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[libc++][hardening][NFC] Introduce _LIBCPP_VERBOSE_TRAP macro. #148262

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Merged
merged 2 commits into from
Jul 14, 2025
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[libc++][hardening][NFC] Introduce _LIBCPP_VERBOSE_TRAP macro. #148262

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[libc++][hardening][NFC] Introduce `_LIBCPP_VERBOSE_TRAP` macro.
var-const Jul 11, 2025
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var-const Jul 12, 2025
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1 change: 1 addition & 0 deletions libcxx/include/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -944,6 +944,7 @@ set(files
__vector/vector_bool.h
__vector/vector_bool_formatter.h
__verbose_abort
__verbose_trap
algorithm
any
array
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36 changes: 36 additions & 0 deletions libcxx/include/__verbose_trap
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Original file line number Diff line number Diff line change
@@ -0,0 +1,36 @@
// -*- 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___VERBOSE_TRAP
#define _LIBCPP___VERBOSE_TRAP

#include <__config>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
#endif

_LIBCPP_BEGIN_NAMESPACE_STD

#if __has_builtin(__builtin_verbose_trap)
// AppleClang shipped a slightly different version of __builtin_verbose_trap from the upstream
// version before upstream Clang actually got the builtin.
// TODO: Remove once AppleClang supports the two-arguments version of the builtin.
# if defined(_LIBCPP_APPLE_CLANG_VER) && _LIBCPP_APPLE_CLANG_VER < 1700
# define _LIBCPP_VERBOSE_TRAP(message) __builtin_verbose_trap(message)
# else
# define _LIBCPP_VERBOSE_TRAP(message) __builtin_verbose_trap("libc++", message)
# endif
#else
# define _LIBCPP_VERBOSE_TRAP(message) ((void)message, __builtin_trap())
#endif

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___VERBOSE_TRAP
14 changes: 2 additions & 12 deletions libcxx/vendor/llvm/default_assertion_handler.in
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Expand Up @@ -15,6 +15,7 @@
# include <__cxx03/__verbose_abort>
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Question: what do we do about the frozen headers?

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This is a good question, and actually it's not easy to answer IMO. CCing @philnik777 .

I see a few options:

  1. Introduce <__cxx03/__verbose_trap>. This seems to go against the notion of having "frozen headers".
  2. Define _LIBCPP_VERBOSE_TRAP inline inside default_assertion_handler.in inside this #if block. This is equivalent to (1) except we don't introduce a new header under __cxx03.
  3. Always use the non-C++03 headers from default_assertion_handler.in. So this means get rid of the #if and always include <__config> and <__verbose_abort>, never their <__cxx03/...> counterparts. This would be doable since <__verbose_abort> doesn't actually need C++11, but it goes against the principle of the split and may cause other conflicts if we somehow end up including both <__config> and <__cxx03/__config>.
  4. Have two copies of default_assertion_handler.in, one normal and one for the frozen headers. This is a bit tricky since default_assertion_handler.in is something we set up from CMake and expect vendors to be able to override.

I think (1) is the best outcome here. We'll have to support these hardening improvements in frozen-C++03 anyways otherwise this'll be a regression in functionality.

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Thanks for the comprehensive overview of alternatives, this is super useful!

From what I gather, the frozen headers are still undergoing development, so them being completely frozen is the end goal rather than a description of the current state. If that perspective is valid, I also think that option (1) is the best, or at least the lesser evil out of the available options.

#else
# include <__config>
# include <__verbose_trap>
# include <__verbose_abort>
#endif

Expand All @@ -28,18 +29,7 @@

#else

# if __has_builtin(__builtin_verbose_trap)
// AppleClang shipped a slightly different version of __builtin_verbose_trap from the upstream
// version before upstream Clang actually got the builtin.
// TODO: Remove once AppleClang supports the two-arguments version of the builtin.
# if defined(_LIBCPP_APPLE_CLANG_VER) && _LIBCPP_APPLE_CLANG_VER < 1700
# define _LIBCPP_ASSERTION_HANDLER(message) __builtin_verbose_trap(message)
# else
# define _LIBCPP_ASSERTION_HANDLER(message) __builtin_verbose_trap("libc++", message)
# endif
# else
# define _LIBCPP_ASSERTION_HANDLER(message) ((void)message, __builtin_trap())
# endif
# define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_TRAP(message)

#endif // _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG

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