JS: Model Array#toString

javascript/ql/lib/semmle/javascript/internal/flow_summaries/AmbiguousCoreMethods.qll

@@ -24,6 +24,7 @@
 private import javascript
 private import semmle.javascript.dataflow.internal.DataFlowNode
 private import semmle.javascript.dataflow.FlowSummary
+private import Arrays
 private import FlowSummaryUtil
 
 class At extends SummarizedCallable {
@@ -41,15 +42,18 @@ class At extends SummarizedCallable {
 }
 
 class Concat extends SummarizedCallable {
-  Concat() { this = "Array#concat / String#concat" }
+  Concat() { this = "Array#concat / String#concat / Buffer.concat" }
 
   override InstanceCall getACallSimple() { result.getMethodName() = "concat" }
 
   override predicate propagatesFlow(string input, string output, boolean preservesValue) {
+    // Array#concat.
+    // Also models Buffer.concat as this happens to out work well with our toString() model.
     preservesValue = true and
     input = "Argument[this,0..].ArrayElement" and
     output = "ReturnValue.ArrayElement"
     or
+    // String#concat
     preservesValue = false and
     input = "Argument[this,0..]" and
     output = "ReturnValue"
@@ -149,3 +153,16 @@ class Values extends SummarizedCallable {
     output = "ReturnValue.IteratorElement"
   }
 }
+
+class ToString extends SummarizedCallable {
+  ToString() { this = "Object#toString / Array#toString" }
+
+  override DataFlow::MethodCallNode getACallSimple() { result.getMethodName() = "toString" }
+
+  override predicate propagatesFlow(string input, string output, boolean preservesValue) {
+    preservesValue = false and
+    // Arrays stringify their contents and joins by ","
+    input = "Argument[this].ArrayElementDeep" and
+    output = "ReturnValue"
+  }
+}

javascript/ql/test/library-tests/TripleDot/buffer.js

@@ -3,5 +3,5 @@ import 'dummy';
 function t1() {
     const b1 = Buffer.from(source("t1.1"));
     const b2 = Buffer.from(source("t1.2"));
-    sink(Buffer.concat([b1, b2]).toString("utf8")); // $ MISSING: hasTaintFlow=t1.1 hasTaintFlow=t1.2
+    sink(Buffer.concat([b1, b2]).toString("utf8")); // $ hasTaintFlow=t1.1 hasTaintFlow=t1.2
 }