Merge pull request #14777 from yoff/python/remove-ssa-nodes-from-dataflow-graph

Python: remove EssaNodes

Author: tausbn

python/ql/lib/change-notes/2023-11-20-remove-essa-nodes-from-dataflow-graph.md

@@ -0,0 +1,5 @@
+---
+category: fix
+---
+
+- The dataflow graph no longer contains SSA variables. Instead, flow is directed via the corresponding controlflow nodes. This should make the graph and the flow simpler to understand. Minor improvements in flow computation has been observed, but in general negligible changes to alerts are expected.

python/ql/lib/semmle/python/Flow.qll

@@ -126,7 +126,10 @@ class ControlFlowNode extends @py_flow_node {
   cached
   string toString() {
     Stages::AST::ref() and
-    exists(Scope s | s.getEntryNode() = this | result = "Entry node for " + s.toString())
+    // Since modules can have ambigous names, entry nodes can too, if we do not collate them.
+    exists(Scope s | s.getEntryNode() = this |
+      result = "Entry node for " + concat( | | s.toString(), ",")
+    )
     or
     exists(Scope s | s.getANormalExit() = this | result = "Exit node for " + s.toString())
     or

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

@@ -281,28 +281,33 @@ class DataFlowExpr = Expr;
 /**
  * A module to compute local flow.
  *
- * Flow will generally go from control flow nodes into essa variables at definitions,
+ * Flow will generally go from control flow nodes for expressions into
+ * control flow nodes for variables at definitions,
  * and from there via use-use flow to other control flow nodes.
  *
  * Some syntaxtic constructs are handled separately.
  */
 module LocalFlow {
-  /** Holds if `nodeFrom` is the control flow node defining the essa variable `nodeTo`. */
+  /** Holds if `nodeFrom` is the expression defining the value for the variable `nodeTo`. */
   predicate definitionFlowStep(Node nodeFrom, Node nodeTo) {
     // Definition
     //   `x = f(42)`
-    //   nodeFrom is `f(42)`, cfg node
-    //   nodeTo is `x`, essa var
-    nodeFrom.(CfgNode).getNode() =
-      nodeTo.(EssaNode).getVar().getDefinition().(AssignmentDefinition).getValue()
+    //   nodeFrom is `f(42)`
+    //   nodeTo is `x`
+    exists(AssignmentDefinition def |
+      nodeFrom.(CfgNode).getNode() = def.getValue() and
+      nodeTo.(CfgNode).getNode() = def.getDefiningNode()
+    )
     or
     // With definition
     //   `with f(42) as x:`
-    //   nodeFrom is `f(42)`, cfg node
-    //   nodeTo is `x`, essa var
-    exists(With with, ControlFlowNode contextManager, ControlFlowNode var |
+    //   nodeFrom is `f(42)`
+    //   nodeTo is `x`
+    exists(With with, ControlFlowNode contextManager, WithDefinition withDef, ControlFlowNode var |
+      var = withDef.getDefiningNode()
+    |
       nodeFrom.(CfgNode).getNode() = contextManager and
-      nodeTo.(EssaNode).getVar().getDefinition().(WithDefinition).getDefiningNode() = var and
+      nodeTo.(CfgNode).getNode() = var and
       // see `with_flow` in `python/ql/src/semmle/python/dataflow/Implementation.qll`
       with.getContextExpr() = contextManager.getNode() and
       with.getOptionalVars() = var.getNode() and
@@ -313,34 +318,6 @@ module LocalFlow {
       // * `foo = x.foo(); await foo.async_method(); foo.close()` and
       // * `async with x.foo() as foo: await foo.async_method()`.
     )
-    or
-    // Async with var definition
-    //  `async with f(42) as x:`
-    //  nodeFrom is `x`, cfg node
-    //  nodeTo is `x`, essa var
-    //
-    // This makes the cfg node the local source of the awaited value.
-    //
-    // We have this step in addition to the step above, to handle cases where the QL
-    // modeling of `f(42)` requires a `.getAwaited()` step (in API graphs) when not
-    // using `async with`, so you can do both:
-    // * `foo = await x.foo(); await foo.async_method(); foo.close()` and
-    // * `async with x.foo() as foo: await foo.async_method()`.
-    exists(With with, ControlFlowNode var |
-      nodeFrom.(CfgNode).getNode() = var and
-      nodeTo.(EssaNode).getVar().getDefinition().(WithDefinition).getDefiningNode() = var and
-      with.getOptionalVars() = var.getNode() and
-      with.isAsync()
-    )
-    or
-    // Parameter definition
-    //   `def foo(x):`
-    //   nodeFrom is `x`, cfgNode
-    //   nodeTo is `x`, essa var
-    exists(ParameterDefinition pd |
-      nodeFrom.(CfgNode).getNode() = pd.getDefiningNode() and
-      nodeTo.(EssaNode).getVar() = pd.getVariable()
-    )
   }
 
