pythongh-115382: Remove foreign import paths from cross compiles

Previously, when a build was configured to use a host interpreter via
--with-build-python, the PYTHON_FOR_BUILD config value included a path
in PYTHONPATH that pointed to the target's built external modules.

For "normal" foreign architecture cross compiles, when loading compiled
external libraries, the target libraries were processed first due to
their precedence in sys.path. These libraries were then ruled out due to
a mismatch in the SOABI so the import mechanism continues searching for
modules until it found the host's native modules.

However, if the host interpreter and the target python are on the same
version + SOABI combination, the host interpreter would attempt to load
the target's external modules due to their precedence in sys.path.

Despite the "match", the target build may be linked against a different
libc or may include instructions that are not supported on the host, so
loading/executing the target's external modules can lead to crashes.

Now, the path to the target's external modules is no longer defined in
PYTHONPATH to prevent accidentally loading these foreign modules.

One caveat is that during certain build stages, the target's sysconfig
module requires higher precedence than the host's version in order to
accurately query the target build's configuration.

This worked previously due to the target's sysconfig data module having
precedence over the host's (see above). In order to keep this desired
behavior, a new environment variable, _PYTHON_SYSCONFIGDATA_PATH, has
been defined so sysconfig can search this directory for the target's
sysconfig data.

Signed-off-by: Vincent Fazio <vfazio@gmail.com>

Lib/sysconfig.py

@@ -533,7 +533,25 @@ def _init_posix(vars):
     """Initialize the module as appropriate for POSIX systems."""
     # _sysconfigdata is generated at build time, see _generate_posix_vars()
     name = _get_sysconfigdata_name()
-    _temp = __import__(name, globals(), locals(), ['build_time_vars'], 0)
+
+    # For cross builds, this path used to be in PYTHONPATH, however, for
+    # host SOABI == target SOABI situations, the target modules would be loaded
+    # instead of host modules due to their precedence in sys.path. This could
+    # cause build problems as the target may link against a different libc or
+    # attempt to execute unsupported instructions.
+    # 
+    # The sysconfig data is a special case. Certain build steps need this to
+    # query the target's info so it needs to preempt the host's sysconfig data.
+    # Since it is a simple pure python module, it is safe to load from file.
+    path = os.environ.get('_PYTHON_SYSCONFIGDATA_PATH')
+    if path is not None:
+        from importlib.machinery import FileFinder, SourceFileLoader, SOURCE_SUFFIXES
+        from importlib.util import module_from_spec
+        spec = FileFinder(path, (SourceFileLoader, SOURCE_SUFFIXES)).find_spec(name)
+        _temp = module_from_spec(spec)
+        spec.loader.exec_module(_temp)
+    else:
+        _temp = __import__(name, globals(), locals(), ['build_time_vars'], 0)
     build_time_vars = _temp.build_time_vars
     vars.update(build_time_vars)
 

Lib/test/libregrtest/main.py

@@ -519,6 +519,7 @@ def _add_cross_compile_opts(self, regrtest_opts):
                 '_PYTHON_PROJECT_BASE',
                 '_PYTHON_HOST_PLATFORM',
                 '_PYTHON_SYSCONFIGDATA_NAME',
+                "_PYTHON_SYSCONFIGDATA_PATH",
                 'PYTHONPATH'
             }
             old_environ = os.environ

Lib/test/pythoninfo.py

@@ -326,6 +326,7 @@ def format_groups(groups):
         "_PYTHON_HOST_PLATFORM",
         "_PYTHON_PROJECT_BASE",
         "_PYTHON_SYSCONFIGDATA_NAME",
+        "_PYTHON_SYSCONFIGDATA_PATH",
         "__PYVENV_LAUNCHER__",
 
         # Sanitizer options

Tools/scripts/run_tests.py

@@ -42,6 +42,7 @@ def main(regrtest_args):
             '_PYTHON_PROJECT_BASE',
             '_PYTHON_HOST_PLATFORM',
             '_PYTHON_SYSCONFIGDATA_NAME',
+            "_PYTHON_SYSCONFIGDATA_PATH",
             'PYTHONPATH'
         }
         environ = {

configure.ac

@@ -164,7 +164,7 @@ AC_ARG_WITH([build-python],
     dnl Build Python interpreter is used for regeneration and freezing.
     ac_cv_prog_PYTHON_FOR_REGEN=$with_build_python
     PYTHON_FOR_FREEZE="$with_build_python"
-    PYTHON_FOR_BUILD='_PYTHON_PROJECT_BASE=$(abs_builddir) _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`:)$(srcdir)/Lib _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_$(ABIFLAGS)_$(MACHDEP)_$(MULTIARCH) '$with_build_python
+    PYTHON_FOR_BUILD='_PYTHON_PROJECT_BASE=$(abs_builddir) _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(srcdir)/Lib _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_$(ABIFLAGS)_$(MACHDEP)_$(MULTIARCH) _PYTHON_SYSCONFIGDATA_PATH=$(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`) '$with_build_python
     AC_MSG_RESULT([$with_build_python])
   ], [
     AS_VAR_IF([cross_compiling], [yes],