selftests/lkdtm: Add way to repeat a test

Some LKDTM tests need to be run more than once (usually to setup and
then later trigger). Until now, the only case was the SOFT_LOCKUP test,
which wasn't useful to run in the bulk selftests. The coming stack canary
checking needs to run twice, so support this with a new test output prefix
"repeat".
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211022223826.330653-2-keescook@chromium.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/testing/selftests/lkdtm/run.sh

@@ -56,8 +56,14 @@ if echo "$test" | grep -q '^#' ; then
 fi
 
 # If no expected output given, assume an Oops with back trace is success.
+repeat=1
 if [ -z "$expect" ]; then
 	expect="call trace:"
+else
+	if echo "$expect" | grep -q '^repeat:' ; then
+		repeat=$(echo "$expect" | cut -d' ' -f1 | cut -d: -f2)
+		expect=$(echo "$expect" | cut -d' ' -f2-)
+	fi
 fi
 
 # Prepare log for report checking
@@ -83,7 +89,9 @@ dmesg > "$DMESG"
 # the signal that killed the subprocess, we must ignore the failure and
 # continue. However we don't silence stderr since there might be other
 # useful details reported there in the case of other unexpected conditions.
-echo "$test" | cat >"$TRIGGER" || true
+for i in $(seq 1 $repeat); do
+	echo "$test" | cat >"$TRIGGER" || true
+done
 
 # Record and dump the results
 dmesg | comm --nocheck-order -13 "$DMESG" - > "$LOG" || true