFlowStep(mid, node)
+        )
+        or
+        // read
+        exists(Node mid |
+          parameterValueFlow(p, mid, TReadStepTypesNone()) and
+          readStepWithTypes(mid, read.getContainerType(), read.getContent(), node,
+            read.getContentType()) and
+          Cand::parameterValueFlowReturnCand(p, _, true) and
+          compatibleTypes(getNodeType(p), read.getContainerType())
+        )
+        or
+        parameterValueFlow0_0(TReadStepTypesNone(), p, node, read)
+      }
+
+      pragma[nomagic]
+      private predicate parameterValueFlow0_0(
+        ReadStepTypesOption mustBeNone, ParameterNode p, Node node, ReadStepTypesOption read
+      ) {
+        // flow through: no prior read
+        exists(ArgumentNode arg |
+          parameterValueFlowArg(p, arg, mustBeNone) and
+          argumentValueFlowsThrough(arg, read, node)
+        )
+        or
+        // flow through: no read inside method
+        exists(ArgumentNode arg |
+          parameterValueFlowArg(p, arg, read) and
+          argumentValueFlowsThrough(arg, mustBeNone, node)
+        )
+      }
+
+      pragma[nomagic]
+      private predicate parameterValueFlowArg(
+        ParameterNode p, ArgumentNode arg, ReadStepTypesOption read
+      ) {
+        parameterValueFlow(p, arg, read) and
+        Cand::argumentValueFlowsThroughCand(arg, _, _)
+      }
+
+      pragma[nomagic]
+      private predicate argumentValueFlowsThrough0(
+        DataFlowCall call, ArgumentNode arg, ReturnKind kind, ReadStepTypesOption read
+      ) {
+        exists(ParameterNode param | viableParamArg(call, param, arg) |
+          parameterValueFlowReturn(param, kind, read)
+        )
+      }
+
+      /**
+       * Holds if `arg` flows to `out` through a call using only
+       * value-preserving steps and possibly a single read step, not taking
+       * call contexts into account.
+       *
+       * If a read step was taken, then `read` captures the `Content`, the
+       * container type, and the content type.
+       */
+      pragma[nomagic]
+      predicate argumentValueFlowsThrough(ArgumentNode arg, ReadStepTypesOption read, Node out) {
+        exists(DataFlowCall call, ReturnKind kind |
+          argumentValueFlowsThrough0(call, arg, kind, read) and
+          out = getAnOutNode(call, kind)
+        |
+          // normal flow through
+          read = TReadStepTypesNone() and
+          compatibleTypes(getNodeType(arg), getNodeType(out))
+          or
+          // getter
+          compatibleTypes(getNodeType(arg), read.getContainerType()) and
+          compatibleTypes(read.getContentType(), getNodeType(out))
+        )
+      }
+
+      /**
+       * Holds if `arg` flows to `out` through a call using only
+       * value-preserving steps and a single read step, not taking call
+       * contexts into account, thus representing a getter-step.
+       */
+      predicate getterStep(ArgumentNode arg, Content c, Node out) {
+        argumentValueFlowsThrough(arg, TReadStepTypesSome(_, c, _), out)
+      }
+
+      /**
+       * Holds if `p` can flow to a return node of kind `kind` in the same
+       * callable using only value-preserving steps and possibly a single read
+       * step.
+       *
+       * If a read step was taken, then `read` captures the `Content`, the
+       * container type, and the content type.
+       */
+      private predicate parameterValueFlowReturn(
+        ParameterNode p, ReturnKind kind, ReadStepTypesOption read
+      ) {
+        exists(ReturnNode ret |
+          parameterValueFlow(p, ret, read) and
+          kind = ret.getKind()
+        )
+      }
+    }
+
+    import Final
+  }
+
+  import FlowThrough
+
+  cached
+  private module DispatchWithCallContext {
+    /**
+     * Holds if the call context `ctx` reduces the set of viable run-time
+     * dispatch targets of call `call` in `c`.
+     */
+    cached
+    predicate reducedViableImplInCallContext(DataFlowCall call, DataFlowCallable c, DataFlowCall ctx) {
+      exists(int tgts, int ctxtgts |
+        mayBenefitFromCallContext(call, c) and
+        c = viableCallable(ctx) and
+        ctxtgts = count(viableImplInCallContext(call, ctx)) and
+        tgts = strictcount(viableCallable(call)) and
+        ctxtgts < tgts
+      )
+    }
+
+    /**
+     * Gets a viable run-time dispatch target for the call `call` in the
+     * context `ctx`. This is restricted to those calls for which a context
+     * makes a difference.
