Determine GC type based on BUILT_KERNEL_VERSION_FILE.

How it works:
1. build/make/core/Makefile generates a txt file with the kernel
   version, which is taken from an explicit BOARD_KERNEL_VERSION value,
   or extracted from the kernel image on the source tree, or extracted
   from the kernel image extracted from the prebuilt boot.img.
   The file is saved at
   $ANDROID_PRODUCT_OUT/obj/PACKAGING/check_vintf_all_intermediates/kernel_version.txt.
2. If PRODUCT_ENABLE_UFFD_GC is "default", meaning the GC type needs to
   be determined by the kernel version, build/make/core/Makefile copies
   kernel_version.txt to
   out/soong/dexpreopt/kernel_version_for_uffd_gc.txt.
3. build/soong/dexpreopt/config.go writes the the UFFD GC flag to
   out/soong/dexpreopt/uffd_gc_flag.txt. The flag is determined by an
   explicit PRODUCT_ENABLE_UFFD_GC value or by contruct_uffd_gc_flag.py,
   which reads kernel_version_for_uffd_gc.txt and determines the flag
   accordingly.
4. dex2oat takes the UFFD GC flag from uffd_gc_flag.txt.
5. post_process_props.py mangles ro.dalvik.vm.enable_uffd_gc based on
   the same logic.

Bug: 321751629
Bug: 319554951
Bug: 318763448
Bug: 319648491
Test: m --no-skip-soong-tests nothing
Test: atest uffd_gc_utils_test
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=default m` for device with no
  UFFD support -
  1. Check the existence of `-Xgc:CMC` in
     out/soong/dexpreopt_arm64/dex_bootjars/android/system/framework/arm64/boot.invocation
     (dex2oat invocation for a boot image)
  2. Check the existence of `-Xgc:CMC` in
     out/soong/.intermediates/packages/apps/Settings/Settings/android_common/dexpreopt/Settings/oat/arm64/package.invocation
     (dex2oat invocation for an app defined in .bp)
  3. Check the existence of `-Xgc:CMC` in
     $ANDROID_PRODUCT_OUT/obj/APPS/Dialer_intermediates/oat/arm64/package.invocation
     (dex2oat invocation for an app defined in .mk)
  4. Check the value of ro.dalvik.vm.enable_uffd_gc in
     $ANDROID_PRODUCT_OUT/product/etc/build.prop
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=default m` for device with
  UFFD support, and do the steps above.
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=true m`, and do the steps
  above.
Test: Build with `OVERRIDE_ENABLE_UFFD_GC=false m`, and do the steps
  above.

Change-Id: I035ad32233e49e2a30ce11f6c7c318a648511ef8

scripts/Android.bp

@@ -142,6 +142,39 @@ python_test_host {
     test_suites: ["general-tests"],
 }
 
+python_library_host {
+    name: "uffd_gc_utils",
+    srcs: [
+        "uffd_gc_utils.py",
+    ],
+    visibility: [
+        "//build/make/tools:__subpackages__",
+    ],
+}
+
+python_test_host {
+    name: "uffd_gc_utils_test",
+    main: "uffd_gc_utils_test.py",
+    srcs: [
+        "uffd_gc_utils_test.py",
+    ],
+    libs: [
+        "uffd_gc_utils",
+    ],
+    test_suites: ["general-tests"],
+}
+
+python_binary_host {
+    name: "construct_uffd_gc_flag",
+    main: "construct_uffd_gc_flag.py",
+    srcs: [
+        "construct_uffd_gc_flag.py",
+    ],
+    libs: [
+        "uffd_gc_utils",
+    ],
+}
+
 python_library_host {
     name: "ninja_rsp",
     srcs: ["ninja_rsp.py"],

scripts/construct_uffd_gc_flag.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python
+#
+# Copyright (C) 2024 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+"""A tool for constructing UFFD GC flag."""
+
+import argparse
+import os
+
+from uffd_gc_utils import should_enable_uffd_gc
+
+
+def parse_args():
+  parser = argparse.ArgumentParser()
+  parser.add_argument('kernel_version_file')
+  parser.add_argument('output')
+  return parser.parse_args()
+
+def main():
+  args = parse_args()
+  enable_uffd_gc = should_enable_uffd_gc(args.kernel_version_file)
+  flag = '--runtime-arg -Xgc:CMC' if enable_uffd_gc else ''
+  # Prevent the file's mtime from being changed if the contents don't change.
+  # This avoids unnecessary dexpreopt reruns.
+  if os.path.isfile(args.output):
+    with open(args.output, 'r') as f:
+      if f.read() == flag:
+        return
+  with open(args.output, 'w') as f:
+    f.write(flag)
+
+
+if __name__ == '__main__':
+  main()

