[libc++][hardening] Introduce assertion semantics.

Assertion semantics closely mimic C++26 Contracts evaluation semantics.
This brings our implementation closer in line with C++26 Library
Hardening (one particular benefit is that using the `observe` semantic
makes adopting hardening easier for users).

Author: var-const

.github/workflows/libcxx-build-and-test.yaml

@@ -128,6 +128,7 @@ jobs:
           'generic-abi-unstable',
           'generic-hardening-mode-debug',
           'generic-hardening-mode-extensive',
+          'generic-hardening-mode-extensive-observe-semantic',
           'generic-hardening-mode-fast',
           'generic-hardening-mode-fast-with-abi-breaks',
           'generic-merged',

libcxx/cmake/caches/Generic-hardening-mode-extensive-observe-semantic.cmake

@@ -0,0 +1,2 @@
+set(LIBCXX_HARDENING_MODE "extensive" CACHE STRING "")
+set(LIBCXX_TEST_PARAMS "assertion_semantic=observe" CACHE STRING "")

libcxx/docs/Hardening.rst

@@ -39,6 +39,8 @@ modes are:
 
    Enabling hardening has no impact on the ABI.
 
+.. _notes-for-users:
+
 Notes for users
 ---------------
 
@@ -72,6 +74,19 @@ to control the level by passing **one** of the following options to the compiler
    pre-built components. Most libc++ code is header-based, so a user-provided
    value for ``_LIBCPP_HARDENING_MODE`` will be mostly respected.
 
+.. warning::
+
+Assertion semantics are currently an experimental feature.
+
+.. note::
+
+Assertion semantics are not available in the C++03 mode.
+
+In some cases, users might want to override the assertion semantic used by the
+library. This can be done similarly to setting the hardening mode; please refer
+to the :ref:`relevant section <assertion-semantics>`. Note that this feature is
+currently experimental.
+
 Notes for vendors
 -----------------
 
@@ -260,6 +275,76 @@ output. This is less secure and increases the size of the binary (among other
 things, it has to store the error message strings) but makes the failure easier
 to debug. It also allows testing the error messages in our test suite.
 
+.. warning::
+
+Assertion semantics are currently an experimental feature.
+
+.. note::
+
+Assertion semantics are not available in the C++03 mode.
+
+Experimentally, this default behavior can be customized by users via
+:ref:`assertion semantics <assertion-semantics>`; it can also be completely
+overridden by vendors by providing a :ref:`custom assertion failure handler
+<override-assertion-handler>`.
+
+.. _assertion-semantics:
+
+Assertion semantics
+-------------------
+
+.. warning::
+
+Assertion semantics are currently an experimental feature.
+
+.. note::
+
+Assertion semantics are not available in the C++03 mode.
+
+What happens when an assertion fails depends on the assertion semantic being
+used. Four assertion semantics are available, based on C++26 Contracts
+evaluation semantics:
+
+- ``ignore`` evaluates the assertion but has no effect if it fails (note that it
+  differs from the Contracts ``ignore`` semantic which would not evaluate
+  the assertion at all);
+- ``observe`` logs an error (indicating, if possible on the platform, that the
+  error is fatal) but continues execution;
+- ``quick-enforce`` terminates the program as fast as possible via a trap
+  instruction. It is the default semantic for the production modes (``fast`` and
+  ``extensive``);
+- ``enforce`` logs an error and then terminates the program. It is the default
+  semantic for the ``debug`` mode.
+
+Notes:
+
+- Continuing execution after a hardening check fails results in undefined
+  behavior; the ``observe`` semantic is meant to make adopting hardening easier
+  but should not be used outside of the adoption period;
+- C++26 wording for Library Hardening precludes a conforming Hardened
+  implementation from using the Contracts ``ignore`` semantic when evaluating
+  hardened preconditions in the Library. Libc++ allows using this semantic for
+  hardened preconditions, but please be aware that using ``ignore`` does not
+  produce a conforming "Hardened" implementation, unlike the other semantics
+  above.
+
+The default assertion semantics are as follows:
+
+- ``fast``: ``quick-enforce``;
+- ``extensive``: ``quick-enforce``;
+- ``debug``: ``enforce``.
+
+The default assertion semantics can be overridden by passing **one** of the
+following options to the compiler:
+
+- ``-D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_IGNORE``
+- ``-D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_OBSERVE``
+- ``-D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE``
+- ``-D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_ENFORCE``
+
+All the :ref:`same notes <notes-for-users>` apply to setting this macro as for
+setting ``_LIBCPP_HARDENING_MODE``.
+
 .. _override-assertion-handler:
 
 Overriding the assertion failure handler

libcxx/docs/ReleaseNotes/21.rst

@@ -88,6 +88,12 @@ Improvements and New Features
 
 - ``ctype::tolower`` and ``ctype::toupper`` have been optimized, resulting in a 2x performance improvement.
 
