Move 2020 workshop into its own folder

Author: adityasharad

README.md workshop-2020/README.mdREADME.md renamed to workshop-2020/README.md

@@ -0,0 +1,36 @@
+/**
+ * @name Use after free
+ * @kind path-problem
+ * @id cpp/workshop/use-after-free
+ */
+
+import cpp
+
+import semmle.code.cpp.dataflow.TaintTracking
+import DataFlow::PathGraph
+
+
+class Config extends DataFlow::Configuration {
+  Config() { this = "Use after free config (doesn't matter)"}
+
+  override predicate isSource(DataFlow::Node arg) {
+    exists(FunctionCall call |
+      call.getArgument(0) = arg.asDefiningArgument() and
+      call.getTarget().hasGlobalOrStdName("free")
+    )
+  }
+
+  override predicate isSink(DataFlow::Node sink) {
+    dereferenced(sink.asExpr()) // depends on DataFlow1
+  }
+
+  override predicate isBarrier(DataFlow::Node barrier) {
+    none()
+  }
+}
+
+from DataFlow::PathNode source, DataFlow::PathNode sink, Config config
+where config.hasFlowPath(source, sink)
+select sink, source, sink,
+  "Potential use-after-free vulnerability: memory is $@ and $@.",
+  source, "freed here", sink, "used here"