+     */
+    cached
+    DataFlowCallable prunedViableImplInCallContext(DataFlowCall call, DataFlowCall ctx) {
+      result = viableImplInCallContext(call, ctx) and
+      reducedViableImplInCallContext(call, _, ctx)
+    }
+
+    /**
+     * Holds if flow returning from callable `c` to call `call` might return
+     * further and if this path restricts the set of call sites that can be
+     * returned to.
+     */
+    cached
+    predicate reducedViableImplInReturn(DataFlowCallable c, DataFlowCall call) {
+      exists(int tgts, int ctxtgts |
+        mayBenefitFromCallContext(call, _) and
+        c = viableCallable(call) and
+        ctxtgts = count(DataFlowCall ctx | c = viableImplInCallContext(call, ctx)) and
+        tgts = strictcount(DataFlowCall ctx | viableCallable(ctx) = call.getEnclosingCallable()) and
+        ctxtgts < tgts
+      )
+    }
+
+    /**
+     * Gets a viable run-time dispatch target for the call `call` in the
+     * context `ctx`. This is restricted to those calls and results for which
+     * the return flow from the result to `call` restricts the possible context
+     * `ctx`.
+     */
+    cached
+    DataFlowCallable prunedViableImplInCallContextReverse(DataFlowCall call, DataFlowCall ctx) {
+      result = viableImplInCallContext(call, ctx) and
+      reducedViableImplInReturn(result, call)
+    }
+  }
+
+  import DispatchWithCallContext
+
+  /**
+   * Holds if `p` can flow to the pre-update node associated with post-update
+   * node `n`, in the same callable, using only value-preserving steps.
+   */
+  cached
+  predicate parameterValueFlowsToPreUpdate(ParameterNode p, PostUpdateNode n) {
+    parameterValueFlow(p, n.getPreUpdateNode(), TReadStepTypesNone())
+  }
+
+  private predicate store(
+    Node node1, Content c, Node node2, DataFlowType contentType, DataFlowType containerType
+  ) {
+    storeStep(node1, c, node2) and
+    readStep(_, c, _) and
+    contentType = getNodeType(node1) and
+    containerType = getNodeType(node2)
+    or
+    exists(Node n1, Node n2 |
+      n1 = node1.(PostUpdateNode).getPreUpdateNode() and
+      n2 = node2.(PostUpdateNode).getPreUpdateNode()
+    |
+      argumentValueFlowsThrough(n2, TReadStepTypesSome(containerType, c, contentType), n1)
+      or
+      readStep(n2, c, n1) and
+      contentType = getNodeType(n1) and
+      containerType = getNodeType(n2)
+    )
+  }
+
+  /**
+   * Holds if data can flow from `node1` to `node2` via a direct assignment to
+   * `f`.
+   *
+   * This includes reverse steps through reads when the result of the read has
+   * been stored into, in order to handle cases like `x.f1.f2 = y`.
+   */
+  cached
+  predicate store(Node node1, TypedContent tc, Node node2, DataFlowType contentType) {
+    store(node1, tc.getContent(), node2, contentType, tc.getContainerType())
+  }
+
+  /**
+   * Holds if the call context `call` either improves virtual dispatch in
+   * `callable` or if it allows us to prune unreachable nodes in `callable`.
+   */
+  cached
+  predicate recordDataFlowCallSite(DataFlowCall call, DataFlowCallable callable) {
+    reducedViableImplInCallContext(_, callable, call)
+    or
+    exists(Node n | n.getEnclosingCallable() = callable | isUnreachableInCall(n, call))
+  }
+
+  cached
+  newtype TCallContext =
+    TAnyCallContext() or
+    TSpecificCall(DataFlowCall call) { recordDataFlowCallSite(call, _) } or
+    TSomeCall() or
+    TReturn(DataFlowCallable c, DataFlowCall call) { reducedViableImplInReturn(c, call) }
+
+  cached
+  newtype TReturnPosition =
+    TReturnPosition0(DataFlowCallable c, ReturnKindExt kind) {
+      exists(ReturnNodeExt ret |
+        c = returnNodeGetEnclosingCallable(ret) and
+        kind = ret.getKind()
+      )
+    }
+
+  cached
+  newtype TLocalFlowCallContext =
+    TAnyLocalCall() or
+    TSpecificLocalCall(DataFlowCall call) { isUnreachableInCall(_, call) }
+
+  cached
+  newtype TReturnKindExt =
+    TValueReturn(ReturnKind kind) or
+    TParamUpdate(int pos) { exists(ParameterNode p | p.isParameterOf(_, pos)) }
+
+  cached
+  newtype TBooleanOption =
+    TBooleanNone() or
+    TBooleanSome(boolean b) { b = true or b = false }
+
+  cached
+  newtype TTypedContent = MkTypedContent(Content c, DataFlowType t) { store(_, c, _, _, t) }
+
+  cached
+  newtype TAccessPathFront =
+    TFrontNil(DataFlowType t) or
+    TFrontHead(TypedContent tc)
+
+  cached
+  newtype TAccessPathFrontOption =
+    TAccessPathFrontNone() or
+    TAccessPathFrontSome(AccessPathFront apf)
+}
+
+/**
+ * A `Node` at which a cast can occur such that the type should be checked.
