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

Exactly what it says on the tin! These were written ages ago (2010s),
but are still functional, only the docs are missing.


![image](https://github.com/user-attachments/assets/d6b89611-4064-41dd-8482-8643e9e68836)

---------

Co-authored-by: Donát Nagy <[email protected]>
Co-authored-by: Balazs Benics <[email protected]>
Co-authored-by: isuckatcs <[email protected]>

Author: 4 people

clang/include/clang/Basic/Attr.td

@@ -2776,7 +2776,7 @@ def Ownership : InheritableAttr {
   let Args = [IdentifierArgument<"Module">,
               VariadicParamIdxArgument<"Args">];
   let Subjects = SubjectList<[HasFunctionProto]>;
-  let Documentation = [Undocumented];
+  let Documentation = [OwnershipDocs];
 }
 
 def Packed : InheritableAttr {

clang/include/clang/Basic/AttrDocs.td

@@ -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
+annotated 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>`_.
+  }];
+}
+
 def ObjCMethodFamilyDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{