[clang-tools-extra] [clang-tidy] Add new `bugprone-suspicious-pointer-arithmetics-using-sizeof` (`cert-arr39-c`) check (PR #106061)

[Nicolas van Kempen via cfe-commits]

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@@ -0,0 +1,50 @@
+.. title:: clang-tidy - bugprone-suspicious-pointer-arithmetics-using-sizeof
+
+bugprone-suspicious-pointer-arithmetics-using-sizeof
+====================================================
+
+Finds suspicious pointer arithmetic calculations where the pointer is offset by
+an ``alignof()``, ``offsetof()``, or ``sizeof()`` expression.
+
+Pointer arithmetic expressions implicitly scale the offset added to or
+subtracted from the address by the size of the pointee type.
+Using an offset expression that is already scaled by the size of the underlying
+type effectively results in a squared offset, which is likely an invalid
+pointer that points beyond the end of the intended array.
+
+.. code-block:: c
+
+  void printEveryEvenIndexElement(int *Array, size_t N) {
+    int *P = Array;
+    while (P <= Array + N * sizeof(int)) { // Suspicious pointer arithmetics using sizeof()!
+      printf("%d ", *P);
+
+      P += 2 * sizeof(int); // Suspicious pointer arithmetics using sizeof()!
+    }
+  }
+
+The above example should be in the following, correct form:
+
+.. code-block:: c
+
+  void printEveryEvenIndexElement(int *Array, size_t N) {
+    int *P = Array;
+    while (P <= Array + N) {
+      printf("%d ", *P);
+
+      P += 2;
+    }
+  }
+
+`cert-arr39-c` redirects here as an alias of this check.
+
+This check corresponds to the CERT C Coding Standard rule
+`ARR39-C. Do not add or subtract a scaled integer to a pointer
+<http://wiki.sei.cmu.edu/confluence/display/c/ARR39-C.+Do+not+add+or+subtract+a+scaled+integer+to+a+pointer>`_.
+
+Limitations
+-----------
+
+While incorrect from a technically rigorous point of view, the check does not
+warn for pointer arithmetics where the pointee type is ``char``
+(``sizeof(char) == 1``, by definition) on purpose.
----------------
nicovank wrote:

Just curious, why limit this? Does it introduce noise? Or just because it won't make a difference in the operation? I think it's fine to also warn on those cases.

https://github.com/llvm/llvm-project/pull/106061