scripts/uffd_gc_utils.py

@@ -0,0 +1,68 @@
+#!/usr/bin/env python
+#
+# Copyright (C) 2024 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+"""Utils to determine whether to enable UFFD GC."""
+
+import re
+import sys
+
+
+def should_enable_uffd_gc(kernel_version_file):
+  with open(kernel_version_file, 'r') as f:
+    kernel_version = f.read().strip()
+  return should_enable_uffd_gc_impl(kernel_version)
+
+def should_enable_uffd_gc_impl(kernel_version):
+  # See https://source.android.com/docs/core/architecture/kernel/gki-versioning#determine-release
+  p = r"^(?P<w>\d+)[.](?P<x>\d+)[.](?P<y>\d+)(-android(?P<z>\d+)-(?P<k>\d+).*$)?"
+  m = re.match(p, kernel_version)
+  if m is not None:
+    if m.group('z') is not None:
+      android_release = int(m.group('z'))
+      # No need to check w, x, y because all Android 12 kernels have backports.
+      return android_release >= 12
+    else:
+      # Old kernel or non-GKI kernel.
+      version = int(m.group('w'))
+      patch_level = int(m.group('x'))
+      if version < 5:
+        # Old kernel.
+        return False
+      elif (version == 5 and patch_level >= 7) or version >= 6:
+        # New non-GKI kernel. 5.7 supports MREMAP_DONTUNMAP without the need for
+        # backports.
+        return True
+      else:
+        # Non-GKI kernel between 5 and 5.6. It may have backports.
+        raise exit_with_error(kernel_version)
+  elif kernel_version == '<unknown-kernel>':
+    # The kernel information isn't available to the build system, probably
+    # because PRODUCT_OTA_ENFORCE_VINTF_KERNEL_REQUIREMENTS is set to false. We
+    # assume that the kernel supports UFFD GC because it is the case for most of
+    # the products today and it is the future.
+    return True
+  else:
+    # Unrecognizable non-GKI kernel.
+    raise exit_with_error(kernel_version)
+
+def exit_with_error(kernel_version):
+  sys.exit(f"""
+Unable to determine UFFD GC flag for kernel version "{kernel_version}".
+You can fix this by explicitly setting PRODUCT_ENABLE_UFFD_GC to "true" or
+"false" based on the kernel version.
+1. Set PRODUCT_ENABLE_UFFD_GC to "true" if the kernel supports userfaultfd(2)
+   and MREMAP_DONTUNMAP.
+2. Set PRODUCT_ENABLE_UFFD_GC to "false" otherwise.""")

scripts/uffd_gc_utils_test.py

@@ -0,0 +1,61 @@
+#!/usr/bin/env python
+#
+# Copyright (C) 2024 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+"""Unit tests for uffd_gc_utils.py."""
+
+import unittest
+
+from uffd_gc_utils import should_enable_uffd_gc_impl
+
+
+class UffdGcUtilsTest(unittest.TestCase):
+  def test_should_enable_uffd_gc_impl(self):
+    # GKI kernels in new format.
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "6.1.25-android14-11-g34fde9ec08a3-ab10675345"))
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "5.4.42-android12-0-something"))
+    self.assertFalse(should_enable_uffd_gc_impl(
+        "5.4.42-android11-0-something"))
+    # GKI kernels in old format.
+    self.assertFalse(should_enable_uffd_gc_impl(
+        "4.19.282-g4b749a433956-ab10893502"))
+    # Non GKI kernels.
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "6.1.25-foo"))
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "6.1.25"))
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "5.10.19-foo"))
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "5.10.19"))
+    with self.assertRaises(SystemExit):
+        should_enable_uffd_gc_impl("5.4.42-foo")
+    with self.assertRaises(SystemExit):
+        should_enable_uffd_gc_impl("5.4.42")
+    self.assertFalse(should_enable_uffd_gc_impl(
+        "4.19.282-foo"))
+    self.assertFalse(should_enable_uffd_gc_impl(
+        "4.19.282"))
+    with self.assertRaises(SystemExit):
+        should_enable_uffd_gc_impl("foo")
+    # No kernel.
+    self.assertTrue(should_enable_uffd_gc_impl(
+        "<unknown-kernel>"))
+
+
+if __name__ == '__main__':
+  unittest.main(verbosity=2)