+- As an experimental feature, Hardening now supports assertion semantics that allow customizing how a hardening
+  assertion failure is handled. The four available semantics, modeled on C++26 Contracts, are ``ignore``, ``observe``,
+  ``quick-enforce`` and ``enforce``. The ``observe`` semantic is intended to make it easier to adopt Hardening in
+  production but should not be used outside of this scenario. Please refer to the :ref:`Hardening documentation
+  <hardening>` for details.
+
 Deprecations and Removals
 -------------------------
 

libcxx/docs/UserDocumentation.rst

@@ -72,6 +72,11 @@ when ``-fexperimental-library`` is passed:
 * ``std::chrono::tzdb`` and related time zone functionality
 * ``<syncstream>``
 
+Additionally, assertion semantics are an experimental feature that can be used
+to customize the behavior of Hardening (see :ref:`here <assertion-semantics>`).
+Assertion semantics mirror the evaluation semantics of C++26 Contracts but are
+not a standard feature.
+
 .. note::
   Experimental libraries are experimental.
     * The contents of the ``<experimental/...>`` headers and the associated static

libcxx/include/__config

@@ -147,6 +147,48 @@ _LIBCPP_HARDENING_MODE_EXTENSIVE, \
 _LIBCPP_HARDENING_MODE_DEBUG
 #  endif
 
+// Hardening assertion semantics generally mirror the evaluation semantics of C++26 Contracts:
+// - `ignore` evaluates the assertion but doesn't do anything if it fails (note that it differs from the Contracts
+//   `ignore` semantic which wouldn't evaluate the assertion at all);
+// - `observe` logs an error (indicating, if possible, that the error is fatal) and continues execution;
+// - `quick-enforce` terminates the program as fast as possible (via trapping);
+// - `enforce` logs an error and then terminates the program.
+//
+// Notes:
+// - Continuing execution after a hardening check fails results in undefined behavior; the `observe` semantic is meant
+//   to make adopting hardening easier but should not be used outside of this scenario;
+// - C++26 wording for Library Hardening precludes a conforming Hardened implementation from using the Contracts
+//   `ignore` semantic when evaluating hardened preconditions in the Library. Libc++ allows using this semantic for
+//   hardened preconditions, however, be aware that using `ignore` does not produce a conforming "Hardened"
+//   implementation, unlike the other semantics above.
+// clang-format off
+#  define _LIBCPP_ASSERTION_SEMANTIC_IGNORE        (1 << 1)
+#  define _LIBCPP_ASSERTION_SEMANTIC_OBSERVE       (1 << 2)
+#  define _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE (1 << 3)
+#  define _LIBCPP_ASSERTION_SEMANTIC_ENFORCE       (1 << 4)
+// clang-format on
+
+// Allow users to define an arbitrary assertion semantic; otherwise, use the default mapping from modes to semantics.
+// The default is for production-capable modes to use `quick-enforce` (i.e., trap) and for the `debug` mode to use
+// `enforce` (i.e., log and abort).
+#  ifndef _LIBCPP_ASSERTION_SEMANTIC
+
+#    if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
+#      define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_ENFORCE
+#    else
+#      define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE
+#    endif
+
+#  else
+#    if !_LIBCPP_HAS_EXPERIMENTAL_LIBRARY
+#      error "Assertion semantics are an experimental feature."
+#    endif // ! _LIBCPP_HAS_EXPERIMENTAL_LIBRARY
+#    if defined(_LIBCPP_CXX03_LANG)
+#      error "Assertion semantics are not available in the C++03 mode."
+#    endif // defined(_LIBCPP_CXX03_LANG)
+
+#  endif // _LIBCPP_ASSERTION_SEMANTIC
+
 // } HARDENING
 
 #  define _LIBCPP_TOSTRING2(x) #x

libcxx/test/libcxx/thread/thread.barrier/assert.arrive.pass.cpp

@@ -8,6 +8,8 @@
 // UNSUPPORTED: no-threads
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // REQUIRES: libcpp-hardening-mode={{extensive|debug}}
+// Without the assertion, the test will most likely time out.
+// UNSUPPORTED: libcpp-assertion-semantic={{ignore|observe}}
 
 // XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing
 

libcxx/test/libcxx/thread/thread.latch/assert.arrive_and_wait.pass.cpp

@@ -18,6 +18,8 @@
 
 // REQUIRES: has-unix-headers
 // REQUIRES: libcpp-hardening-mode={{extensive|debug}}
+// Without the assertion, the test will most likely time out.
+// UNSUPPORTED: libcpp-assertion-semantic={{ignore|observe}}
 // XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing
 
 #include <latch>

libcxx/test/libcxx/thread/thread.latch/assert.ctor.pass.cpp

@@ -24,11 +24,12 @@
 
 #include "check_assertion.h"
 
-int main(int, char **) {
+int main(int, char**) {
   {
-    TEST_LIBCPP_ASSERT_FAILURE([]{ std::latch l(-1); }(),
-                               "latch::latch(ptrdiff_t): latch cannot be "
-                               "initialized with a negative value");
+    TEST_LIBCPP_ASSERT_FAILURE(
+        [] { [[maybe_unused]] std::latch l(-1); }(),
+        "latch::latch(ptrdiff_t): latch cannot be "
+        "initialized with a negative value");
   }
 
   // We can't check the precondition for max() because there's no value

libcxx/test/libcxx/thread/thread.semaphore/assert.ctor.pass.cpp

@@ -26,7 +26,7 @@
 int main(int, char**) {
   {
     TEST_LIBCPP_ASSERT_FAILURE(
-        [] { std::counting_semaphore<> s(-1); }(),
+        [] { [[maybe_unused]] std::counting_semaphore<> s(-1); }(),
         "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
         "initialized with a negative value");
   }