+ */
+class CastingNode extends Node {
+  CastingNode() {
+    this instanceof ParameterNode or
+    this instanceof CastNode or
+    this instanceof OutNodeExt or
+    // For reads, `x.f`, we want to check that the tracked type after the read (which
+    // is obtained by popping the head of the access path stack) is compatible with
+    // the type of `x.f`.
+    readStep(_, _, this)
+  }
+}
+
+private predicate readStepWithTypes(
+  Node n1, DataFlowType container, Content c, Node n2, DataFlowType content
+) {
+  readStep(n1, c, n2) and
+  container = getNodeType(n1) and
+  content = getNodeType(n2)
+}
+
+private newtype TReadStepTypesOption =
+  TReadStepTypesNone() or
+  TReadStepTypesSome(DataFlowType container, Content c, DataFlowType content) {
+    readStepWithTypes(_, container, c, _, content)
+  }
+
+private class ReadStepTypesOption extends TReadStepTypesOption {
+  predicate isSome() { this instanceof TReadStepTypesSome }
+
+  DataFlowType getContainerType() { this = TReadStepTypesSome(result, _, _) }
+
+  Content getContent() { this = TReadStepTypesSome(_, result, _) }
+
+  DataFlowType getContentType() { this = TReadStepTypesSome(_, _, result) }
+
+  string toString() { if this.isSome() then result = "Some(..)" else result = "None()" }
+}
+
+/**
+ * A call context to restrict the targets of virtual dispatch, prune local flow,
+ * and match the call sites of flow into a method with flow out of a method.
+ *
+ * There are four cases:
+ * - `TAnyCallContext()` : No restrictions on method flow.
+ * - `TSpecificCall(DataFlowCall call)` : Flow entered through the
+ *    given `call`. This call improves the set of viable
+ *    dispatch targets for at least one method call in the current callable
+ *    or helps prune unreachable nodes in the current callable.
+ * - `TSomeCall()` : Flow entered through a parameter. The
+ *    originating call does not improve the set of dispatch targets for any
+ *    method call in the current callable and was therefore not recorded.
+ * - `TReturn(Callable c, DataFlowCall call)` : Flow reached `call` from `c` and
+ *    this dispatch target of `call` implies a reduced set of dispatch origins
+ *    to which data may flow if it should reach a `return` statement.
+ */
+abstract class CallContext extends TCallContext {
+  abstract string toString();
+
+  /** Holds if this call context is relevant for `callable`. */
+  abstract predicate relevantFor(DataFlowCallable callable);
+}
+
+abstract class CallContextNoCall extends CallContext { }
+
+class CallContextAny extends CallContextNoCall, TAnyCallContext {
+  override string toString() { result = "CcAny" }
+
+  override predicate relevantFor(DataFlowCallable callable) { any() }
+}
+
+abstract class CallContextCall extends CallContext {
+  /** Holds if this call context may be `call`. */
+  bindingset[call]
+  abstract predicate matchesCall(DataFlowCall call);
+}
+
+class CallContextSpecificCall extends CallContextCall, TSpecificCall {
+  override string toString() {
+    exists(DataFlowCall call | this = TSpecificCall(call) | result = "CcCall(" + call + ")")
+  }
+
+  override predicate relevantFor(DataFlowCallable callable) {
+    recordDataFlowCallSite(getCall(), callable)
+  }
+
+  override predicate matchesCall(DataFlowCall call) { call = this.getCall() }
+
+  DataFlowCall getCall() { this = TSpecificCall(result) }
+}
+
+class CallContextSomeCall extends CallContextCall, TSomeCall {
+  override string toString() { result = "CcSomeCall" }
+
+  override predicate relevantFor(DataFlowCallable callable) {
+    exists(ParameterNode p | p.getEnclosingCallable() = callable)
+  }
+
+  override predicate matchesCall(DataFlowCall call) { any() }
+}
+
+class CallContextReturn extends CallContextNoCall, TReturn {
+  override string toString() {
+    exists(DataFlowCall call | this = TReturn(_, call) | result = "CcReturn(" + call + ")")
+  }
+
+  override predicate relevantFor(DataFlowCallable callable) {
+    exists(DataFlowCall call | this = TReturn(_, call) and call.getEnclosingCallable() = callable)
+  }
+}
+
+/**
+ * A call context that is relevant for pruning local flow.
