[RFC PATCH bpf-next v2 2/2] selftests/livepatch: update test-ftrace.sh for deprecated ftrace_enabled

kernel.ftrace_enabled=0 is now refused on kernels that deprecate the
knob, so the old disable/reload flow no longer applies there. Probe
for this with ftrace_disable_supported() and keep the full original
scenario (disable, fail to load a livepatch, re-enable, load, confirm
disable is refused while loaded) on kernels where it still works;
otherwise just confirm the write is refused.

Suggested-by: Steven Rostedt <[email protected]>
Suggested-by: Miroslav Benes <[email protected]>
Assisted-by: Claude:claude-sonnet-5
Signed-off-by: Andrey Grodzovsky <[email protected]>
---
 .../testing/selftests/livepatch/functions.sh  | 13 ++++++
 .../selftests/livepatch/test-ftrace.sh        | 45 ++++++++++++-------
 2 files changed, 41 insertions(+), 17 deletions(-)

diff --git a/tools/testing/selftests/livepatch/functions.sh b/tools/testing/selftests/livepatch/functions.sh
index 30dc677b2f45..7fa4bda70221 100644
--- a/tools/testing/selftests/livepatch/functions.sh
+++ b/tools/testing/selftests/livepatch/functions.sh
@@ -126,6 +126,19 @@ function set_ftrace_enabled() {
 	echo "livepatch: kernel.ftrace_enabled = $result" > /dev/kmsg
 }
 
+# ftrace_disable_supported() - probe whether kernel.ftrace_enabled=0
+#	can still disable ftrace on this kernel. Newer kernels deprecate
+#	the knob and always refuse the write with -EOPNOTSUPP.
+function ftrace_disable_supported() {
+	local result
+
+	sysctl -q kernel.ftrace_enabled=0 &> /dev/null
+	result=$(sysctl --values kernel.ftrace_enabled)
+	sysctl -q kernel.ftrace_enabled=1 &> /dev/null
+
+	[[ "$result" == "0" ]]
+}
+
 function cleanup() {
 	pop_config
 }
diff --git a/tools/testing/selftests/livepatch/test-ftrace.sh b/tools/testing/selftests/livepatch/test-ftrace.sh
index d2c3dea63104..cd27148510f8 100755
--- a/tools/testing/selftests/livepatch/test-ftrace.sh
+++ b/tools/testing/selftests/livepatch/test-ftrace.sh
@@ -12,29 +12,32 @@ setup_config
 # - turn ftrace_enabled OFF and verify livepatches can't load
 # - turn ftrace_enabled ON and verify livepatch can load
 # - verify that ftrace_enabled can't be turned OFF while a livepatch is loaded
+# (skipped on kernels where the sysctl is deprecated and always refuses 0)
 
 start_test "livepatch interaction with ftrace_enabled sysctl"
 
-set_ftrace_enabled 0
-load_failing_mod $MOD_LIVEPATCH
+if ftrace_disable_supported; then
 
-set_ftrace_enabled 1
-load_lp $MOD_LIVEPATCH
-if [[ "$(cat /proc/cmdline)" != "$MOD_LIVEPATCH: this has been live patched" ]] ; then
-	echo -e "FAIL\n\n"
-	die "livepatch kselftest(s) failed"
-fi
+	set_ftrace_enabled 0
+	load_failing_mod $MOD_LIVEPATCH
 
-# Check that ftrace could not get disabled when a livepatch is enabled
-set_ftrace_enabled --fail 0
-if [[ "$(cat /proc/cmdline)" != "$MOD_LIVEPATCH: this has been live patched" ]] ; then
-	echo -e "FAIL\n\n"
-	die "livepatch kselftest(s) failed"
-fi
-disable_lp $MOD_LIVEPATCH
-unload_lp $MOD_LIVEPATCH
+	set_ftrace_enabled 1
+	load_lp $MOD_LIVEPATCH
+	if [[ "$(cat /proc/cmdline)" != "$MOD_LIVEPATCH: this has been live patched" ]] ; then
+		echo -e "FAIL\n\n"
+		die "livepatch kselftest(s) failed"
+	fi
 
-check_result "livepatch: kernel.ftrace_enabled = 0
+	# Check that ftrace could not get disabled when a livepatch is enabled
+	set_ftrace_enabled --fail 0
+	if [[ "$(cat /proc/cmdline)" != "$MOD_LIVEPATCH: this has been live patched" ]] ; then
+		echo -e "FAIL\n\n"
+		die "livepatch kselftest(s) failed"
+	fi
+	disable_lp $MOD_LIVEPATCH
+	unload_lp $MOD_LIVEPATCH
+
+	check_result "livepatch: kernel.ftrace_enabled = 0
 % insmod test_modules/$MOD_LIVEPATCH.ko
 livepatch: enabling patch '$MOD_LIVEPATCH'
 livepatch: '$MOD_LIVEPATCH': initializing patching transition
@@ -60,6 +63,14 @@ livepatch: '$MOD_LIVEPATCH': completing unpatching transition
 livepatch: '$MOD_LIVEPATCH': unpatching complete
 % rmmod $MOD_LIVEPATCH"
 
+else
+
+	set_ftrace_enabled --fail 0
+	check_result "livepatch: sysctl: setting key \"kernel.ftrace_enabled\": \
+Operation not supported"
+
+fi
+
 
 # - verify livepatch can load
 # - check if traces have a patched function
-- 
2.34.1