[libc++][hardening] Finish documenting hardening.

Author: var-const

libcxx/docs/Hardening.rst

@@ -1,4 +1,4 @@
-.. _hardening-modes:
+.. _hardening:
 
 ===============
 Hardening Modes
@@ -29,8 +29,11 @@ modes are:
   rigour impacts performance more than fast mode: we recommend benchmarking to
   determine if that is acceptable for your program.
 - **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.
+  including heuristic checks that might have significant performance overhead as
+  well as internal library assertions. This mode should be used in
+  non-production environments (such as test suites, CI, local development). We
+  don’t commit to a particular level of performance in this mode and it’s *not*
+  intended to be used in production.
 
 .. note::
 
@@ -72,17 +75,302 @@ to control the level by passing **one** of the following options to the compiler
 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).
+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
-------------------------
+Vendors can also override the termination handler by :ref:`providing a custom
+header <override-assertion-handler>`.
 
-TODO(hardening)
+Assertion categories
+====================
+
+Inside the library, individual assertions are grouped into different
+_categories_. Each hardening mode enables a different set of assertion
+categories; categories provide an additional layer of abstraction that makes it
+easier to reason about the high-level semantics of a hardening mode.
+
+- ``valid-element-access`` -- checks that any attempts to access a container
+  element, whether through the container object or through an iterator, are
+  valid and do not attempt to go out of bounds or otherwise access
+  a non-existent element. For iterator checks to work, bounded iterators must be
+  enabled in the ABI. Types like ``optional`` and ``function`` are considered
+  one-element containers for the purposes of this check.
+
+- ``valid-input-range`` -- checks that ranges (whether expressed as an iterator
+  pair, an iterator and a sentinel, an iterator and a count, or
+  a ``std::range``) given as input to library functions are valid:
+  - the sentinel is reachable from the begin iterator;
+  - TODO(hardening): both iterators refer to the same container.
+
+  ("input" here refers to "an input given to an algorithm", not to an iterator
+  category)
+
+  Violating assertions in this category leads to an out-of-bounds access.
+
+- ``non-null`` -- checks that the pointer being dereferenced is not null. On
+  most modern platforms zero address does not refer to an actual location in
+  memory, so a null pointer dereference would not compromize the memory security
+  of a program (however, it is still undefined behavior that can result in
+  strange errors due to compiler optimizations).
+
+- ``non-overlapping-ranges`` -- for functions that take several ranges as
+  arguments, checks that the given ranges do not overlap.
+
+- ``valid-deallocation`` -- checks that an attempt to deallocate memory is valid
+  (e.g. the given object was allocated by the given allocator). Violating this
+  category typically results in a memory leak.
+
+- ``valid-external-api-call`` -- checks that a call to an external API doesn't
+  fail in an unexpected manner. This includes triggering documented cases of
+  undefined behavior in an external library (like attempting to unlock an
+  unlocked mutex in pthreads). Any API external to the library falls under this
+  category (from system calls to compiler intrinsics). We generally don't expect
+  these failures to compromize memory safety or otherwise create an immediate
+  security issue.
+
+- ``compatible-allocator`` -- checks any operations that exchange nodes between
+  containers to make sure the containers have compatible allocators.
+
+- ``argument-within-domain`` -- checks that the given argument is within the
+  domain of valid arguments for the function. Violating this typically produces
+  an incorrect result (e.g. the clamp algorithm returns the original value
+  without clamping it due to incorrect functors) or puts an object into an
+  invalid state (e.g. a string view where only a subset of elements is possible
+  to access). This category is for assertions violating which doesn't cause any
+  immediate issues in the library -- whatever the consequences are, they will
+  happen in the user code.
+
+- ``pedantic`` -- checks prerequisites that are imposed by the Standard, but
+  violating which happens to be benign in our implementation.
+
+- ``semantic-requirement`` -- checks that the given argument satisfies the
+  semantic requirements imposed by the Standard. Typically, there is no simple
+  way to completely prove that a semantic requirement is satisfied; thus, this
+  would often be a heuristic check and it might be quite expensive.
+
+- ``internal`` -- checks that internal invariants of the library hold. These
+  assertions don't depend on user input.
+
+- ``uncategorized`` -- for assertions that haven't been properly classified yet.
+  This is an escape hatch used for some existing assertions in the library; all
+  new code should have its assertions properly classified.
+
+Mapping between the hardening modes and the assertion categories
+================================================================
+
+.. list-table::
+    :header-rows: 1
+    :widths: auto
+
+    * -
+      - ``fast``
+      - ``extensive``
+      - ``debug``
+    * - ``valid-element-access``
+      - ✅
+      - ✅
+      - ✅
+    * - ``valid-input-range``
+      - ✅
+      - ✅
+      - ✅
+    * - ``non-null``
+      - ❌
+      - ✅
+      - ✅
+    * - ``non-overlapping-ranges``
+      - ❌
+      - ✅
+      - ✅
+    * - ``valid-deallocation``
+      - ❌
+      - ✅
+      - ✅
+    * - ``valid-external-api-call``
+      - ❌
+      - ✅
+      - ✅
+    * - ``compatible-allocator``
+      - ❌
+      - ✅
+      - ✅
+    * - ``argument-within-domain``
+      - ❌
+      - ✅
