Add a simplified linux worktree mechanism

Also fix two invalid regex escape warnings on python 3.12.7.

README.adoc

@@ -28222,6 +28222,24 @@ You can also build <<kernel-modules>> against a specific prebuilt kernel with:
 
 This will then allow you to insmod the kernel modules on your newly built kernel.
 
+===== Linux worktree
+
+If you are working with multiple Linux versions at once, it can be helpful to have their source code checked out all the time, in particular if you are going to be <<gdb,GDB step debugging>>.
+
+We have some automation for that. Suppose that you are also interested in the kernel v6.14.4. Then you could do:
+
+....
+git -C submodules/linux worktree add ../../data/wt/linux/v6.14.4
+git -C data/wt/linux/v6.14.4 reset --hard v6.14.4
+....
+
+Then you can use a specific source for a specific <<linux-kernel-build-variants>> with:
+
+....
+./build-linux --linux-worktree v6.14.4 --linux-build-id v6.14.4
+./run --linux-build-id v6.14.4
+....
+
 ==== QEMU build variants
 
 Analogous to the <<linux-kernel-build-variants>> but with the `--qemu-build-id` option instead:

common.py

@@ -46,11 +46,14 @@ consts['root_dir'] = os.path.dirname(os.path.abspath(__file__))
 consts['data_dir'] = os.path.join(consts['root_dir'], 'data')
 consts['p9_dir'] = os.path.join(consts['data_dir'], '9p')
 consts['gem5_non_default_source_root_dir'] = os.path.join(consts['data_dir'], 'gem5')
+consts['worktree_basename'] = 'wt'
+consts['worktree_dir'] = os.path.join(consts['data_dir'], consts['worktree_basename'])
+consts['linux_worktree_root_dir'] = os.path.join(consts['worktree_dir'], 'linux')
 if consts['in_docker']:
     # fatal: unsafe repository ('/root/lkmc' is owned by someone else)
     # Fuck these error checks, let me shoot my feet in peace.
     # The best solution would be to actually get Docker to mount
-    # the current diretory as root. But I've never been able to do that:
+    # the current directory as root. But I've never been able to do that:
     # * https://stackoverflow.com/questions/51973179/docker-mount-volumes-as-root
     # * https://unix.stackexchange.com/questions/523492/how-to-mount-files-as-specific-user-when-using-docker-namespace-remapping
     # * https://stackoverflow.com/questions/35291520/docker-and-userns-remap-how-to-manage-volume-permissions-to-share-data-betwee
@@ -93,12 +96,11 @@ consts['submodules_dir'] = os.path.join(consts['root_dir'], 'submodules')
 consts['buildroot_source_dir'] = os.path.join(consts['submodules_dir'], 'buildroot')
 consts['crosstool_ng_source_dir'] = os.path.join(consts['submodules_dir'], 'crosstool-ng')
 consts['crosstool_ng_supported_archs'] = set(['arm', 'aarch64'])
-consts['linux_source_dir'] = os.path.join(consts['submodules_dir'], 'linux')
 consts['linux_config_dir'] = os.path.join(consts['root_dir'], 'linux_config')
+consts['linux_default_source_dir'] = os.path.join(consts['submodules_dir'], 'linux')
 consts['gem5_default_source_dir'] = os.path.join(consts['submodules_dir'], 'gem5')
 consts['googletest_source_dir'] = os.path.join(consts['submodules_dir'], 'googletest')
 consts['rootfs_overlay_dir'] = os.path.join(consts['root_dir'], 'rootfs_overlay')
-consts['extract_vmlinux'] = os.path.join(consts['linux_source_dir'], 'scripts', 'extract-vmlinux')
 consts['qemu_source_dir'] = os.path.join(consts['submodules_dir'], 'qemu')
 consts['parsec_benchmark_source_dir'] = os.path.join(consts['submodules_dir'], 'parsec-benchmark')
 consts['ccache_dir'] = os.path.join('/usr', 'lib', 'ccache')
@@ -116,7 +118,7 @@ for key in consts['arch_short_to_long_dict']:
     consts['arch_choices'].add(key)
     consts['arch_choices'].add(consts['arch_short_to_long_dict'][key])
 consts['default_arch'] = 'x86_64'
-consts['gem5_cpt_prefix'] = '^cpt\.'
+consts['gem5_cpt_prefix'] = r'^cpt\.'
 def git_sha(repo_path):
     return subprocess.check_output(['git', '-C', repo_path, 'log', '-1', '--format=%H']).decode().rstrip()
 consts['sha'] = common.git_sha(consts['root_dir'])
@@ -144,7 +146,7 @@ consts['userland_out_exts'] = [
     consts['obj_ext'],
 ]
 consts['default_config_file'] = os.path.join(consts['data_dir'], 'config.py')
-consts['serial_magic_exit_status_regexp_string'] = b'lkmc_exit_status_(\d+)'
+consts['serial_magic_exit_status_regexp_string'] = br'lkmc_exit_status_(\d+)'
 consts['baremetal_lib_basename'] = 'lib'
 consts['emulator_userland_only_short_to_long_dict'] = collections.OrderedDict([
     ('n', 'native'),
@@ -429,6 +431,12 @@ https://cirosantilli.com/linux-kernel-module-cheat#vmlinux-vs-bzimage-vs-zimage-
             '--linux-source-dir',
             help='''\
 Use the given directory as the Linux source tree.
+'''
+        )
+        self.add_argument(
+            '--linux-worktree',
+            help='''\
+Use custom linux source from given worktree.
 '''
         )
         self.add_argument(
@@ -868,9 +876,6 @@ Incompatible archs are skipped.
         env['staging_dir'] = join(env['out_dir'], 'staging', env['arch'])
         env['buildroot_staging_dir'] = join(env['buildroot_build_dir'], 'staging')
         env['buildroot_target_dir'] = join(env['buildroot_build_dir'], 'target')
-        if not env['_args_given']['linux_source_dir']:
-            env['linux_source_dir'] = os.path.join(consts['submodules_dir'], 'linux')
-        common.extract_vmlinux = os.path.join(env['linux_source_dir'], 'scripts', 'extract-vmlinux')
         env['linux_buildroot_build_dir'] = join(env['buildroot_build_build_dir'], 'linux-custom')
 
         # QEMU
@@ -1018,8 +1023,16 @@ Incompatible archs are skipped.
         env['run_cmd_file'] = join(env['run_dir'], env['run_cmd_file_basename'])
 
         # Linux kernel.
+        if env['_args_given']['gem5_source_dir']:
+            assert os.path.exists(env['gem5_source_dir'])
+        else:
+            if env['_args_given']['linux_worktree']:
+                env['linux_source_dir'] = join(env['linux_worktree_root_dir'], env['linux_worktree'])
+            else:
+                env['linux_source_dir'] = env['linux_default_source_dir']
         if not env['_args_given']['linux_build_dir']:
             env['linux_build_dir'] = join(env['out_dir'], 'linux', env['linux_build_id'], env['arch'])
+        env['extract_vmlinux'] = os.path.join(env['linux_source_dir'], 'scripts', 'extract-vmlinux')
         env['lkmc_vmlinux'] = join(env['linux_build_dir'], 'vmlinux')
         if env['arch'] == 'arm':
             env['android_arch'] = 'arm'