Fix 18-1 amendment use-use flow

cpp/common/src/codingstandards/cpp/rules/donotusepointerarithmetictoaddressdifferentarrays/DoNotUsePointerArithmeticToAddressDifferentArrays.qll

@@ -33,7 +33,7 @@ abstract class ArrayLikeAccess extends Expr {
  */
 class ArrayVariableAccess extends ArrayLikeAccess, VariableAccess {
   int size;
- 
+
   ArrayVariableAccess() { size = getType().(ArrayType).getArraySize() }
 
   override Variable getElement() { result = getTarget() }
@@ -47,7 +47,7 @@ class ArrayVariableAccess extends ArrayLikeAccess, VariableAccess {
 
 /**
  * Get the size of the object pointed to by a type (pointer or array).
- * 
+ *
  * Depth of type unwrapping depends on the type. Pointer will be dereferenced only once: the element
  * size of `T*` is `sizeof(T)` while the element size of `T**` is `sizeof(T*)`. However, array types
  * will be deeply unwrapped, as the pointed to size of `T[][]` is `sizeof(T)`. These processes
@@ -88,7 +88,17 @@ class CastedToBytePointer extends ArrayLikeAccess, Conversion {
 
   override int getSize() { result = size }
 
-  override DataFlow::Node getNode() { result.asConvertedExpr() = this }
+  override DataFlow::Node getNode() {
+    // Carefully avoid use-use flow, which would mean any later usage of the original pointer value
+    // after the cast would be considered a usage of the byte pointer value.
+    //
+    // To fix this, we currently assume the value is assigned to a variable, and find that variable
+    // with `.asDefinition()` like so:
+    exists(DataFlow::Node conversion |
+      conversion.asConvertedExpr() = this and
+      result.asDefinition() = conversion.asExpr()
+    )
+  }
 }
 
 /**
@@ -272,4 +282,4 @@ query predicate problems(Expr arrayPointerCreation, string message, Element arra
       "Array pointer " + derivedArrayPointerOrPointerOperand.getName() + " points " +
         difference.toString() + " " + denomination + " past the end of $@."
   )
-}
+}

cpp/common/test/rules/donotusepointerarithmetictoaddressdifferentarrays/DoNotUsePointerArithmeticToAddressDifferentArrays.expected

@@ -6,4 +6,3 @@
 | test.cpp:25:15:25:21 | & ... | Array pointer p14 points 1 element past the end of $@. | test.cpp:2:7:2:8 | l1 | l1 |
 | test.cpp:30:15:30:21 | & ... | Array pointer p17 points 1 element past the end of $@. | test.cpp:28:24:28:42 | (unsigned char *)... | cast to btye pointer (unsigned char *)... |
 | test.cpp:35:15:35:21 | & ... | Array pointer p20 points 1 element past the end of $@. | test.cpp:33:24:33:43 | (unsigned char *)... | cast to btye pointer (unsigned char *)... |
-| test.cpp:43:15:43:23 | & ... | Array pointer p23 points 96 elements past the end of $@. | test.cpp:28:24:28:42 | (unsigned char *)... | cast to btye pointer (unsigned char *)... |

cpp/common/test/rules/donotusepointerarithmetictoaddressdifferentarrays/test.cpp

@@ -36,9 +36,7 @@ void f1() {
 
   // Casting to a pointer to a differently sized type that isn't char
   // invalidates analysis
-  int l3[3];
-  long *p21 = (long*)&l1;
+  long *p21 = (long *)&l1;
   void *p22 = &p21[0];   // COMPLIANT
-  // Not compliant, but we shouldn't detect it, but we do for the wrong reason:
-  void *p23 = &p21[100]; // NON_COMPLIANT[FALSE_NEGATIVE][FALSE_POSITIVE]
+  void *p23 = &p21[100]; // NON_COMPLIANT[FALSE_NEGATIVE]
 }