selftests/livepatch: fix and refactor new dmesg message code

The livepatching kselftests rely on comparing expected vs. observed
dmesg output. After each test, new dmesg entries are determined by the
'comm' utility comparing a saved, pre-test copy of dmesg to post-test
dmesg output.

Alexander reports that the 'comm --nocheck-order -13' invocation used by
the tests can be confused when dmesg entry timestamps vary in magnitude
(ie, "[ 98.820331]" vs. "[ 100.031067]"), in which case, additional
messages are reported as new. The unexpected entries then spoil the
test results.

Instead of relying on 'comm' or 'diff' to determine new testing dmesg
entries, refactor the code:

- pre-test : log a unique canary dmesg entry
- test : run tests, log messages
- post-test : filter dmesg starting from pre-test message

Reported-by: Alexander Gordeev <[e-mail]>
Closes: https://lore.kernel.org/live-patching/[e-mail]/
Signed-off-by: Joe Lawrence <[e-mail]>
Acked-by: Alexander Gordeev <[e-mail]>
Tested-by: Marcos Paulo de Souza <[e-mail]>
Reviewed-by: Marcos Paulo de Souza <[e-mail]>
Acked-by: Miroslav Benes <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/livepatch/functions.sh

···
 	exit 1
 }
 
-# save existing dmesg so we can detect new content
-function save_dmesg() {
-	SAVED_DMESG=$(mktemp --tmpdir -t klp-dmesg-XXXXXX)
-	dmesg > "$SAVED_DMESG"
-}
-
-# cleanup temporary dmesg file from save_dmesg()
-function cleanup_dmesg_file() {
-	rm -f "$SAVED_DMESG"
-}
-
 function push_config() {
 	DYNAMIC_DEBUG=$(grep '^kernel/livepatch' /sys/kernel/debug/dynamic_debug/control | \
 			awk -F'[: ]' '{print "file " $1 " line " $2 " " $4}')
···
 
 function cleanup() {
 	pop_config
-	cleanup_dmesg_file
 }
 
 # setup_config - save the current config and set a script exit trap that
···
 function start_test {
 	local test="$1"
 
-	save_dmesg
+	# Dump something unique into the dmesg log, then stash the entry
+	# in LAST_DMESG.  The check_result() function will use it to
+	# find new kernel messages since the test started.
+	local last_dmesg_msg="livepatch kselftest timestamp: $(date --rfc-3339=ns)"
+	log "$last_dmesg_msg"
+	loop_until 'dmesg | grep -q "$last_dmesg_msg"' ||
+		die "buffer busy? can't find canary dmesg message: $last_dmesg_msg"
+	LAST_DMESG=$(dmesg | grep "$last_dmesg_msg")
+
 	echo -n "TEST: $test ... "
 	log "===== TEST: $test ====="
 }
···
 	local expect="$*"
 	local result
 
-	# Note: when comparing dmesg output, the kernel log timestamps
-	# help differentiate repeated testing runs.  Remove them with a
-	# post-comparison sed filter.
-
-	result=$(dmesg | comm --nocheck-order -13 "$SAVED_DMESG" - | \
+	# Test results include any new dmesg entry since LAST_DMESG, then:
+	# - include lines matching keywords
+	# - exclude lines matching keywords
+	# - filter out dmesg timestamp prefixes
+	result=$(dmesg | awk -v last_dmesg="$LAST_DMESG" 'p; $0 == last_dmesg { p=1 }' | \
 		 grep -e 'livepatch:' -e 'test_klp' | \
 		 grep -v '\(tainting\|taints\) kernel' | \
 		 sed 's/^\[[ 0-9.]*\] //')
 
 	if [[ "$expect" == "$result" ]] ; then
 		echo "ok"
+	elif [[ "$result" == "" ]] ; then
+		echo -e "not ok\n\nbuffer overrun? can't find canary dmesg entry: $LAST_DMESG\n"
+		die "livepatch kselftest(s) failed"
 	else
 		echo -e "not ok\n\n$(diff -upr --label expected --label result <(echo "$expect") <(echo "$result"))\n"
 		die "livepatch kselftest(s) failed"
 	fi
-
-	cleanup_dmesg_file
 }
 
 # check_sysfs_rights(modname, rel_path, expected_rights) - check sysfs