[PATCH] selftests/livepatch: support BusyBox diff

[Joe Lawrence]

BusyBox diff doesn't support the GNU diff '--LTYPE-line-format' options
that were used in the selftests to filter older kernel log messages from
dmesg output.

Bite the bullet and code this into the selftest:

  - test start: remember the last dmesg entry (has unique [timestamp]
    prefix) in $SAVED_DMESG
  - test end: awk-filter dmesg output up to and including $SAVED_DMESG

Account for kernel log overflow in case awk doesn't find $SAVED_DMESG,
exit and fail the test with a recommendation that the user expand the
kernel log buffer.

Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
---
 tools/testing/selftests/livepatch/functions.sh | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/tools/testing/selftests/livepatch/functions.sh b/tools/testing/selftests/livepatch/functions.sh
index 36648ca367c2..365ce4110265 100644
--- a/tools/testing/selftests/livepatch/functions.sh
+++ b/tools/testing/selftests/livepatch/functions.sh
@@ -41,15 +41,16 @@ function die() {
 	exit 1
 }
 
-# save existing dmesg so we can detect new content
+# save last dmesg entry so we can detect new content
 function save_dmesg() {
-	SAVED_DMESG=$(mktemp --tmpdir -t klp-dmesg-XXXXXX)
-	dmesg > "$SAVED_DMESG"
+	SAVED_DMESG="$(dmesg | tail -n1)"
 }
 
-# cleanup temporary dmesg file from save_dmesg()
-function cleanup_dmesg_file() {
-	rm -f "$SAVED_DMESG"
+# filter all dmesg content before and including $SAVED_DMESG
+function new_dmesg() {
+	if ! dmesg | awk -v last="$SAVED_DMESG" 'p; $0 == last{p=1} END {exit !p}'; then
+		die "dmesg overflow, try increasing kernel log buffer size"
+	fi
 }
 
 function push_config() {
@@ -81,7 +82,6 @@ function set_ftrace_enabled() {
 
 function cleanup() {
 	pop_config
-	cleanup_dmesg_file
 }
 
 # setup_config - save the current config and set a script exit trap that
@@ -277,7 +277,7 @@ function check_result {
 	# help differentiate repeated testing runs.  Remove them with a
 	# post-comparison sed filter.
 
-	result=$(dmesg | diff --changed-group-format='%>' --unchanged-group-format='' "$SAVED_DMESG" - | \
+	result=$(new_dmesg | \
 		 grep -e 'livepatch:' -e 'test_klp' | \
 		 grep -v '\(tainting\|taints\) kernel' | \
 		 sed 's/^\[[ 0-9.]*\] //')
@@ -288,6 +288,4 @@ function check_result {
 		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
 }
-- 
2.21.3