[clang] [NFC][analyzer][docs] Document MallocChecker's ownership attributes (PR #121759)

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=?utf-8?q?Kristóf?= Umann <dkszelethus at gmail.com>,
=?utf-8?q?Kristóf?= Umann <dkszelethus at gmail.com>,
=?utf-8?q?Kristóf?= Umann <dkszelethus at gmail.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/121759 at github.com>


================
@@ -1389,6 +1389,81 @@ Query for this attribute with ``__has_attribute(overloadable)``.
   }];
 }
 
+def OwnershipDocs : Documentation {
+  let Heading = "ownership_holds, ownership_returns, ownership_takes (Clang "
+                "Static Analyzer)";
+  let Category = DocCatFunction;
+  let Content = [{
+
+.. note::
+
+  In order for the Clang Static Analyzer to acknowledge these attributes, the
+  ``Optimistic`` config needs to be set to true for the checker
+  ``unix.DynamicMemoryModeling``:
+
+  ``-Xclang -analyzer-config -Xclang unix.DynamicMemoryModeling:Optimistic=true``
+
+These attributes are used by the Clang Static Analyzer's dynamic memory modeling
+facilities to mark custom allocating/deallocating functions.
+
+All 3 attributes' first parameter of type string is the type of the allocation:
+``malloc``, ``new``, etc. to allow for catching :ref:`mismatched deallocation
+<unix-MismatchedDeallocator>` bugs. The allocation type can be any string, e.g.
+a function annotated with
+returning a piece of memory of type ``lasagna`` but freed with a function
+annoteted to release ``cheese`` typed memory will result in mismatched
+deallocation warning.
+
+The (currently) only allocation type having special meaning is ``malloc`` --
+the Clang Static Analyzer makes sure that allocating functions annotated with
+``malloc`` are treated like they used the standard ``malloc()``, and can be
+safely deallocated with the standard ``free()``.
+
+* Use ``ownership_returns`` to mark a function as an allocating function. Takes
+  1 parameter to denote the allocation type.
+* Use ``ownership_takes`` to mark a function as a deallocating function. Takes 2
+  parameters: the allocation type, and the index of the parameter that is being
+  deallocated (counting from 1).
+* Use ``ownership_holds`` to mark that a function takes over the ownership of a
+  piece of memory and will free it at some unspecified point in the future. Like
+  ``ownership_takes``, this takes 2 parameters: the allocation type, and the
+  index of the parameter whose ownership will be taken over (counting from 1).
+
+The annotations ``ownership_takes`` and ``ownership_holds`` both prevent memory
+leak reports (concerning the specified argument); the difference between them
+is that using taken memory is a use-after-free error, while using held memory
+is assumed to be legitimate.
+
+Example:
+
+.. code-block:: c
+
+  // Denotes that my_malloc will return with a dynamically allocated piece of
+  // memory using malloc().
+  void __attribute((ownership_returns(malloc))) *my_malloc(size_t);
+
+  // Denotes that my_free will deallocate its parameter using free().
+  void __attribute((ownership_takes(malloc, 1))) my_free(void *);
+
+  // Denotes that my_hold will take over the ownership of its parameter that was
+  // allocated via malloc().
+  void __attribute((ownership_holds(malloc, 1))) my_hold(void *);
+
+Further reading about dynamic memory modeling in the Clang Static Analyzer is
+found in these checker docs:
+:ref:`unix.Malloc <unix-Malloc>`, :ref:`unix.MallocSizeof <unix-MallocSizeof>`,
+:ref:`unix.MismatchedDeallocator <unix-MismatchedDeallocator>`,
+:ref:`cplusplus.NewDelete <cplusplus-NewDelete>`,
+:ref:`cplusplus.NewDeleteLeaks <cplusplus-NewDeleteLeaks>`,
+:ref:`optin.taint.TaintedAlloc <optin-taint-TaintedAlloc>`.
+Mind that many more checkers are affected by dynamic memory modeling changes to
+some extent.
+
+Further reading for other annotations:
+`Source Annotations in the Clang Static Analyzer <https://clang-analyzer.llvm.org/annotations.html>`_.
+}];
+}
----------------
isuckatcs wrote:

Nit: the closing tags of `Content` are indented in case of the other `Documentation`s
```suggestion
  }];
}
```

https://github.com/llvm/llvm-project/pull/121759