C/C++ : Wrong Uint access

cpp/ql/src/experimental/Best Practices/WrongUintAccess.qhelp

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+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+<overview>
+<p>Find access to an array with a Uint16 when the array has a size lower than 256.</p>
+</overview>
+
+<recommendation>
+<p>Use a Uint8 instead</p>
+</recommendation>
+
+<example>
+<sample src="WrongUintAcess.cpp" />
+</example>
+
+</qhelp>

cpp/ql/src/experimental/Best Practices/WrongUintAccess.ql

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+/**
+ * @id cpp/wrong-uint-access
+ * @name Wrong Uint
+ * @descripion Acess an array of size lower than 256 with a uint16.
+ * @kind problem
+ * @problem.severity recommendation
+ * @tags efficiency
+ */
+
+import cpp
+import semmle.code.cpp.controlflow.SSA
+
+from
+  Variable E, ArrayExpr useExpr, ArrayType defExpr, VariableDeclarationEntry def, VariableAccess use
+where
+  def = defExpr.getATypeNameUse() and
+  E = def.getDeclaration() and
+  use = useExpr.getArrayBase() and
+  E = use.getTarget() and
+  useExpr.getArrayOffset().getType() instanceof UInt16_t and
+  defExpr.getArraySize() <= 256
+select useExpr, "Using a UInt16_t to acess the array $@ of size " + defExpr.getArraySize() + ".", E,
+  E.getName()

cpp/ql/src/experimental/Best Practices/WrongUintAcess.cpp

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+void test()
+{
+    uint16_t j = 256;
+    char testSubject[122];
+
+    testSubject[j] = 12; // You can use a uint8 here
+}