+ */
+abstract class LocalCallContext extends TLocalFlowCallContext {
+  abstract string toString();
+
+  /** Holds if this call context is relevant for `callable`. */
+  abstract predicate relevantFor(DataFlowCallable callable);
+}
+
+class LocalCallContextAny extends LocalCallContext, TAnyLocalCall {
+  override string toString() { result = "LocalCcAny" }
+
+  override predicate relevantFor(DataFlowCallable callable) { any() }
+}
+
+class LocalCallContextSpecificCall extends LocalCallContext, TSpecificLocalCall {
+  LocalCallContextSpecificCall() { this = TSpecificLocalCall(call) }
+
+  DataFlowCall call;
+
+  DataFlowCall getCall() { result = call }
+
+  override string toString() { result = "LocalCcCall(" + call + ")" }
+
+  override predicate relevantFor(DataFlowCallable callable) { relevantLocalCCtx(call, callable) }
+}
+
+private predicate relevantLocalCCtx(DataFlowCall call, DataFlowCallable callable) {
+  exists(Node n | n.getEnclosingCallable() = callable and isUnreachableInCall(n, call))
+}
+
+/**
+ * Gets the local call context given the call context and the callable that
+ * the contexts apply to.
+ */
+LocalCallContext getLocalCallContext(CallContext ctx, DataFlowCallable callable) {
+  ctx.relevantFor(callable) and
+  if relevantLocalCCtx(ctx.(CallContextSpecificCall).getCall(), callable)
+  then result.(LocalCallContextSpecificCall).getCall() = ctx.(CallContextSpecificCall).getCall()
+  else result instanceof LocalCallContextAny
+}
+
+/**
+ * A node from which flow can return to the caller. This is either a regular
+ * `ReturnNode` or a `PostUpdateNode` corresponding to the value of a parameter.
+ */
+class ReturnNodeExt extends Node {
+  ReturnNodeExt() {
+    this instanceof ReturnNode or
+    parameterValueFlowsToPreUpdate(_, this)
+  }
+
+  /** Gets the kind of this returned value. */
+  ReturnKindExt getKind() {
+    result = TValueReturn(this.(ReturnNode).getKind())
+    or
+    exists(ParameterNode p, int pos |
+      parameterValueFlowsToPreUpdate(p, this) and
+      p.isParameterOf(_, pos) and
+      result = TParamUpdate(pos)
+    )
+  }
+}
+
+/**
+ * A node to which data can flow from a call. Either an ordinary out node
+ * or a post-update node associated with a call argument.
+ */
+class OutNodeExt extends Node {
+  OutNodeExt() {
+    this instanceof OutNode
+    or
+    this.(PostUpdateNode).getPreUpdateNode() instanceof ArgumentNode
+  }
+}
+
+/**
+ * An extended return kind. A return kind describes how data can be returned
+ * from a callable. This can either be through a returned value or an updated
+ * parameter.
