cm/filesystem: Fix crash with pre-C++11 std::string GNU ABI in C++17

The `remove_filename` and `replace_extension` methods compute an offset
between the whole path in a `std::string` and a part of a path in a
`std::string_view`.  This is done by subtracting their `.data()`
pointers.  However, C++17 adds a non-const `.data()` through which
modification of the string is allowed.  This means the copy-on-write
implementation used by the pre-C++11 std::string GNU ABI must reallocate
if the string has been copied.  Our subtraction then computes an offset
between two different allocations, which is undefined behavior.

The workaround in commit b3ca4f9ad1 (cm/filesystem: Work around crash
when compiled for CYGWIN/MSYS runtime, 2021-04-22, v3.21.0-rc1~271^2~2)
avoided the problem by calling the non-const `.data()` to reallocate
before constructing the `string_view`.  Instead, explicitly call the
const `.data()` method on the string, which does not reallocate.

Fixes: #22090, #23328

2 files changed, 12 insertions, 3 deletions

diff --git a/Source/Checks/cm_cxx_features.cmake b/Source/Checks/cm_cxx_features.cmake
index f20572e06c..7917d41faf 100644
--- a/Source/Checks/cm_cxx_features.cmake
+++ b/Source/Checks/cm_cxx_features.cmake
@@ -80,9 +80,7 @@ if(CMake_HAVE_CXX_MAKE_UNIQUE)
   set(CMake_HAVE_CXX_UNIQUE_PTR 1)
 endif()
 cm_check_cxx_feature(unique_ptr)
-if (NOT CMAKE_CXX_STANDARD LESS "17"
-    AND NOT MSYS # FIXME: RunCMake.cmake_path cases crash with MSYS std::filesystem
-    )
+if (NOT CMAKE_CXX_STANDARD LESS "17")
   if (NOT CMAKE_CROSSCOMPILING OR CMAKE_CROSSCOMPILING_EMULATOR)
     cm_check_cxx_feature(filesystem TRY_RUN)
   else()
diff --git a/Source/Checks/cm_cxx_filesystem.cxx b/Source/Checks/cm_cxx_filesystem.cxx
index ae8acc56a6..732f28f39e 100644
--- a/Source/Checks/cm_cxx_filesystem.cxx
+++ b/Source/Checks/cm_cxx_filesystem.cxx
@@ -23,5 +23,16 @@ int main()
   }
 #endif
 
+  // If std::string is copy-on-write, the std::filesystem::path
+  // implementation may accidentally trigger a reallocation and compute
+  // an offset between two allocations, leading to undefined behavior.
+#if defined(__GLIBCXX__) &&                                                   \
+  (!defined(_GLIBCXX_USE_CXX11_ABI) || !_GLIBCXX_USE_CXX11_ABI)
+  std::string p5s1 = "/path";
+  std::string p5s2 = std::move(p5s1);
+  std::filesystem::path p5 = std::string(p5s2);
+  p5.remove_filename();
+#endif
+
   return 0;
 }