C++: Fix FP in cpp/return-stack-allocated-memory.

Author: MathiasVP

cpp/ql/src/Likely Bugs/Memory Management/ReturnStackAllocatedMemory.ql

@@ -13,6 +13,7 @@
 
 import cpp
 import semmle.code.cpp.dataflow.EscapesTree
+import semmle.code.cpp.models.interfaces.PointerWrapper
 import semmle.code.cpp.dataflow.DataFlow
 
 /**
@@ -39,6 +40,10 @@ predicate hasNontrivialConversion(Expr e) {
     e instanceof ParenthesisExpr
   )
   or
+  // A smart pointer can be stack-allocated while the data it points to is heap-allocated.
+  // So we exclude such "conversions" from this predicate.
+  e = any(PointerWrapper wrapper).getAnUnwrapperFunction().getACallToThisFunction()
+  or
   hasNontrivialConversion(e.getConversion())
 }
 

cpp/ql/test/query-tests/Likely Bugs/Memory Management/ReturnStackAllocatedMemory/ReturnStackAllocatedMemory.expected

@@ -6,4 +6,3 @@
 | test.cpp:112:2:112:12 | return ... | May return stack-allocated memory from $@. | test.cpp:112:9:112:11 | arr | arr |
 | test.cpp:119:2:119:19 | return ... | May return stack-allocated memory from $@. | test.cpp:119:11:119:13 | arr | arr |
 | test.cpp:171:3:171:24 | return ... | May return stack-allocated memory from $@. | test.cpp:170:35:170:41 | myLocal | myLocal |
-| test.cpp:217:3:217:13 | return ... | May return stack-allocated memory from $@. | test.cpp:216:14:216:17 | port | port |

cpp/ql/test/query-tests/Likely Bugs/Memory Management/ReturnStackAllocatedMemory/test.cpp

@@ -214,5 +214,5 @@ auto make_read_port()
 {
   auto port = std::shared_ptr<int>(new int);
   auto ptr = port.get();
-  return ptr; // GOOD [FALSE POSITIVE]
+  return ptr; // GOOD
 }