WSL: Subprocess cmd.exe with /U to output UTF-16LE

cloudinit/sources/DataSourceWSL.py

@@ -90,15 +90,21 @@ def find_home() -> PurePath:
     raises: IOError when no mountpoint with cmd.exe is found
                ProcessExecutionError when either cmd.exe is unable to retrieve
                the user's home directory
+               UnicodeDecodeError when cmd.exe /U outputs invalid UTF16LE
     """
     cmd = cmd_executable()
 
     # cloud-init runs too early to rely on binfmt to execute Windows binaries.
     # But we know that `/init` is the interpreter, so we can run it directly.
     # See /proc/sys/fs/binfmt_misc/WSLInterop[-late]
     # inside any WSL instance for more details.
-    home, _ = subp.subp(["/init", cmd.as_posix(), "/C", "echo %USERPROFILE%"])
-    home = home.rstrip()
+    # Invoking with "/U" makes it output UTF-16LE, which is more predictable
+    # than ANSI Code Pages for anything above the ASCII range.
+    home, _ = subp.subp(
+        ["/init", cmd.as_posix(), "/U", "/C", "echo.%USERPROFILE%"],
+        decode=False,
+    )
+    home = home.decode("utf-16-le").rstrip()
     if not home:
         raise subp.ProcessExecutionError(
             "No output from cmd.exe to show the user profile dir."
@@ -443,7 +449,7 @@ def _get_data(self) -> bool:
 
         try:
             user_home = find_home()
-        except IOError as e:
+        except (IOError, ValueError) as e:
             LOG.debug("Unable to detect WSL datasource: %s", e)
             return False
 

tests/unittests/sources/test_wsl.py

@@ -144,6 +144,19 @@ def test_cmd_exe_no_win_mounts(self, m_mounts, m_os_access):
         with pytest.raises(IOError):
             wsl.cmd_executable()
 
+    @mock.patch("cloudinit.sources.DataSourceWSL.cmd_executable")
+    @mock.patch("cloudinit.util.subp.subp")
+    def test_find_home_raises(self, m_subp, m_cmd):
+        # The value really doesn't matter.
+        m_cmd.return_value = PurePath("/mnt/c/cmd.exe")
+        m_subp.return_value = util.subp.SubpResult(
+            "I am UTF-8 🦄 !".encode("utf-8"), "\r\n".encode("utf-8")
+        )
+        # Checking for ValueError instead of UnicodeDecodeError because
+        # that's what we catch at the call sites.
+        with pytest.raises(ValueError):
+            wsl.find_home()
+
     @pytest.mark.parametrize(
         "linux_distro_value,files",
         (

tools/ds-identify

@@ -1708,7 +1708,9 @@ WSL_path() {
 WSL_run_cmd() {
     local val="" exepath="$1"
     shift
-    _RET=$(/init "$exepath" /c "$@" 2>/dev/null)
+    # Using the '/u' flag to enforce Unicode (UTF-16 LE), thus we need to decode it afterwards.
+    # It's more reliable than the default ANSI Code Pages for anything above the ASCII range.
+    _RET=$(/init "$exepath" /u /c "$@" 2>/dev/null | iconv --from-code UTF-16LE --to-code UTF-8)
 }
 
 WSL_profile_dir() {
@@ -1719,10 +1721,12 @@ WSL_profile_dir() {
     for m in $@; do
         cmdexe="$m/Windows/System32/cmd.exe"
         if command -v "$cmdexe" > /dev/null 2>&1; then
-            # Here WSL's proprietary `/init` is used to start the Windows cmd.exe
+            # Here WSL's `/init` is used to start the Windows cmd.exe
             # to output the Windows user profile directory path, which is
             # held by the environment variable %USERPROFILE%.
-            WSL_run_cmd "$cmdexe" "echo %USERPROFILE%"
+	    # See https://wsl.dev/technical-documentation/interop/ for more information on how /init
+	    # is used to launch Windows binaries.
+            WSL_run_cmd "$cmdexe" "echo.%USERPROFILE%"
             profiledir="${_RET%%[[:cntrl:]]}"
             if [ -n "$profiledir" ]; then
                 # wslpath is a program supplied by WSL itself that translates Windows and Linux paths,