add tracking of strings to compile-sites for poly-redos, in the style of Ruby

python/ql/lib/semmle/python/regex.qll

@@ -85,6 +85,17 @@ predicate used_as_regex(Expr s, string mode) {
   )
 }
 
+private import semmle.python.Concepts
+private import semmle.python.RegexTreeView
+
+/** Gets a parsed regular expression term that is executed at `exec`. */
+RegExpTerm getTermForExecution(RegexExecution exec) {
+  exists(RegexTracking t, DataFlow::Node source | t.hasFlow(source, exec.getRegex()) |
+    result.getRegex() = source.asExpr() and
+    result.isRootTerm()
+  )
+}
+
 /**
  * Gets the canonical name for the API graph node corresponding to the `re` flag `flag`. For flags
  * that have multiple names, we pick the long-form name as a canonical representative.

python/ql/lib/semmle/python/security/dataflow/PolynomialReDoSCustomizations.qll

@@ -13,6 +13,7 @@ private import semmle.python.dataflow.new.RemoteFlowSources
 private import semmle.python.dataflow.new.BarrierGuards
 private import semmle.python.RegexTreeView::RegexTreeView as TreeView
 private import semmle.python.ApiGraphs
+private import semmle.python.regex
 
 /**
  * Provides default sources, sinks and sanitizers for detecting
@@ -66,7 +67,7 @@ module PolynomialReDoS {
 
     RegexExecutionAsSink() {
       exists(RegexExecution re |
-        re.getRegex().asExpr() = t.getRegex() and
+        t = getTermForExecution(re) and
         this = re.getString()
       ) and
       t.isRootTerm()