Move StdBasicRegex to shared StdRegex module; return ExploitableStringLiteral to flow layer

cpp/ql/lib/semmle/code/cpp/regex/RegexFlowConfigs.qll

@@ -30,18 +30,13 @@ import cpp
 private import semmle.code.cpp.dataflow.new.DataFlow
 private import semmle.code.cpp.dataflow.new.TaintTracking
 private import semmle.code.cpp.regex.internal.RegexGrammar as RG
+import semmle.code.cpp.regex.internal.StdRegex
 
 // ---------------------------------------------------------------------------
 // Re-exports from the flow-free grammar/flag module.
 // These are kept in scope for backward compatibility so that consumers can
 // continue to import them from `RegexFlowConfigs`.
 // ---------------------------------------------------------------------------
-/**
- * A `std::basic_regex` class type (or instantiation thereof, e.g. `std::regex`,
- * `std::wregex`). Defined in `RegexGrammar`; re-exported here.
- */
-class StdBasicRegex = RG::StdBasicRegex;
-
 /** See `RegexGrammar::TRegexGrammar`. */
 class TRegexGrammar = RG::TRegexGrammar;
 
@@ -202,10 +197,24 @@ class RegexPatternSink extends DataFlow::Node {
 
 // ---------------------------------------------------------------------------
 // Fast-path: only track literals that look regex-y.
-// `ExploitableStringLiteral` is defined in the flow-free `RegexGrammar`
-// module; re-exported here for backward compatibility.
 // ---------------------------------------------------------------------------
-class ExploitableStringLiteral = RG::ExploitableStringLiteral;
+/**
+ * A string literal that is a plausible ReDoS candidate: it contains at least
+ * one unbounded-repetition quantifier (`+`, `*`, or `{n,}`).
+ *
+ * This is used as the source set of the taint-tracking configuration below:
+ * regexes without such a quantifier are not interesting for the
+ * polynomial-ReDoS analysis, so filtering them out here is a significant
+ * optimisation.
+ */
+class ExploitableStringLiteral extends StringLiteral {
+  ExploitableStringLiteral() {
+    exists(string s | s = this.getValue() |
+      s.regexpMatch(".*[+*].*") or
+      s.regexpMatch(".*\\{[0-9]+,[0-9]*\\}.*")
+    )
+  }
+}
 
 /**
  * A dataflow configuration tracking string literals that reach a regex

cpp/ql/lib/semmle/code/cpp/regex/internal/RegexGrammar.qll

@@ -21,42 +21,27 @@
  */
 
 import cpp
+private import semmle.code.cpp.regex.internal.StdRegex
 
 // ---------------------------------------------------------------------------
 // Fast-path filter: only consider literals that *look* regex-y
 // ---------------------------------------------------------------------------
 /**
- * A string literal that is a plausible ReDoS candidate: it contains at least
- * one unbounded-repetition quantifier (`+`, `*`, or `{n,}`).
+ * Holds if `s` is a plausible ReDoS-candidate string literal value: it
+ * contains at least one unbounded-repetition quantifier (`+`, `*`, or
+ * `{n,}`).
  *
- * This filter is applied by the flag readers below as an optimisation:
- * regexes without such a quantifier are not interesting for the
- * polynomial-ReDoS analysis, and skipping them here mirrors the source
- * restriction of the dataflow configuration in `RegexFlowConfigs`,
- * preserving analysis results exactly.
+ * This mirrors the source restriction of the dataflow configuration in
+ * `RegexFlowConfigs` (whose sources are the `ExploitableStringLiteral`s),
+ * so that the flag readers below consider exactly the same set of regexes
+ * as the taint-tracking configuration. Inlined here as a syntactic check
+ * to keep this module flow-free.
  */
-class ExploitableStringLiteral extends StringLiteral {
-  ExploitableStringLiteral() {
-    exists(string s | s = this.getValue() |
-      s.regexpMatch(".*[+*].*") or
-      s.regexpMatch(".*\\{[0-9]+,[0-9]*\\}.*")
-    )
-  }
-}
-
-// ---------------------------------------------------------------------------
-// std::basic_regex identification
-// ---------------------------------------------------------------------------
-/**
- * A `std::basic_regex` class type (or instantiation thereof, e.g. `std::regex`,
- * `std::wregex`).
- */
-class StdBasicRegex extends Class {
-  StdBasicRegex() {
-    this.hasQualifiedName("std", "basic_regex")
-    or
-    this.(ClassTemplateInstantiation).getTemplate().hasQualifiedName("std", "basic_regex")
-  }
+private predicate isExploitableStringLiteralValue(StringLiteral s) {
+  exists(string v | v = s.getValue() |
+    v.regexpMatch(".*[+*].*") or
+    v.regexpMatch(".*\\{[0-9]+,[0-9]*\\}.*")
+  )
 }
 
 // ---------------------------------------------------------------------------
@@ -124,13 +109,14 @@ private predicate isPatternLiteralOf(StringLiteral regex, Expr patternArg) {
  * This association is purely syntactic (no dataflow): the pattern literal
  * must appear inside the constructor/assign call's first argument.
  *
- * Restricted to `ExploitableStringLiteral` to preserve the exact set of
+ * Restricted to string literals that look like plausible ReDoS candidates
+ * (see `isExploitableStringLiteralValue`) to preserve the exact set of
  * regexes considered by the flag readers when they were routed through
- * the taint-tracking configuration (whose sources were the same
+ * the taint-tracking configuration (whose sources are the same
  * `ExploitableStringLiteral` set).
  */
 private Expr getConstructionFlagArg(StringLiteral regex) {
-  regex instanceof ExploitableStringLiteral and
+  isExploitableStringLiteralValue(regex) and
   (
     // basic_regex(pattern, flags) constructor - both named-variable and
     // temporary constructions are covered because the search for `regex`

cpp/ql/lib/semmle/code/cpp/regex/internal/StdRegex.qll

@@ -0,0 +1,23 @@
+/**
+ * Provides identification of the `std::basic_regex` standard-library API type.
+ *
+ * This is shared infrastructure used by both the flow-free grammar/flag
+ * module (`RegexGrammar`) and the dataflow layer (`RegexFlowConfigs`). It
+ * lives in its own module so that importing it does not pull in the
+ * dataflow libraries: both `RegexGrammar.qll` and `RegexFlowConfigs.qll`
+ * can import it, keeping the parser/tree-view layer flow-free.
+ */
+
+import cpp
+
+/**
+ * A `std::basic_regex` class type (or instantiation thereof, e.g. `std::regex`,
+ * `std::wregex`).
+ */
+class StdBasicRegex extends Class {
+  StdBasicRegex() {
+    this.hasQualifiedName("std", "basic_regex")
+    or
+    this.(ClassTemplateInstantiation).getTemplate().hasQualifiedName("std", "basic_regex")
+  }
+}