[libc++][hardening] Rework how the assertion handler can be overridden.

libcxx/CMakeLists.txt

@@ -69,6 +69,13 @@ if (NOT "${LIBCXX_HARDENING_MODE}" IN_LIST LIBCXX_SUPPORTED_HARDENING_MODES)
   message(FATAL_ERROR
     "Unsupported hardening mode: '${LIBCXX_HARDENING_MODE}'. Supported values are ${LIBCXX_SUPPORTED_HARDENING_MODES}.")
 endif()
+set(LIBCXX_ASSERTION_HANDLER_FILE
+  "${CMAKE_CURRENT_SOURCE_DIR}/vendor/llvm/default_assertion_handler.in"
+  CACHE STRING
+  "Specify the path to a header that contains a custom implementation of the
+   assertion handler that gets invoked when a hardening assertion fails. If
+   provided, the contents of this header will get injected into the library code
+   and override the default assertion handler.")
 option(LIBCXX_ENABLE_RANDOM_DEVICE
   "Whether to include support for std::random_device in the library. Disabling
    this can be useful when building the library for platforms that don't have

libcxx/docs/ReleaseNotes/18.rst

@@ -107,6 +107,10 @@ Deprecations and Removals
   macro is provided to restore the previous behavior, and it will be supported in the LLVM 18 release only.
   In LLVM 19 and beyond, ``_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT`` will not be honored anymore.
 
+- The only supported way to customize the assertion handler that gets invoked when a hardening assertion fails
+  is now by setting the ``LIBCXX_ASSERTION_HANDLER_FILE`` CMake variable and providing a custom header. See
+  the documentation on overriding the default assertion handler for details.
+
 - The ``_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED`` macro is not honored anymore in LLVM 18.
   Please see the updated documentation about the hardening modes in libc++ and in particular the
   ``_LIBCPP_VERBOSE_ABORT`` macro for details.

libcxx/docs/UsingLibcxx.rst

@@ -146,9 +146,9 @@ IWYU, you should run the tool like so:
 If you would prefer to not use that flag, then you can replace ``/path/to/include-what-you-use/share/libcxx.imp``
 file with the libc++-provided ``libcxx.imp`` file.
 
-.. _termination-handler:
+.. _assertion-handler:
 
-Overriding the default termination handler
+Overriding the default assertion handler
 ==========================================
 
 When the library wants to terminate due to an unforeseen condition (such as a hardening assertion
@@ -158,57 +158,37 @@ provided by the static or shared library, so it is only available when deploying
 the compiled library is sufficiently recent. On older platforms, the program will terminate in an
 unspecified unsuccessful manner, but the quality of diagnostics won't be great.
 
-However, users can also override that mechanism at two different levels. First, the mechanism can be
-overridden at compile time by defining the ``_LIBCPP_VERBOSE_ABORT(format, args...)`` variadic macro.
-When that macro is defined, it will be called with a format string as the first argument, followed by
-a series of arguments to format using printf-style formatting. Compile-time customization may be
-useful to get precise control over code generation, however it is also inconvenient to use in
-some cases. Indeed, compile-time customization of the verbose termination function requires that all
-translation units be compiled with a consistent definition for ``_LIBCPP_VERBOSE_ABORT`` to avoid ODR
-violations, which can add complexity in the build system of users.
-
-Otherwise, if compile-time customization is not necessary, link-time customization of the handler is also
-possible, similarly to how replacing ``operator new`` works. This mechanism trades off fine-grained control
-over the call site where the termination is initiated in exchange for better ergonomics. Link-time
-customization is done by simply defining the following function in exactly one translation unit of your
-program:
-
-.. code-block:: cpp
-
-  void __libcpp_verbose_abort(char const* format, ...)
-
-This mechanism is similar to how one can replace the default definition of ``operator new``
-and ``operator delete``. For example:
-
-.. code-block:: cpp
-
-  // In HelloWorldHandler.cpp
-  #include <version> // must include any libc++ header before defining the function (C compatibility headers excluded)
-
-  void std::__libcpp_verbose_abort(char const* format, ...) {
-    std::va_list list;
-    va_start(list, format);
-    std::vfprintf(stderr, format, list);
-    va_end(list);
-
-    std::abort();
-  }
-
-  // In HelloWorld.cpp
-  #include <vector>
-
-  int main() {
-    std::vector<int> v;
-    int& x = v[0]; // Your termination function will be called here if hardening is enabled.
-  }
-
-Also note that the verbose termination function should never return. Since assertions in libc++
-catch undefined behavior, your code will proceed with undefined behavior if your function is called
-and does return.
-
-Furthermore, exceptions should not be thrown from the function. Indeed, many functions in the
-library are ``noexcept``, and any exception thrown from the termination function will result
-in ``std::terminate`` being called.
+However, vendors can also override that mechanism at CMake configuration time. When a hardening
+assertion fails, the library invokes the ``_LIBCPP_ASSERTION_HANDLER`` macro. A vendor may provide
+a header that contains a custom definition of this macro and specify the path to the header via the
+``LIBCXX_ASSERTION_HANDLER_FILE`` CMake variable. If provided, the contents of this header will be
+injected into library configuration headers, replacing the default implementation. The header must not
+include any standard library headers (directly or transitively) because doing so will almost always
+create a circular dependency.
+
+``_LIBCPP_ASSERTION_HANDLER(error_message, format, args...)`` is a variadic macro that takes the
+following parameters:
+
+* ``error_message`` -- the original error message that explains the hardening failure. In general, it
+  does not contain information about the source location that triggered the failure.
+* ``format`` -- a printf-style format string that contains a general description of the failure with
+  placeholders for the error message as well as details about the source location.
+* ``args...`` -- arguments to substitute in the ``format`` string. The exact order and meaning of the
+  arguments is unspecified and subject to change (but is always in sync with the format string). Note
+  that for convenience, ``args`` contain the error message as well.
+
+Programs that wish to terminate as fast as possible may use the ``error_message`` parameter that
+doesn't require any formatting. Programs that prefer having more information about the failure (such as
+the filename and the line number of the code that triggered the assertion) should use the printf-style
+formatting with ``format`` and ``args...``.
+
+When a hardening assertion fails, it means that the program is about to invoke undefined behavior. For
+this reason, the custom assertion handler is generally expected to terminate the program. If a custom
+assertion handler decides to avoid doing so (e.g. it chooses to log and continue instead), it does so
+at its own risk -- this approach should only be used in non-production builds and with an understanding
+of potential consequences. Furthermore, the custom assertion handler should not throw any exceptions as
+it may be invoked from standard library functions that are marked ``noexcept`` (so throwing will result
+in ``std::terminate`` being called).
 
