FAILED: patch "[PATCH] test_firmware: fix setting old custom fw path back on exit," failed to apply to 4.16-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.16-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From e538409257d0217a9bc715686100a5328db75a15 Mon Sep 17 00:00:00 2001
From: Ben Hutchings <ben.hutchings@codethink.co.uk>
Date: Wed, 4 Apr 2018 22:38:49 +0200
Subject: [PATCH] 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>

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"