C++: Make the 'getBufferSize' a lot more like the pre-use-use flow implementation.

cpp/ql/lib/semmle/code/cpp/commons/Buffer.qll

@@ -73,67 +73,29 @@ private int isSource(Expr bufferExpr, Element why) {
   )
 }
 
-/**
- * Holds if `e2` is an expression that is derived from `e1` such that if `e1[n]` is a
- * well-defined expression for some number `n`, then `e2[n + delta]` is also a well-defined
- * expression.
- */
-private predicate step(Expr e1, Expr e2, int delta) {
-  getBufferSizeCand0(e1) and
-  (
-    exists(Variable bufferVar, Class parentClass, VariableAccess parentPtr, int bufferSize |
-      e1 = parentPtr
-    |
-      bufferVar = e2.(VariableAccess).getTarget() and
-      // buffer is the parentPtr->bufferVar of a 'variable size struct'
-      memberMayBeVarSize(parentClass, bufferVar) and
-      parentPtr = e2.(VariableAccess).getQualifier() and
-      parentPtr.getTarget().getUnspecifiedType().(PointerType).getBaseType() = parentClass and
-      (
-        if exists(bufferVar.getType().getSize())
-        then bufferSize = bufferVar.getType().getSize()
-        else bufferSize = 0
-      ) and
-      delta = bufferSize - parentClass.getSize()
-    )
-    or
-    DataFlow::localExprFlowStep(e1, e2) and
-    delta = 0
-  )
-}
-
-pragma[nomagic]
-private predicate getBufferSizeCand0(Expr e) {
-  exists(isSource(e, _))
-  or
-  exists(Expr e0 |
-    getBufferSizeCand0(e0) and
-    step(e0, e, _)
-  )
-}
-
-/**
- * Get the size in bytes of the buffer pointed to by an expression (if this can be determined).
- *
- * NOTE: There can be multiple `(result, why)` for a given `bufferExpr`.
- */
-private int getBufferSizeCand(Expr bufferExpr, Element why) {
-  getBufferSizeCand0(bufferExpr) and
-  (
-    result = isSource(bufferExpr, why)
-    or
-    exists(Expr e0, int delta, int size |
-      size = getBufferSizeCand(e0, why) and
-      step(e0, bufferExpr, delta) and
-      result = size + delta
-    )
-  )
-}
-
 /**
  * Get the size in bytes of the buffer pointed to by an expression (if this can be determined).
  */
+language[monotonicAggregates]
 int getBufferSize(Expr bufferExpr, Element why) {
-  result = max( | | getBufferSizeCand(bufferExpr, _)) and
-  result = getBufferSizeCand(bufferExpr, why)
+  result = isSource(bufferExpr, why)
+  or
+  exists(Class parentClass, VariableAccess parentPtr, int bufferSize, Variable bufferVar |
+    bufferVar = bufferExpr.(VariableAccess).getTarget() and
+    // buffer is the parentPtr->bufferVar of a 'variable size struct'
+    memberMayBeVarSize(parentClass, bufferVar) and
+    why = bufferVar and
+    parentPtr = bufferExpr.(VariableAccess).getQualifier() and
+    parentPtr.getTarget().getUnspecifiedType().(PointerType).getBaseType() = parentClass and
+    result = getBufferSize(parentPtr, _) + bufferSize - parentClass.getSize()
+  |
+    if exists(bufferVar.getType().getSize())
+    then bufferSize = bufferVar.getType().getSize()
+    else bufferSize = 0
+  )
+  or
+  // dataflow (all sources must be the same size)
+  result = unique(Expr def | DataFlow::localExprFlowStep(def, bufferExpr) | getBufferSize(def, _)) and
+  // find reason
+  exists(Expr def | DataFlow::localExprFlowStep(def, bufferExpr) | exists(getBufferSize(def, why)))
 }