 Libc++ Configuration Macros
 ===========================

libcxx/include/CMakeLists.txt

@@ -1019,9 +1019,12 @@ foreach(feature LIBCXX_ENABLE_FILESYSTEM LIBCXX_ENABLE_LOCALIZATION LIBCXX_ENABL
 endforeach()
 
 configure_file("__config_site.in" "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site" @ONLY)
+file(READ ${LIBCXX_ASSERTION_HANDLER_FILE} _LIBCPP_ASSERTION_HANDLER_OVERRIDE)
+configure_file("__assertion_handler.in" "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__assertion_handler" @ONLY)
 configure_file("module.modulemap.in" "${LIBCXX_GENERATED_INCLUDE_DIR}/module.modulemap" @ONLY)
 
 set(_all_includes "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site"
+                  "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__assertion_handler"
                   "${LIBCXX_GENERATED_INCLUDE_DIR}/module.modulemap")
 foreach(f ${files})
   set(src "${CMAKE_CURRENT_SOURCE_DIR}/${f}")
@@ -1059,6 +1062,12 @@ if (LIBCXX_INSTALL_HEADERS)
     PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
     COMPONENT cxx-headers)
 
+  # Install the generated __assertion_handler file to the per-target include dir.
+  install(FILES "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__assertion_handler"
+    DESTINATION "${LIBCXX_INSTALL_INCLUDE_TARGET_DIR}"
+    PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
+    COMPONENT cxx-headers)
+
   # Install the generated modulemap file to the generic include dir.
   install(FILES "${LIBCXX_GENERATED_INCLUDE_DIR}/module.modulemap"
     DESTINATION "${LIBCXX_INSTALL_INCLUDE_DIR}"

libcxx/include/__assert

@@ -10,8 +10,8 @@
 #ifndef _LIBCPP___ASSERT
 #define _LIBCPP___ASSERT
 
+#include <__assertion_handler>
 #include <__config>
-#include <__verbose_abort>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -20,7 +20,8 @@
 #define _LIBCPP_ASSERT(expression, message)                                                                            \
   (__builtin_expect(static_cast<bool>(expression), 1)                                                                  \
        ? (void)0                                                                                                       \
-       : _LIBCPP_VERBOSE_ABORT(                                                                                        \
+       : _LIBCPP_ASSERTION_HANDLER(                                                                                    \
+             message,                                                                                                  \
              "%s:%d: assertion %s failed: %s\n", __builtin_FILE(), __builtin_LINE(), #expression, message))
 
 // TODO: __builtin_assume can currently inhibit optimizations. Until this has been fixed and we can add

libcxx/include/__assertion_handler.in

@@ -0,0 +1,21 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ASSERTION_HANDLER
+#define _LIBCPP___ASSERTION_HANDLER
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+@_LIBCPP_ASSERTION_HANDLER_OVERRIDE@
+
+#endif // _LIBCPP___ASSERTION_HANDLER

libcxx/test/libcxx/lint/lint_headers.sh.py

@@ -14,6 +14,7 @@ def exclude_from_consideration(path):
         or path.endswith(".modulemap.in")
         or os.path.basename(path) == "__config"
         or os.path.basename(path) == "__config_site.in"
+        or os.path.basename(path) == "__assertion_handler.in"
         or os.path.basename(path) == "libcxx.imp"
         or os.path.basename(path).startswith("__pstl")
         or not os.path.isfile(path)  # TODO: Remove once PSTL integration is finished

libcxx/utils/generate_iwyu_mapping.py

@@ -43,6 +43,8 @@ def generate_map(include):
         public = []
         if i == "__assert":
             continue
+        elif i == "__assertion_handler.in":
+            continue
         elif i == "__availability":
             continue
         elif i == "__bit_reference":

libcxx/utils/libcxx/header_information.py

@@ -168,6 +168,9 @@ def is_modulemap_header(header):
     if header == "__config_site":
         return False
 
+    if header == "__assertion_handler":
+        return False
+
     # exclude libc++abi files
     if header in ["cxxabi.h", "__cxxabi_config.h"]:
         return False

libcxx/vendor/llvm/default_assertion_handler.in

@@ -0,0 +1,22 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <__config>
+#include <__verbose_abort>
+
+#if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
+
+#define _LIBCPP_ASSERTION_HANDLER(error_message, ...) ((void)error_message, _LIBCPP_VERBOSE_ABORT(__VA_ARGS__))
+
+#else
+
+// TODO(hardening): in production, trap rather than abort.
+#define _LIBCPP_ASSERTION_HANDLER(error_message, ...) ((void)error_message, _LIBCPP_VERBOSE_ABORT(__VA_ARGS__))
+
+#endif // #if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG