[libcxx] reorganises the hardening documentation

The reorganisation assists with identifying information that's relevant
to the reader by using sections, note/warning blocks, and highlighted
lists.

Some rewording was necessary to fit the new structure and some to
improve flow. Changes to the intention of the documentation have not
been made.

Author: cjdb

libcxx/docs/Hardening.rst

@@ -15,61 +15,71 @@ assertions that prevent undefined behavior caused by violating preconditions of
 the standard library. Different hardening modes make different trade-offs
 between the amount of checking and runtime performance. The available hardening
 modes are:
-- fast mode;
-- extensive mode;
-- debug mode.
-
-The fast mode contains a set of security-critical checks that can be done with
-relatively little overhead in constant time and are intended to be used in
-production. We recommend most projects to adopt the fast mode.
-
-The extensive mode contains all the checks from the fast mode and additionally
-some checks for undefined behavior that incur relatively little overhead but
-aren't security-critical. While the performance penalty is somewhat more
-significant compared to the fast mode, the extensive mode is still intended to
-be usable in production.
-
-The debug mode enables all the available checks in the library, including
-internal assertions, some of which might be very expensive. This mode is
-intended to be used for testing, not in production.
-
-Vendors can set the default hardening mode by using the
-``LIBCXX_HARDENING_MODE`` variable at CMake configuration time with the possible
-values of ``none``, ``fast``, ``extensive`` and ``debug``. The default value is
-``none`` which doesn't enable any hardening checks (this mode is sometimes
-called the ``unchecked`` mode).
-
-When hardening is enabled, the compiled library is built with the corresponding
-mode enabled, **and** user code will be built with the same mode enabled by
-default. If the mode is set to "none" at the CMake configuration time, the
-compiled library will not contain any assertions and the default when building
-user code will be to have assertions disabled. As a user, you can consult your
-vendor to know which level of hardening is enabled by default.
-
-Furthermore, independently of any vendor-selected default, users can always
-control which level of hardening is enabled in their code by defining the macro
-``_LIBCPP_HARDENING_MODE`` before including any libc++ headers (preferably by
-passing ``-D_LIBCPP_HARDENING_MODE=X`` to the compiler). The macro can be
-set to one of the following possible values:
-
-- ``_LIBCPP_HARDENING_MODE_NONE``;
-- ``_LIBCPP_HARDENING_MODE_FAST``;
-- ``_LIBCPP_HARDENING_MODE_EXTENSIVE``;
-- ``_LIBCPP_HARDENING_MODE_DEBUG``.
-
-The exact numeric values of these macros are unspecified and users should not
-rely on them (e.g. expect the values to be sorted in any way).
-
-Note that if the compiled library was built by the vendor with the hardening
-mode set to "none", functions compiled inside the static or shared library won't
-have any hardening enabled even if the user compiles with hardening enabled (the
-same is true for the inverse case where the static or shared library was
-compiled **with** hardening enabled but the user tries to disable it). However,
-most of the code in libc++ is in the headers, so the user-selected value for
-``_LIBCPP_HARDENING_MODE``, if any, will usually be respected.
-
-Enabling hardening has no impact on the ABI.
+
+- **Unchecked mode/none**, which disables all hardening checks.
+- **Fast mode**, which contains a set of security-critical checks that can be
+  done with relatively little overhead in constant time and are intended to be
+  used in production. We recommend most projects to adopt the fast mode.
+- **Extensive mode**, which contains all the checks from the fast mode and
+  additionally some checks for undefined behavior that incur relatively little
+  overhead but aren't security-critical. While the performance penalty is
+  somewhat more significant compared to the fast mode, the extensive mode is
+  still intended to be usable in production.
+- **Debug mode**, which enables all the available checks in the library,
+  including internal assertions, some of which might be very expensive. This
+  mode is intended to be used for testing, not in production.
+
+.. note::
+
+   Enabling hardening has no impact on the ABI.
+
+Notes for users
+---------------
+
+As a user, you can consult your vendor to know which level of hardening is
+enabled by default.
+
+Users wishing for a different hardening level to their vendor default are able
+to control the level by passing **one** of the following options to the compiler:
+
+- ``-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE``
+- ``-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST``
+- ``-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE``
+- ``-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG``
+
+.. warning::
+
+   The exact numeric values of these macros are unspecified and users should not
+   rely on them (e.g. expect the values to be sorted in any way).
+
+.. warning::
+
+   If you would prefer to override the hardening level on a per-translation-unit
+   basis, you must do so **before** including any headers to avoid `ODR issues`_.
+
+.. _`ODR issues`: https://en.cppreference.com/w/cpp/language/definition#:~:text=is%20ill%2Dformed.-,One%20Definition%20Rule,-Only%20one%20definition
+
+.. note::
+
+   Since the static and shared library components of libc++ are built by the
+   vendor, setting this macro will have no impact on the hardening mode for the
+   pre-built components. Most libc++ code is header-based, so a user-provided
+   value for ``_LIBCPP_HARDENING_MODE`` will be mostly respected.
+
+Notes for vendors
+-----------------
+
+Vendors can set the default hardening mode by providing ``LIBCXX_HARDENING_MODE``
+as a configuration option, with the possible values of ``none``, ``fast``,
+``extensive`` and ``debug``. The default value is ``none`` which doesn't enable
+any hardening checks (this mode is sometimes called the ``unchecked`` mode).
+
+This option controls both the hardening mode that the precompiled library is
+built with and the default hardening mode that users will build with. If set to
+``none``, the precompiled library will not contain any assertions, and user code
+will default to building without assertions.
 
 Iterator bounds checking
 ------------------------
+
 TODO(hardening)