+      - ✅
+    * - ``pedantic``
+      - ❌
+      - ✅
+      - ✅
+    * - ``semantic-requirement``
+      - ❌
+      - ❌
+      - ✅
+    * - ``internal``
+      - ❌
+      - ❌
+      - ✅
+    * - ``uncategorized``
+      - ❌
+      - ✅
+      - ✅
+
+.. note::
+
+  At the moment, each subsequent hardening mode is a strict superset of the
+  previous one (in other words, each subsequent mode only enables additional
+  assertion categories without disabling any), but this won't necessarily be
+  true for any hardening modes that might potentially be added in the future.
+
+Hardening assertion failure
+===========================
+
+In production modes (``fast`` and ``extensive``), a hardening assertion failure
+immediately traps the program. This is the safest approach that also minimizes
+the code size penalty as the failure handler maps to a single instruction. The
+downside is that the failure provides no additional details other than the stack
+trace (which might also be affected by optimizations).
+TODO(hardening): describe ``__builtin_verbose_trap`` once we can use it.
+
+In the ``debug`` mode, an assertion failure terminates the program in an
+unspecified manner and also outputs the associated error message to the error
+output. This is less secure and increases the size of the binary (among other
+things, to store the error message strings) but makes the failure easier to
+debug. It also allows us to test the error messages in our test suite.
+
+.. _override-assertion-handler:
+
+Overriding the assertion failure handler
+----------------------------------------
+
+Vendors can override the default termination handler mechanism by following
+these steps:
+
+- create a header file that provides a definition of a macro called
+  ``_LIBCPP_ASSERTION_HANDLER``. The macro will be invoked when a hardening
+  assertion fails, with a single parameter containing a null-terminated string
+  with the error message.
+- when configuring the library, provide the path to custom header (relative to
+  the root of the repository) via the CMake variable
+  ``LIBCXX_ASSERTION_HANDLER_FILE``.
+
+ABI
+===
+
+Setting a hardening mode does **not** affect the ABI. Each mode uses the subset
+of checks available in the current ABI configuration which is determined by the
+platform.
+
+It is important to stress that whether a particular check is enabled depends on
+the combination of the selected hardening mode and the hardening-related ABI
+options. Some checks require changing the ABI from the "default" to store
+additional information in the library classes -- e.g. checking whether an
+iterator is valid upon dereference generally requires storing data about bounds
+inside the iterator object. Using ``std::span`` as an example, setting the
+hardening mode to ``fast`` will always enable the ``valid-element-access``
+checks when accessing elements via a ``span`` object, but whether dereferencing
+a ``span`` iterator does the equivalent check depends on the ABI configuration.
+
+ABI options
+-----------
+
+Vendors can use the following ABI options to enable additional hardening checks:
+
+- ``_LIBCPP_ABI_BOUNDED_ITERATORS`` -- changes the iterator type of select
+  containers (see below) to a bounded iterator that keeps track of whether it's
+  within the bounds of the original container and asserts it on every
+  dereference.
+
+  ABI impact: changes the iterator type of the relevant containers.
+
+  Supported containers:
+  - ``span``;
+  - ``string_view``;
+  - ``array``.
+
+ABI tags
+--------
+
+We use ABI tags to allow translation units built with different hardening modes
+to interact with each other without causing ODR violations. Knowing how
+hardening modes are encoded into the ABI tags might be useful to examine
+a binary and determine whether it was built with hardening enabled.
+
+.. warning::
+  We don't commit to the ABI tags being stable between different releases of
+  libc++. The following describes the state of the latest release and is for
+  informational purposes only.
+
+The first character of an ABI tag encodes the hardening mode:
+- ``f`` -- [f]ast mode;
+- ``s`` -- extensive ("[s]afe") mode;
+- ``d`` -- [d]ebug mode;
+- ``n`` -- [n]one mode.
+
+Hardened containers
+===================
+
+.. list-table::
+    :header-rows: 1
+    :widths: auto
+
+    * - Name
+      - Member functions
+      - Iterators (ABI-dependent)
+    * - ``span``
+      - ✅
+      - ✅
+    * - ``string_view``
+      - ✅
+      - ✅
+    * - ``array``
+      - ✅
+      - ❌
+    * - ``vector``
+      - ✅
+      - ❌
+    * - ``string``
+      - ✅
+      - ❌
+    * - ``list``
+      - ✅
+      - ❌
+    * - ``forward_list``
+      - ✅
+      - ❌
+    * - ``deque``
+      - ✅
+      - ❌
+    * - ``mdspan``
+      - ✅
+      - ❌
+    * - ``optional``
+      - ✅
+      - N/A
+
+TODO(hardening): make this table exhaustive.
+
+Testing
+=======
+
+Each hardening assertion should be tested using death tests (via the
+``TEST_LIBCPP_ASSERT_FAILURE`` macro). Use the ``libcpp-hardening-mode`` Lit
+feature to make sure the assertion is enabled in (and only in) the intended
+modes. Note that error messages are only tested (matched) if the ``debug``
+hardening mode is used.
+
+Further reading
+===============
+
+- ``_Hardening RFC <https://discourse.llvm.org/t/rfc-hardening-in-libc/73925>``:
+  contains some of the design rationale.
+
+:ref:`hardening mode <using-hardening-modes>`

libcxx/docs/ReleaseNotes/18.rst

@@ -40,7 +40,7 @@ and C++26 features.
 
 New hardened modes for the library have been added, replacing the legacy debug mode that was
 removed in the LLVM 17 release. Unlike the legacy debug mode, some of these hardening modes are
-also intended to be used in production. See :ref:`hardening-modes` for more details.
+also intended to be used in production. See :ref:`hardening` for more details.
 
 Work on the ranges support has progressed. See
 :ref:`ranges-status` for the current status.