   predicate expressionFlowStep(Node nodeFrom, Node nodeTo) {
@@ -372,9 +349,12 @@ module LocalFlow {
     // First use after definition
     //   `y = 42`
     //   `x = f(y)`
-    //   nodeFrom is `y` on first line, essa var
-    //   nodeTo is `y` on second line, cfg node
-    defToFirstUse(nodeFrom.asVar(), nodeTo.asCfgNode())
+    //   nodeFrom is `y` on first line
+    //   nodeTo is `y` on second line
+    exists(EssaDefinition def |
+      nodeFrom.(CfgNode).getNode() = def.(EssaNodeDefinition).getDefiningNode() and
+      AdjacentUses::firstUse(def, nodeTo.(CfgNode).getNode())
+    )
     or
     // Next use after use
     //   `x = f(y)`
@@ -565,11 +545,7 @@ predicate neverSkipInPathGraph(Node n) {
   // ```
   // we would end up saying that the path MUST not skip the x in `y = x`, which is just
   // annoying and doesn't help the path explanation become clearer.
-  n.asVar() instanceof EssaDefinition and
-  // For a parameter we have flow from ControlFlowNode to SSA node, and then onwards
-  // with use-use flow, and since the CFN is already part of the path graph, we don't
-  // want to force showing the SSA node as well.
-  not n.asVar() instanceof ParameterDefinition
+  n.asCfgNode() = any(EssaNodeDefinition def).getDefiningNode()
 }
 
 /**
@@ -916,7 +892,7 @@ predicate subscriptReadStep(CfgNode nodeFrom, Content c, CfgNode nodeTo) {
 predicate forReadStep(CfgNode nodeFrom, Content c, Node nodeTo) {
   exists(ForTarget target |
     nodeFrom.asExpr() = target.getSource() and
-    nodeTo.asVar().(EssaNodeDefinition).getDefiningNode() = target
+    nodeTo.asCfgNode() = target
   ) and
   (
     c instanceof ListElementContent

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

@@ -24,13 +24,13 @@ private import semmle.python.frameworks.data.ModelsAsData
  * The current implementation of these cross flows can be seen in `EssaTaintTracking`.
  */
 newtype TNode =
-  /** A node corresponding to an SSA variable. */
-  TEssaNode(EssaVariable var) or
   /** A node corresponding to a control flow node. */
   TCfgNode(ControlFlowNode node) {
     isExpressionNode(node)
     or
     node.getNode() instanceof Pattern
+    or
+    node = any(ScopeEntryDefinition def).getDefiningNode()
   } or
   /**
    * A synthetic node representing the value of an object before a state change.
@@ -156,9 +156,6 @@ class Node extends TNode {
     this.getLocation().hasLocationInfo(filepath, startline, startcolumn, endline, endcolumn)
   }
 
-  /** Gets the ESSA variable corresponding to this node, if any. */
-  EssaVariable asVar() { none() }
-
   /** Gets the control-flow node corresponding to this node, if any. */
   ControlFlowNode asCfgNode() { none() }
 
@@ -171,25 +168,6 @@ class Node extends TNode {
   LocalSourceNode getALocalSource() { result.flowsTo(this) }
 }
 
-/** A data-flow node corresponding to an SSA variable. */
-class EssaNode extends Node, TEssaNode {
-  EssaVariable var;
-
-  EssaNode() { this = TEssaNode(var) }
-
-  /** Gets the `EssaVariable` represented by this data-flow node. */
-  EssaVariable getVar() { result = var }
-
-  override EssaVariable asVar() { result = var }
-
-  /** Gets a textual representation of this element. */
-  override string toString() { result = var.toString() }
-
-  override Scope getScope() { result = var.getScope() }
-
-  override Location getLocation() { result = var.getLocation() }
-}
-
 /** A data-flow node corresponding to a control-flow node. */
 class CfgNode extends Node, TCfgNode {
   ControlFlowNode node;
@@ -412,8 +390,8 @@ class ModuleVariableNode extends Node, TModuleVariableNode {
   }
 
