test_firmware: fix setting old custom fw path back on exit, second try

Commit 65c79230576 tried to clear the custom firmware path on exit by
writing a single space to the firmware_class.path parameter.  This
doesn't work because nothing strips this space from the value stored
and fw_get_filesystem_firmware() only ignores zero-length paths.

Instead, write a null byte.

Fixes: 0a8adf58475 ("test: add firmware_class loader test")
Fixes: 65c79230576 ("test_firmware: fix setting old custom fw path back on exit")
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 6 insertions, 4 deletions

diff --git a/tools/testing/selftests/firmware/fw_lib.sh b/tools/testing/selftests/firmware/fw_lib.sh
index 9ea31b57d71a..962d7f4ac627 100755
--- a/tools/testing/selftests/firmware/fw_lib.sh
+++ b/tools/testing/selftests/firmware/fw_lib.sh
@@ -154,11 +154,13 @@ test_finish()
         if [ "$HAS_FW_LOADER_USER_HELPER" = "yes" ]; then
                 echo "$OLD_TIMEOUT" >/sys/class/firmware/timeout
         fi
-        if [ "$OLD_FWPATH" = "" ]; then
-                OLD_FWPATH=" "
-        fi
         if [ "$TEST_REQS_FW_SET_CUSTOM_PATH" = "yes" ]; then
-                echo -n "$OLD_FWPATH" >/sys/module/firmware_class/parameters/path
+                if [ "$OLD_FWPATH" = "" ]; then
+                        # A zero-length write won't work; write a null byte
+                        printf '\000' >/sys/module/firmware_class/parameters/path
+                else
+                        echo -n "$OLD_FWPATH" >/sys/module/firmware_class/parameters/path
+                fi
         fi
         if [ -f $FW ]; then
                 rm -f "$FW"