+ */
+abstract class ReturnKindExt extends TReturnKindExt {
+  /** Gets a textual representation of this return kind. */
+  abstract string toString();
+
+  /** Gets a node corresponding to data flow out of `call`. */
+  abstract OutNodeExt getAnOutNode(DataFlowCall call);
+}
+
+class ValueReturnKind extends ReturnKindExt, TValueReturn {
+  private ReturnKind kind;
+
+  ValueReturnKind() { this = TValueReturn(kind) }
+
+  ReturnKind getKind() { result = kind }
+
+  override string toString() { result = kind.toString() }
+
+  override OutNodeExt getAnOutNode(DataFlowCall call) {
+    result = getAnOutNode(call, this.getKind())
+  }
+}
+
+class ParamUpdateReturnKind extends ReturnKindExt, TParamUpdate {
+  private int pos;
+
+  ParamUpdateReturnKind() { this = TParamUpdate(pos) }
+
+  int getPosition() { result = pos }
+
+  override string toString() { result = "param update " + pos }
+
+  override OutNodeExt getAnOutNode(DataFlowCall call) {
+    exists(ArgumentNode arg |
+      result.(PostUpdateNode).getPreUpdateNode() = arg and
+      arg.argumentOf(call, this.getPosition())
+    )
+  }
+}
+
+/** A callable tagged with a relevant return kind. */
+class ReturnPosition extends TReturnPosition0 {
+  private DataFlowCallable c;
+  private ReturnKindExt kind;
+
+  ReturnPosition() { this = TReturnPosition0(c, kind) }
+
+  /** Gets the callable. */
+  DataFlowCallable getCallable() { result = c }
+
+  /** Gets the return kind. */
+  ReturnKindExt getKind() { result = kind }
+
+  /** Gets a textual representation of this return position. */
+  string toString() { result = "[" + kind + "] " + c }
+}
+
+pragma[noinline]
+private DataFlowCallable returnNodeGetEnclosingCallable(ReturnNodeExt ret) {
+  result = ret.getEnclosingCallable()
+}
+
+pragma[noinline]
+private ReturnPosition getReturnPosition0(ReturnNodeExt ret, ReturnKindExt kind) {
+  result.getCallable() = returnNodeGetEnclosingCallable(ret) and
+  kind = result.getKind()
+}
+
+pragma[noinline]
+ReturnPosition getReturnPosition(ReturnNodeExt ret) {
+  result = getReturnPosition0(ret, ret.getKind())
+}
+
+bindingset[cc, callable]
+predicate resolveReturn(CallContext cc, DataFlowCallable callable, DataFlowCall call) {
+  cc instanceof CallContextAny and callable = viableCallable(call)
+  or
+  exists(DataFlowCallable c0, DataFlowCall call0 |
+    call0.getEnclosingCallable() = callable and
+    cc = TReturn(c0, call0) and
+    c0 = prunedViableImplInCallContextReverse(call0, call)
+  )
+}
+
+bindingset[call, cc]
+DataFlowCallable resolveCall(DataFlowCall call, CallContext cc) {
+  exists(DataFlowCall ctx | cc = TSpecificCall(ctx) |
+    if reducedViableImplInCallContext(call, _, ctx)
+    then result = prunedViableImplInCallContext(call, ctx)
+    else result = viableCallable(call)
+  )
+  or
+  result = viableCallable(call) and cc instanceof CallContextSomeCall
+  or
+  result = viableCallable(call) and cc instanceof CallContextAny
+  or
+  result = viableCallable(call) and cc instanceof CallContextReturn
+}
+
+predicate read = readStep/3;
+
+/** An optional Boolean value. */
+class BooleanOption extends TBooleanOption {
+  string toString() {
+    this = TBooleanNone() and result = "<none>"
+    or
+    this = TBooleanSome(any(boolean b | result = b.toString()))
+  }
+}
+
+/** Content tagged with the type of a containing object. */
+class TypedContent extends MkTypedContent {
+  private Content c;
+  private DataFlowType t;
+
+  TypedContent() { this = MkTypedContent(c, t) }
+
+  /** Gets the content. */
+  Content getContent() { result = c }
+
+  /** Gets the container type. */
+  DataFlowType getContainerType() { result = t }
+
+  /** Gets a textual representation of this content. */
+  string toString() { result = c.toString() }
+}
+
+/**
+ * The front of an access path. This is either a head or a nil.
+ */
+abstract class AccessPathFront extends TAccessPathFront {
+  abstract string toString();
+
+  abstract DataFlowType getType();
+
+  abstract boolean toBoolNonEmpty();
+
+  TypedContent getHead() { this = TFrontHead(result) }
+
+  predicate isClearedAt(Node n) { clearsContent(n, getHead().getContent()) }
+}
+
+class AccessPathFrontNil extends AccessPathFront, TFrontNil {
+  private DataFlowType t;
+
+  AccessPathFrontNil() { this = TFrontNil(t) }
+
+  override string toString() { result = ppReprType(t) }
+
+  override DataFlowType getType() { result = t }
+
+  override boolean toBoolNonEmpty() { result = false }
+}
+
+class AccessPathFrontHead extends AccessPathFront, TFrontHead {
+  private TypedContent tc;
+
+  AccessPathFrontHead() { this = TFrontHead(tc) }
+
+  override string toString() { result = tc.toString() }
+
+  override DataFlowType getType() { result = tc.getContainerType() }
+
+  override boolean toBoolNonEmpty() { result = true }
+}
+
+/** An optional access path front. */
+class AccessPathFrontOption extends TAccessPathFrontOption {
+  string toString() {
+    this = TAccessPathFrontNone() and result = "<none>"
+    or
+    this = TAccessPathFrontSome(any(AccessPathFront apf | result = apf.toString()))
+  }
+}