   /** Gets an `EssaNode` that corresponds to an assignment of this global variable. */
-  EssaNode getAWrite() {
-    result.getVar().getDefinition().(EssaNodeDefinition).definedBy(var, any(DefinitionNode defn))
+  Node getAWrite() {
+    any(EssaNodeDefinition def).definedBy(var, result.asCfgNode().(DefinitionNode))
   }
 
   /** Gets the possible values of the variable at the end of import time */

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

@@ -112,7 +112,7 @@ module ImportResolution {
       not allowedEssaImportStep(_, firstDef)
     |
       not LocalFlow::defToFirstUse(firstDef, _) and
-      val.asVar() = firstDef
+      val.asCfgNode() = firstDef.getDefinition().(EssaNodeDefinition).getDefiningNode()
       or
       exists(ControlFlowNode mid, ControlFlowNode end |
         LocalFlow::defToFirstUse(firstDef, mid) and
@@ -320,11 +320,11 @@ module ImportResolution {
     // name as a submodule, we always consider that this attribute _could_ be a
     // reference to the submodule, even if we don't know that the submodule has been
     // imported yet.
-    exists(string submodule, Module package |
-      submodule = result.asVar().getName() and
-      SsaSource::init_module_submodule_defn(result.asVar().getSourceVariable(),
-        package.getEntryNode()) and
-      m = getModuleFromName(package.getPackageName() + "." + submodule)
+    exists(string submodule, Module package, EssaVariable var |
+      submodule = var.getName() and
+      SsaSource::init_module_submodule_defn(var.getSourceVariable(), package.getEntryNode()) and
+      m = getModuleFromName(package.getPackageName() + "." + submodule) and
+      result.asCfgNode() = var.getDefinition().(EssaNodeDefinition).getDefiningNode()
     )
   }
 

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

@@ -87,13 +87,13 @@
  *    This is adequate as the route through `TIterableElement(sequence)` does not transfer precise content.
  *
  * 5. [Read] Content is read from `sequence` to its elements.
- *    a) If the element is a plain variable, the target is the corresponding essa node.
+ *    a) If the element is a plain variable, the target is the corresponding control flow node.
  *
  *    b) If the element is itself a sequence, with control-flow node `seq`, the target is `TIterableSequence(seq)`.
  *
  *    c) If the element is a starred variable, with control-flow node `v`, the target is `TIterableElement(v)`.
  *
- * 6. [Store] Content is stored from `TIterableElement(v)` to the essa variable for `v`, with
+ * 6. [Store] Content is stored from `TIterableElement(v)` to the control flow node for variable `v`, with
  *    content type `ListElementContent`.
  *
  * 7. [Flow, Read, Store] Steps 2 through 7 are repeated for all recursive elements which are sequences.
@@ -313,7 +313,7 @@ predicate iterableUnpackingConvertingStoreStep(Node nodeFrom, Content c, Node no
  * Step 5
  * For a sequence node inside an iterable unpacking, data flows from the sequence to its elements. There are
  * three cases for what `toNode` should be:
- *    a) If the element is a plain variable, `toNode` is the corresponding essa node.
+ *    a) If the element is a plain variable, `toNode` is the corresponding control flow node.
  *
  *    b) If the element is itself a sequence, with control-flow node `seq`, `toNode` is `TIterableSequence(seq)`.
  *
@@ -351,20 +351,25 @@ predicate iterableUnpackingElementReadStep(Node nodeFrom, Content c, Node nodeTo
           nodeTo = TIterableElementNode(element)
         else
           // Step 5a
-          nodeTo.asVar().getDefinition().(MultiAssignmentDefinition).getDefiningNode() = element
+          exists(MultiAssignmentDefinition mad | element = mad.getDefiningNode() |
+            nodeTo.(CfgNode).getNode() = element
+          )
     )
   )
 }
 
 /**
  * Step 6
- * Data flows from `TIterableElement(v)` to the essa variable for `v`, with
+ * Data flows from `TIterableElement(v)` to the control flow node for variable `v`, with
  * content type `ListElementContent`.
  */
 predicate iterableUnpackingStarredElementStoreStep(Node nodeFrom, Content c, Node nodeTo) {
-  exists(ControlFlowNode starred | starred.getNode() instanceof Starred |
+  exists(ControlFlowNode starred, MultiAssignmentDefinition mad |
+    starred.getNode() instanceof Starred and
+    starred = mad.getDefiningNode()
+  |
     nodeFrom = TIterableElementNode(starred) and
-    nodeTo.asVar().getDefinition().(MultiAssignmentDefinition).getDefiningNode() = starred and
+    nodeTo.asCfgNode() = starred and
     c instanceof ListElementContent
   )
 }

