selftest/lkdtm: Skip stack-entropy test if lkdtm is not available

Exit with return code 4 if lkdtm is not available like other tests
in order to properly skip the test.

Signed-off-by: Misono Tomohiro <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/testing/selftests/lkdtm/stack-entropy.sh

···
 # Measure kernel stack entropy by sampling via LKDTM's REPORT_STACK test.
 set -e
 samples="${1:-1000}"
+TRIGGER=/sys/kernel/debug/provoke-crash/DIRECT
+KSELFTEST_SKIP_TEST=4
+
+# Verify we have LKDTM available in the kernel.
+if [ ! -r $TRIGGER ] ; then
+	/sbin/modprobe -q lkdtm || true
+	if [ ! -r $TRIGGER ] ; then
+		echo "Cannot find $TRIGGER (missing CONFIG_LKDTM?)"
+	else
+		echo "Cannot write $TRIGGER (need to run as root?)"
+	fi
+	# Skip this test
+	exit $KSELFTEST_SKIP_TEST
+fi
 
 # Capture dmesg continuously since it may fill up depending on sample size.
 log=$(mktemp -t stack-entropy-XXXXXX)
 dmesg --follow >"$log" & pid=$!
 report=-1
 for i in $(seq 1 $samples); do
-        echo "REPORT_STACK" >/sys/kernel/debug/provoke-crash/DIRECT
+        echo "REPORT_STACK" > $TRIGGER
 	if [ -t 1 ]; then
 		percent=$(( 100 * $i / $samples ))
 		if [ "$percent" -ne "$report" ]; then