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

@@ -71,7 +71,7 @@ class LocalSourceNode extends Node {
     or
     // We include all scope entry definitions, as these act as the local source within the scope they
     // enter.
-    this.asVar() instanceof ScopeEntryDefinition
+    this.asCfgNode() = any(ScopeEntryDefinition def).getDefiningNode()
   }
 
   /** Holds if this `LocalSourceNode` can flow to `nodeTo` in one or more local flow steps. */
@@ -165,7 +165,7 @@ class LocalSourceNodeNotModuleVariableNode extends LocalSourceNode {
   LocalSourceNodeNotModuleVariableNode() {
     this instanceof ExprNode
     or
-    this.asVar() instanceof ScopeEntryDefinition
+    this.asCfgNode() = any(ScopeEntryDefinition def).getDefiningNode()
   }
 }
 

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

@@ -89,8 +89,9 @@ predicate matchAsFlowStep(Node nodeFrom, Node nodeTo) {
     or
     // the interior pattern flows to the alias
     nodeFrom.(CfgNode).getNode().getNode() = subject.getPattern() and
-    nodeTo.(EssaNode).getVar().getDefinition().(PatternAliasDefinition).getDefiningNode().getNode() =
-      alias
+    exists(PatternAliasDefinition pad | pad.getDefiningNode().getNode() = alias |
+      nodeTo.(CfgNode).getNode() = pad.getDefiningNode()
+    )
   )
 }
 
@@ -123,13 +124,9 @@ predicate matchLiteralFlowStep(Node nodeFrom, Node nodeTo) {
 predicate matchCaptureFlowStep(Node nodeFrom, Node nodeTo) {
   exists(MatchCapturePattern capture, Name var | capture.getVariable() = var |
     nodeFrom.(CfgNode).getNode().getNode() = capture and
-    nodeTo
-        .(EssaNode)
-        .getVar()
-        .getDefinition()
-        .(PatternCaptureDefinition)
-        .getDefiningNode()
-        .getNode() = var
+    exists(PatternCaptureDefinition pcd | pcd.getDefiningNode().getNode() = var |
+      nodeTo.(CfgNode).getNode() = pcd.getDefiningNode()
+    )
   )
 }
 

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

@@ -216,8 +216,10 @@ predicate awaitStep(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
  */
 predicate asyncWithStep(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
   exists(With with, ControlFlowNode contextManager, ControlFlowNode var |
+    var = any(WithDefinition wd).getDefiningNode()
+  |
     nodeFrom.(DataFlow::CfgNode).getNode() = contextManager and
-    nodeTo.(DataFlow::EssaNode).getVar().getDefinition().(WithDefinition).getDefiningNode() = var and
+    nodeTo.(DataFlow::CfgNode).getNode() = var and
     // see `with_flow` in `python/ql/src/semmle/python/dataflow/Implementation.qll`
     with.getContextExpr() = contextManager.getNode() and
     with.getOptionalVars() = var.getNode() and

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

@@ -50,8 +50,20 @@ predicate jumpStep(Node nodeFrom, Node nodeTo) {
 }
 
 predicate capturedJumpStep(Node nodeFrom, Node nodeTo) {
-  exists(SsaSourceVariable var, DefinitionNode def | var.hasDefiningNode(def) |
-    nodeTo.asVar().(ScopeEntryDefinition).getSourceVariable() = var and
+  // Jump into a capturing scope.
+  //
+  // var = expr
+  // ...
+  // def f():
+  // ..var is used..
+  //
+  // nodeFrom is `expr`
+  // nodeTo is entry node for `f`
+  exists(ScopeEntryDefinition e, SsaSourceVariable var, DefinitionNode def |
+    e.getSourceVariable() = var and
+    var.hasDefiningNode(def)
+  |
+    nodeTo.asCfgNode() = e.getDefiningNode() and
     nodeFrom.asCfgNode() = def.getValue() and
     var.getScope().getScope*() = nodeFrom.getScope()
   )
@@ -228,8 +240,7 @@ private module SummaryTypeTrackerInput implements SummaryTypeTracker::Input {
     |
       param = FlowSummary::SummaryComponent::parameter(apos) and
       DataFlowDispatch::parameterMatch(ppos, apos) and
-      // pick the SsaNode rather than the CfgNode
-      result.asVar().getDefinition().(ParameterDefinition).getParameter() = p and
+      result.asCfgNode().getNode() = p and
       (
         exists(int i | ppos.isPositional(i) |
           p = callable.getALocalSource().asExpr().(CallableExpr).getInnerScope().getArg(i)